Designing plant-based bigels for controlled nutrient delivery: structural and digestive perspectives

1 Introduction

The global prevalence of chronic health conditions, including obesity, cardiovascular diseases, and type 2 diabetes, has been strongly linked to dietary habits, particularly the high intake of saturated and trans fatty acids (De Souza et al., 2015). Consequently, consumers and regulatory bodies are increasingly demanding healthier, clean-label, minimally processed, and plant-based alternatives that can replace conventional solid fats without compromising the sensory and structural qualities of food (Aschemann-Witzel et al., 2021; Sanpedro-Díaz et al., 2025).

The growing demand for functional foods enriched with bioactive compounds, such as polyphenols, carotenoids, ω-3 fatty acids, phytosterols, vitamins, and probiotics, has highlighted persistent delivery problems. Most of these molecules are poorly soluble, chemically unstable, and have low bioavailability during gastrointestinal transit when incorporated into conventional food matrices (Manzoor et al., 2022; Pinto et al., 2021).

An ideal carrier protects bioactive compounds from processing stress (heat and mechanical stress) and gastrointestinal degradation (oxidation, hydrolysis, and pH fluctuations) while ensuring efficient intestinal absorption. A system with low gastric stability (rapid disintegration in the stomach) or high lipase resistance (resistance to small intestinal enzymatic lipolysis) will effectively fail as a bioactive delivery system, despite its structural strength. Because simple, single-phase systems lack the necessary structural integrity and capacity for site-specific release, researchers have relied on multiphase, structured colloidal systems to achieve effective encapsulation (Julian McClements and Mahdi Jafari, 2017; Osullivan et al., 2017).

Traditional delivery vehicles exemplify the challenges in addressing this issue, as each structured matrix fails at distinct junctions. Liquid emulsions are thermodynamically unstable because of the tendency of their mobile droplets to coalesce, cream, and coarsen through Ostwald ripening, which gradually erodes the dispersed structure essential for delivery (Julian McClements and Mahdi Jafari, 2017). Emulsion gels, or emulgels, enhance stability by gelling the continuous phase; however, the mechanical disparity between a rigid matrix and liquid droplets facilitates droplet migration and phase separation over time (Shakeel et al., 2019). Solid lipid nanoparticles may undergo polymorphic transitions during storage, wherein the crystallizing lipid lattice expels the encapsulated payload (Gonçalves et al., 2018). Hydrogels effectively transport hydrophilic bioactives but require prior emulsification to accommodate hydrophobic compounds for their delivery. Their high water content renders them incompatible with low-moisture foods and makes them susceptible to microbial spoilage (Dimakopoulou-Papazoglou et al., 2025; Wang et al., 2025c). Oleogels serve as efficient fat replacers and lipophilic carriers; however, their hydrophobic continuous phase excludes hydrophilic bioactives and limits their compatibility with high-moisture products (Jin et al., 2026). The underlying issue is that each system either lacks a second compatible phase for co-delivery or depends on a dispersed phase that remains mobile. Bigels were developed to address both failure modes simultaneously by gelling both phases, thereby preventing the aqueous and lipid domains from flowing, coalescing, or separating from each other.

To overcome these limitations, bigels (also known as biphasic or hybrid gels) have emerged as a highly promising class of semi-solid soft materials constructed by combining two distinct, immiscible gel phases: aqueous hydrogels and lipid-based oleogels. In a bigel, both the internally dispersed phase and the externally continuous phase are independently structured into three-dimensional (3D) networks. This dual-gelation architecture eliminates the mobility of the dispersed phase, conferring exceptional physical and quasi-thermodynamic stability that can extend the shelf life for months without the use of synthetic emulsifiers (Alves Barroso et al., 2022; Dangi et al., 2025). Unlike emulsions, bigels do not depend on emulsifiers for kinetic stability, as both the internal and external phases are gelled. Furthermore, the reported shelf life exceeds 6 months without phase separation (Zampouni et al., 2024). Unlike single-phase gels, bigels permit the simultaneous encapsulation of hydrophilic and lipophilic bioactives in their respective compatible phases, with the possibility of independent control over the release kinetics of each (Cheng et al., 2025; Liu et al., 2024; Mao et al., 2020).

The primary advantage of bigels lies in their ability to simultaneously encapsulate and co-deliver hydrophilic and lipophilic bioactives within their distinct phases, thereby overcoming the solubility constraints inherent to monophasic hydrogels and oleogels (Table 1). Furthermore, the physicochemical, thermal, and rheological properties of bigels can be tailored by controlling the hydrogel-to-oleogel mass ratio, hydrogelator and oleogelator chemistry, interfacial composition, and processing conditions, producing structural morphologies that vary from oleogel-in-hydrogel to bicontinuous and hydrogel-in-oleogel architectures (Gonçalves et al., 2024; Xie et al., 2023; Zhu et al., 2021).

TABLE 1. Encapsulation efficiency and bioactive retention of bigels.

Although bigels were originally popularized in the pharmaceutical and cosmetic industries for transdermal drug delivery, their fat-mimicking texture and structural versatility have driven their rapid expansion into food applications. Plant-based bigels are currently being investigated as solid fat replacers in meat, bakery, and dairy products (Akcicek et al., 2025; Ghiasi and Golmakani, 2022; Hou et al., 2024), inks for 3D food printing (Cai and Lu, 2026; Fernandes et al., 2026; Xie et al., 2023), and advanced delivery matrices for nutraceuticals, antioxidants, carotenoids, and probiotics (Li et al., 2025; Qian et al., 2025; Wei et al., 2025). This review provides a comprehensive analysis of the theoretical foundations and structural design principles of plant-based bigels, explores the mechanistic relationships between bigel microstructure and digestive behavior, highlights the current applications across various bioactive classes, and highlights the research gaps and future research opportunities in the development of healthy functional foods, which will determine whether plant-based bigels transition from laboratory demonstration to commercial deployment.

This review distinguishes itself from existing bigel reviews in several ways. This review integrates structural design with digestive fate, considering bigel architecture as a regulator of release mechanisms rather than merely a descriptive attribute. This approach contrasts with that of prior reviews, which typically address structure and digestion separately. Additionally, the review adopts a critical, comparative perspective, emphasizing contradictions in the literature, such as the differing effects of oleogel content and gelator chemistry on the release of various bioactives, rather than presenting the studies in a sequential manner. Collectively, this review addresses a specific gap by linking the structural aspects of plant-based bigels to their digestive functions and identifying unexplored areas.

To ensure transparency, the relevant literature was identified through searches of Scopus, Web of Science, PubMed, and Google Scholar, focusing on studies published over the past decade, with an emphasis on the most recent 5 years. Search terms combined core descriptors (bigels, biphasic gels, oleogel–hydrogel systems) with terms related to composition and function (plant-based, food-grade, encapsulation, controlled release, bioactive delivery) and to digestive behaviour and characterisation (in vitro digestion, bioaccessibility, lipolysis, rheology). Peer-reviewed primary and review articles published in English and relevant to food or oral nutraceutical applications were considered, while studies on bigels developed exclusively for pharmaceutical, cosmetic, or topical use were excluded unless they provided transferable structural or mechanistic insight. Although the review focuses on plant-based gelator systems, a limited number of non-plant model systems including gelatin-based and beeswax-containing formulations are discussed where they provide mechanistic evidence not yet available from plant-based studies.

2 Conceptual framework

2.1 Definition and classification

A gel is broadly defined as a coherent two-component system in which a structured network immobilizes a liquid phase to produce a material with solid-like mechanical behavior (Julian McClements and Mahdi Jafari, 2017). A bigel (also known as a biphasic gel or hybrid gel) is formally defined as a soft material containing two immiscible liquid phases that are individually and independently stabilized by their own gelators and combined into a single semi-solid matrix (Wakhet et al., 2015; Zampouni et al., 2024). This defining criterion, where both phases are individually gelled, distinguishes a bigel from an emulsion, emulgel, or any other biphasic system, where only one phase forms a structured network.

Bigels are primarily classified according to their phase architecture, which is largely determined by the volumetric or mass ratio of the two phases (Figure 1). Oleogel-in-hydrogel (O/H) systems contain oleogel phase dispersed within a continuous hydrogel matrix which is hydrogel-rich compositions, whereas hydrogel-in-oleogel (H/O) systems consist of hydrogel phase dispersed within a continuous lipid matrix as a oleogel-rich compositions. Consequently, intermediate phase ratios can induce phase inversion, triggering a transition to bicontinuous architectures. In this state, both the aqueous and lipid phases form co-continuous interpenetrating networks rather than isolated droplets. (Han et al., 2025; Wei et al., 2025).

The ratio at which inversion occurs varies depending on the gelator used. A bicontinuous structure was developed at a 5:5 oleogel-to-hydrogel ratio in a monoglyceride system and inverted to H/O only above approximately 60% oleogel (Tian et al., 2024). In another study, the oleogel-in-hydrogel structure remained intact up to 50% oleogel, became bicontinuous at 60%–70%, and inverted at 80%–90% (Wei et al., 2025). Because inversion thresholds span a broad range rather than a single value, architectures from different studies can be compared only when the gelator type and concentration are known.

A further morphological distinction separates bulk (monolithic) bigels from bigel beads, which are discrete millimeter-scale spherical particles (typically ranging from 1.8 to 2.5 mm in diameter). Fabricated by dripping or extruding an oleogel-in-hydrogel emulsion, these bigel beads typically rely on sodium alginate as the hydrogelator. Introducing this matrix into a crosslinking bath containing divalent cations (such as Ca²⁺) induces rapid ionotropic gelation within the continuous aqueous phase. (Lin et al., 2024; Liu G. et al., 2025; Yang et al., 2024a; Zhang et al., 2025).

Finally, plant-based bigels are defined as those in which all structural components, including the hydrogelator, oleogelator, base oil, and interfacial stabilizers, are derived exclusively from botanical sources. This excludes systems utilizing animal-derived proteins (such as gelatin or whey protein isolate) and insect-derived structurants (such as beeswax), ensuring that the formulations fully align with clean-label, vegan, and sustainable dietary preferences.

2.2 Bigels distinguished from emulsions and emulgels

The distinction between bigels, emulgels, and structured emulsions has direct implications for formulation stability, encapsulation performance, and regulatory classifications. Conventional and structured emulsions (high-internal-phase emulsions and Pickering emulsions) are kinetically stabilized dispersions of one liquid phase within another, maintained by surfactants or particles that reduce the interfacial tension. These systems are thermodynamically unstable and highly susceptible to gravitational separation, coalescence and Ostwald ripening. Although emulgels address this issue by cross-linking the continuous phase, the ungelled state of the dispersed liquid droplets introduces a distinct mechanical mismatch between the rigid matrix and mobile internal phase (Behera et al., 2014; Sagiri et al., 2015).

The conceptual advancement of bigels is to resolve this problem by simultaneously gelating both phases. Because the dispersed inclusions are rigid gel networks rather than mobile liquids, they restrict the internal flow and eliminate Ostwald ripening, which requires solute diffusion through liquid droplets (Kodela et al., 2017; Zhu et al., 2021). Physical stability is substantially enhanced, and well-formulated bigels have been shown to retain the fatty-acid composition over 6 months in fish-oil systems (Rehman et al., 2014) and to retain over 92% of encapsulated curcumin over 28 days at 25 °C in plant-protein bigels (Qian et al., 2025). Reliance on emulsifiers is reduced, with surfactant-free formulations being feasible in agar/stearyl alcohol bigel systems (Kodela et al., 2017). The mechanical properties can be independently controlled through composition, with the viscoelastic response of the composite reflecting contributions from both networks and their modes of contact (Behera et al., 2014; Sagiri et al., 2015). However, these advantages should not be overstated: in the absence of interfacial stabilization, bigels can still exhibit slow phase coarsening, and many published formulations incorporate amphiphilic gelators, such as monoglycerides or lecithin, which simultaneously perform bulk structuring and interfacial stabilization (Li et al., 2025; Lu et al., 2022; Machado et al., 2023). The distinction between bigels and emulgels is best understood as a continuum rather than an absolute boundary.

2.3 Stabilisation mechanisms

The stability of a bigel can be explained by bulk and interfacial stabilization mechanisms, as shown in Figure 2. Bulk stabilization refers to the immobilization of gelator networks within each phase. In the dispersed phase, gelation arrests internal flow and eliminates coalescence pathways. In the continuous phase, gelation imparts a yield stress that prevents droplet sedimentation and creaming (Kodela et al., 2017; Sagiri et al., 2015). This is generally considered to be the primary contributor to long-term shelf life and the principal reason surfactant-free formulations are feasible.

Interfacial stabilization at the oil–water boundary involves three types of stabilizers such as amphiphilic gelators (monoglycerides, lecithin), small-molecule surfactants (Tween-80, PGPR, sucrose esters), and solid colloidal particles via Pickering mechanisms (whey protein microgels, milled corn-silk particles, plant-protein nanoparticles) (Cai and Lu, 2026; Liu et al., 2024; Luo et al., 2025; Yang et al., 2023). Each of these either lowers the interfacial tension or builds a physical barrier against coalescence. Although it is not essential once both phases gelled, it is crucial during hot-mixing or homogenization for initial droplet formation, when the dispersed phase is still molten and most easily disrupted. Pickering particles offer an additional benefit because they adsorb almost irreversibly and form a rigid shell, they also prevent lipase from reaching the lipid interface, which improves storage stability and slows lipolysis during digestion Combining Pickering-particle stabilization with bulk gelation enhances the rheological reproducibility and storage stability of bigels and slows lipase access (Cai and Lu, 2026; Liu et al., 2024), Once both phases are gelled, interfacial stabilization is no longer essential; however, the two mechanisms still complement each other rather than substituting for one another. The partitioning of amphiphilic gelator molecules between the bulk and interface affects the network strength and interfacial coverage. The effect of glycerol monostearate on curcumin release depends on the hydrogel chemistry, which illustrates the combination of bulk and interfacial stabilization mechanisms (Gray et al., 2025; Lu et al., 2022). However, the balance between these two mechanisms remains unclear. Few studies separate the bulk and interfacial contributions, and the view that bulk stabilization dominates is based mainly on a handful of surfactant-free bigel systems, even though most food bigels still use amphiphilic gelators.

2.4 Functional rationale for the biphasic gelled architecture

The biphasic gelled architecture offers unique functional benefits that are not available in single-phase systems. The two phases carry hydrophilic bioactives in the hydrogel and lipophilic bioactives in the oleogel, enabling the co-delivery of bioactive compounds (Table 1), such as curcumin and EGCG (Lin et al., 2024; Lu et al., 2022; Yang et al., 2023), curcumin and N-acetylneuraminic acid (Qian et al., 2025), and ascorbic acid and astaxanthin (Wei et al., 2025).

These phases have distinct release mechanisms: hydrogels use swelling and hydrolysis, whereas oleogels use lipolysis and erosion to release bioactive compounds. Release kinetics can be modulated independently, for instance, varying the glycerol monostearate (GMS) content systematically alters curcumin release while leaving epigallocatechin gallate (EGCG) release unaffected (Lu et al., 2022). and phase-architecture-modulated bioaccessibility of ascorbic acid (AA) and astaxanthin (AST) (Wei et al., 2025). Both gel networks protect unstable bioactives from oxygen, light, and acids more effectively than single-phase gels. Co-encapsulated antioxidants offer synergistic protection (Qian et al., 2025; Wei et al., 2025). Bigels can also be mechanically stronger than single-phase gels (Table 2), with higher storage modulus, hardness, and yield stress (Huang et al., 2026; Martins et al., 2023; Zhu et al., 2021). However, improvements in mechanical properties are not universal. Because the storage modulus follows the phase continuity and phase ratio (Tian et al., 2024; Wei et al., 2025), the improvement over the single-phase gels depends on the composition of the bigel.

TABLE 2. Physicochemical and functional characteristics of bigels.

3 Structural design principles

3.1 Oleogel components and gelation mechanisms

The oleogel phase of a plant-based bigel comprises a continuous edible vegetable oil and a structuring agent (oleogelator) that organizes into a 3D network capable of immobilizing liquid oil through capillary forces, surface adsorption, and physical entrapment (Pascuta et al., 2022; Sanpedro-Díaz et al., 2025). Vegetable oils reported in plant-based bigels include soybean, sunflower, olive, corn, flaxseed, coconut, avocado, and pomegranate seed oil (Akcicek et al., 2025; Gonçalves et al., 2024; Kavya et al., 2025; Luo et al., 2025; Machado et al., 2023). Fatty acid saturation, polarity, and viscosity are important properties that determine oleogel firmness.

The relationship between oil composition and oleogel strength is system-dependent and cannot be predicted solely from bulk physicochemical properties. In direct wax-based oleogels, studies have reported that oils with higher unsaturation produce stronger crystalline networks (Han et al., 2022). For bigels intended for dysphagia, corn and linseed oils showed that linseed (most polyunsaturated) gelled at the lowest beeswax concentration, producing a denser microstructure and stronger van der Waals interactions, consistent with weaker hydrophobic gelator-solvent interactions favoring gelator-gelator self-association during nucleation. However, the opposite order has been reported for emulsion-templated systems with hydrocolloid stabilizers (Wang et al., 2020). Vitamin C loaded HPMC oleogels with corn oil exhibited higher Vitamin C loading, better oxidation stability, and superior physical properties than those with linseed or camellia oil, which was attributed to differences in the saturated fatty acid content interacting with emulsion templating. These contradictory findings suggest that the oil saturation–strength relationship cannot be reduced to a single rule. Minor components, fatty acid composition specific to the gelator system, and solvent–gelator molecular compatibility must be considered for a better understanding of these relationships.

Plant-derived oleogelators are classified into four categories based on their gelation mechanisms. Fatty acid crystallization forms crystalline networks from natural waxes (candelilla, carnauba, rice bran, and sunflower; beeswax is often included), monoglycerides (monopalmitin and monostearin), diglycerides, fatty acids (stearic and palmitic), and fatty alcohols (stearyl). Waxes are ideal for plant-based bigels, as they gel at low concentrations: approximately 1 wt.% for rice bran wax and two to five wt.% for candelilla and carnauba waxes, depending on the oil phase (Dassanayake et al., 2009; Wang Z. et al., 2024). The crystal morphology varies among waxes and is sensitive to the processing. Cryo-SEM imaging shows that rice bran, sunflower, and candelilla waxes form microplatelet networks that appear needle-like in polarized-light microscopy owing to edge-on projection (Blake and Marangoni, 2015; Scharfe et al., 2022). Carnauba wax formed larger branched crystals, whereas beeswax formed compact needle-like networks (Wang Z. et al., 2024). The cooling rate affects the crystal size and packing density, with faster cooling yielding smaller crystals and stronger gels. Monoglycerides, the most studied plant-compatible gelators, are valued for their amphiphilic behavior, which enables them to serve as bulk structures and interfacial stabilizers in many bigel formulations (Lu et al., 2022; Luo et al., 2025; Machado et al., 2023; Yang et al., 2023).

Self-assembled fibrillar networks (SAFINs) are formed via hydrogen bonding, π–π stacking, van der Waals interactions, and hydrophobic interactions. Notable gelators include 12-hydroxystearic acid, β-sitosterol/γ-oryzanol mixtures, and lecithin–sorbitan ester combinations, among others. The β-sitosterol/γ-oryzanol system is notable for plant-based use: neither gel alone, but together they form oleogels at 4–10 wt% concentration, with β-sitosterol lowering cholesterol. β-Sitosterol-based oleogel emulsions carry hydrophobic bioactives (Wang S. et al., 2024). Polymeric networks, such as ethylcellulose, form when dispersed in oil above their glass transition temperature and cooled. They are used in pharmaceutical and food bigels, such as fish oil/ethylcellulose for transdermal drug delivery (Rehman et al., 2014) and ethylcellulose/xanthan-guar bigels for lutein and ascorbic acid delivery (Kaimal and Singhal, 2023). Lecithin-based reverse spherical micelles are a smaller class of micelles, mainly used for drug delivery. Crystalline wax- and monoglyceride-based oleogels are at the top of the hierarchy owing to their low cost, low gelation concentration, food-grade status, and scalability (Akcicek et al., 2025; Martins et al., 2023; Sanpedro-Díaz et al., 2025). Phytosterol-based SAFINs and polymeric oleogels are viable alternatives, but their use is limited by cost and ingredient acceptance.

Oleogel firmness typically increases positively with gelator concentration; however, the relationship between oleogel firmness and bigel mechanical properties is not direct. Martins et al. (2023) explicitly demonstrated this decoupling: in a glyceryl monostearate/sunflower oil oleogel combined with a κ-carrageenan/locust bean gum (1:1) hydrogel, increasing GM from 5 to 20 wt% increased the post-cooling storage modulus from 18,758 to 40,184 Pa (a 2.14-fold enhancement); however, bigel hardness showed no proportional increase above 5 wt% GM, an outcome attributed to the lack of interfacial stabilization (Martins et al., 2023). This disparity indicates that robust oleogels do not necessarily produce strong bigels. The mechanical properties of the composite were influenced by the size, shape, packing density, and interfacial adhesion of the dispersed oleogel phase to the continuous hydrogel phase. These factors are determined by the emulsification conditions and interfacial composition, rather than the inherent strength of the bulk oleogelator itself.

3.2 Hydrogel components and network formation

The hydrogel phase of plant-based bigels mainly originates from hydrocolloids, primarily polysaccharides, with plant proteins as a growing secondary component (Table 3). The variety of hydrogelators exceeds that of oleogelators, with gelation mechanisms such as ionic, thermal, hydrogen-bonded, or hydrophobically associated, affecting the properties of hydrogels and bigels. Polysaccharide gums are categorized based on their source and structure into linear unbranched polymers (alginate, carrageenan), linear polymers with short branches (guar gum, xanthan gum), and branched-on-branch polymers (gum arabic) (Sanpedro-Díaz et al., 2025). Alginate from brown seaweed is widely used for its ionic gelation with divalent cations (usually Ca²⁺) through the ‘egg-box’ mechanism. Alginate hydrogels respond to pH, swelling in alkaline conditions, and contracting in acidic conditions, providing gastric protection in bigel-bead formulations for oral delivery (Huang et al., 2026; Lin et al., 2024; Liu G. et al., 2025). κ-Carrageenan from red seaweed gels, via a coil-to-helix transition upon cooling, forms a strong gel at low concentrations with K⁺ ions (Necas and Bartosikova, 2013), making it a common hydrogel component in bigels (Gray et al., 2025; Liu et al., 2024; Qian et al., 2025; Yang et al., 2023). Locust bean gum and κ-carrageenan in approximately equal proportions show synergistic gel formation and have been employed in food-grade bigels for co-delivery applications (Wang et al., 2025c).

TABLE 3. Bigel formulations and application in bioactive component delivery.

Gellan gum, produced by Sphingomonas elodea, is used to form transparent gels at low concentrations (0.1–0.5 wt%) and offers low-acyl (brittle, strong) and high-acyl (elastic, soft) variants for textural adjustment (Luo et al., 2025; Sanpedro-Díaz et al., 2025; Zhu et al., 2021). Xanthan gum, derived from Xanthomonas campestris, is valued for its pH- and temperature-stable viscosity and acts as a thickener and synergist in bigels, such as gellan/xanthan hydrogels (Luo et al., 2025). Agar from red algae forms thermally reversible gels through helix–coil transitions and is used in plant-based bigel formulations, either alone or in combination with other polysaccharides such as κ-carrageenan (Dimakopoulou-Papazoglou et al., 2025; Fernandes et al., 2026; Kodela et al., 2017).

Pectin, particularly low-methoxyl pectin (LMP), gels ionically through Ca²⁺ bridging between unesterified carboxyl groups and has been used in plant-based bigels, where ionic gelation and pH responsiveness are desired (Huang et al., 2026; Liu G. et al., 2025). The degree of esterification determines the gelation mechanism and pH responsiveness, providing additional design considerations. In particular, LMP-based bigels have been used to encapsulate lipophilic bioactives, such as lutein, via Ca²⁺-modulated network density (Huang et al., 2026). Starches and their derivatives provide cost-effective, abundant, and functionally diverse hydrogel networks, including potato starch/GMS systems for foundational structural studies and maltodextrin-based formulations for probiotic delivery (Alves Barroso et al., 2022; Behera et al., 2014). Seed-derived gums and mucilages represent an emerging class with strong sustainability credentials; psyllium husk and flaxseed gum, in particular, are well documented in plant-based bigel formulations (Akcicek et al., 2025; Li et al., 2025). Psyllium husk powder, used at 5–10 wt% in the hydrogel phase combined with a 7.5 wt% beeswax oleogel, has been demonstrated as an effective palm-oil substitute in cakes, with the additional benefit of contributing ∼80% soluble dietary fiber content of psyllium (Akcicek et al., 2025).

Plant proteins are favored in bigel research because of their nutritional value, amphiphilic nature (allowing them to act as bulk gelators and interfacial stabilizers), and alignment with consumer preferences for animal-free products (Sanpedro-Díaz et al., 2025). Gonçalves et al. (2024) developed potato protein/candelilla wax bigels and found that increased protein concentration improved the hardness and complex modulus. In this system, curcumin had 16.3% bioaccessibility and 43.8% chemical stability, making these bigels slow-release, colon-targeted carriers (Gonçalves et al., 2024). Pea protein–alginate bigel beads use pea protein as an emulsifier to stabilize oleogel droplets, while alginate controls bead strength and release for the co-delivery of curcumin and EGCG (Lin et al., 2024). Soy protein isolate with κ-carrageenan forms emulsion gels and bigels, achieving curcumin encapsulation efficiencies of 98%–99% and improving curcumin retention over 21 days at 25 °C. Glycerol monostearate concentration delays gastric release and increases intestinal lipolysis (Gray et al., 2025). Hydrogelator combinations and networks are crucial for bigel formulations. Examples include locust bean gum/κ-carrageenan, which forms networks through galactomannan–helix interactions and is used for capsaicin and gallic acid co-delivery (Wang et al., 2025b), konjac glucomannan/gelatin networks with oral sensation and digestive behavior (Tian et al., 2023; Tian et al., 2024), sodium alginate/κ-carrageenan using ionic and thermal gelation in bigel-bead systems (Liu G. et al., 2025), and protein–polysaccharide composites such as protein/alginate and soy protein isolate/κ-carrageenan systems (Gray et al., 2025; Lin et al., 2024). The design space for hydrogelator pairs is vast, with limited systematic structure–property mapping, leaving room for predictive models.

3.3 Oleogel-to-hydrogel ratio and structural optimisation

Among the design considerations, the hydrogel-to-oleogel mass ratio (H:O) is the key variable that affects architecture, bioactive partitioning, rheology, texture, processability, and digestion (Table 1). The storage modulus G′ increases significantly with biopolymer concentration; in κ-carrageenan/locust-bean-gum/GMS bigels at a 90:10 H:O ratio, G′ increased nearly nine-fold from 109 Pa to 982 Pa as biopolymer concentration increased from 1 to 2 wt%, with denser hydrogels better arresting oleogel droplets (Martins et al., 2023). In agar-based bigels with stearyl alcohol oleogels, a higher hydrogel fraction increases G′ (Kodela et al., 2017), and in psyllium husk powder/beeswax bigels, a higher PHP fraction boosts G′ below 60 °C, with gradual gel–sol transitions (Akcicek et al., 2025).

The apparent viscosity is dominated by the continuous hydrogel phase in O/H architectures (Rehman et al., 2014), and bigels with higher hydrogel contents exhibit several-fold higher viscosities than their oleogel-rich counterparts under low shear conditions (Güneş et al., 2025). The hardness follows the harder phase. When the hydrogel is the harder component, as in psyllium-husk/beeswax bigels, more hydrogel results in a harder bigel (Akcicek et al., 2025). Where the hydrogel is soft, as in many wax/monoglyceride systems. The hardness increases with the oleogel content because of the dense crystalline oleogel network acting as an active filler (Gonçalves et al., 2024). Zhu et al. reported that with 0.1% high-acyl gellan gum, bigel firmness increased from 49.7 g (10 wt% oleogel) to 185.2 g (60 wt% oleogel) (Zhu et al., 2021). Springiness is maximized at intermediate H:O ratios, balancing the matrix continuity and filler reinforcement (Akcicek et al., 2025). Differential scanning calorimetry revealed that the melting endotherm of the oleogel network was preserved in the bigel, with its enthalpy (ΔH) and width depending on the hydrogel fraction (Akcicek et al., 2025; Martins et al., 2023).

The optimal H:O ratio varies depending on the application of the system. For 3D food printing, oleogel-rich bigels (30%–70% oleogel) are preferred for high yield stress and self-supporting structures, as shown in bigels stabilized by monoglycerides or lecithin (Xie et al., 2023) and bigels with carotenoid extracts from microalgae (Fernandes et al., 2026). Beyond phase inversion, printability fails unless the interfacial stabilization is improved. For gastric protection and intestinal release of lipophilic bioactives, oleogel-rich bigel beads limit gastric release to less than 10% with first-order intestinal release (Huang et al., 2026). For fat replacement in bakery applications, PHP/beeswax bigels at hydrogel-rich ratios (5%-25 PHP) replaced palm oil in cakes (Akcicek et al., 2025). The transition between an oleogel-in-hydrogel (O/H) and a hydrogel-in-oleogel (H/O) is a catastrophic phase inversion. In algal oil/gellan gum/GMS bigels, increasing oleogel content transitions from the O/H phase (20%–50% oleogel) to the bicontinuous phase (60%–70%) to the H/O phase (80%–90%) (Wei et al., 2025). In soy lecithin/beeswax oleogel + flaxseed gum hydrogel bigels for L. plantarum delivery, phase inversion above 70% oleogel reduced probiotic encapsulation efficiency (Li et al., 2025). The inversion point is sensitive to the composition, mixing temperature, shear rate, gelator hydrophilic-lipophilic balance, and the presence of emulsifiers. The predictive control of inversion points in plant-based bigels remains an ongoing area of research.

3.4 Interfacial engineering and emulsification strategies

Bigels do not require interfacial stabilization for thermodynamic stability because both phases are gelled. However, recent studies have shown that the interface is not passive (Figure 2). The interfacial composition affects droplet size, stability, lipase accessibility, and bioactive partitioning (Liu et al., 2024; Luo et al., 2025; Wei et al., 2025). Conventional surfactants are the primary strategy for stabilization. PGPR (low HLB) was compared with Tween-20 in κ-carrageenan/GMS bigels for curcumin and EGCG co-delivery, with PGPR forming H/O architectures and Tween-20 forming O/H architecture (Yang et al., 2023).

Amphiphilic gelators eliminate the need for separate surfactants. Monoglycerides, particularly glyceryl monostearate (GMS), are crystalline oleogel structures and interfacial-active molecules that form viscoelastic films to stabilize droplets without emulsifiers (Lu et al., 2022; Yang et al., 2023). Lu et al. (2022) showed that the GMS content (four to eight wt%) in O/H gelatin-based bigels controls the hardness, viscosity, oil-holding capacity, crystallinity, and curcumin release (gastric release of 20% at 4 wt% GMS dropping to 10% at 8 wt% GMS), with EGCG release unchanged. This finding contrasts with that of Gray et al. (2025), who found that increasing the GMS content from 0 to 20 wt% in SPI/κ-carrageenan bigels increased curcumin intestinal release by over 18%. The contrast between the gelatin matrix in one case and the plant protein–polysaccharide matrix in the other indicates that the effect of monoglyceride content on release depends on the hydrogel chemistry. Lecithin also has dual roles, as seen in soy-lecithin/beeswax/flaxseed-gum bigels for L. plantarum delivery (Li et al., 2025). Plant proteins (soy protein isolate and pea protein) act as macromolecular surfactants, adsorbing onto the oil–water interface through hydrophobic patches and forming viscoelastic films that resist coalescence (Gray et al., 2025; Lin et al., 2024).

The most recent and arguably most promising development in bigel interfacial engineering is the use of Pickering particles (Figure 2), which are solid colloidal particles that adsorb irreversibly at the oil–water interface and form a mechanical barrier stronger than any molecular surfactant film. Plant-based Pickering candidates demonstrated in bigels include whey protein microgels delivering limonene as a flavor-retention model with an enhanced storage modulus, freeze–thaw oil retention, and slowed lipolysis (Liu et al., 2024), milled corn-silk particles for 3D-printable Pickering bigels to improve digestion control (Cai and Lu, 2026), and sericin/sodium alginate/GMS bigel beads with freeze–thaw resistance (Aung et al., 2025). Tea saponin modification of the oleogel–hydrogel interface in alginate/monoglyceride bigel beads enhances the strength, reduces oxidation, and prolongs curcumin release (Zhang et al., 2025). Bulk and interfacial stabilization together enhance rheological properties, storage stability, and digestion control compared to using either method alone (Cai and Lu, 2026; Liu et al., 2024). Polysaccharide-layer modification of alginate-based bigel beads (pectin, carrageenan, chitosan, xanthan, and konjac) altered the release profile of co-encapsulated curcumin and EGCG, with chitosan reducing gastric shrinkage and pectin and carrageenan promoting intestinal release (Liu G. et al., 2025).

Although Pickering bigels present a promising framework for advanced delivery systems, their current validation remains limited. The present research heavily relies on a narrow selection of model formulations and specific bioactive compounds. Furthermore, there is a critical shortage of comparative studies directly evaluating their performance against conventional surfactant- or gelator-stabilized equivalents.

4 Characterization methods for bigel structure

4.1 Macroscopic assessment and the tube-inversion test

The simplest and most universally reported preliminary characterization is the tube (or vial) inversion test, in which a refrigerated bigel sample is inverted under gravity and observed for its flow. Samples that retain their shape, that is, displaying a measurable yield stress capable of resisting their own weight, are classified as gels and advanced for further analysis; samples that flow are excluded as non-gel emulsions or pastes. Although the inversion test is simple, it is crucial for binary gel/non-gel classification and is essential for further characterization (Table 4). It has a defined composition in various plant-based bigel systems. Flaxseed oil/beeswax/pectin/xanthan gum bigels with less than 20% oleogel did not form self-standing gels and were excluded from further study (Kavya et al., 2025). Sunflower oil/beeswax bigels with zein nanoparticles required at least 25% oil for full homogenization, and 40%–50% of the oil leaked during storage (Li et al., 2023). Inversion-based classification of psyllium-husk-powder/beeswax bigels showed that hydrogel-rich (75:25) compositions formed stable structures (Akcicek et al., 2025). These cases show that the inversion test threshold is composition-specific, often 20%–30% for systems without dedicated interfacial stabilization.

TABLE 4. Characterisation techniques for plant-based bigels and their strength and limitations.

Although this method serves as an efficient binary screening tool, it remains strictly qualitative in nature. This merely verifies the presence of a yield stress without quantifying its magnitude, which means that passing formulations may still exhibit structural weakness or metastability. Furthermore, because the outcomes are sensitive to thermal fluctuations and subjective operator evaluation, this approach cannot substitute for definitive instrumental analysis.

Complementary macroscopic methods include visual inspection (consistency, color uniformity, transparency, and surface oiling), colorimetry using the Lab* color system (Akcicek et al., 2025; Qian et al., 2025), and pH measurement of the aqueous phase. Although simple, these observations frequently provide the first indication of phase separation, oxidative changes, or microbial spoilage during the storage. Accelerated stability testing by centrifugation (typically 4,000–10,000 rpm for 10–15 min) quantifies oil leakage as a percolation rate and provides an early indicator of long-term physical stability (Dimakopoulou-Papazoglou et al., 2025). Stable plant-based bigel formulations typically exhibit liquid release in the 0.3%–3.4% range under these conditions. However, because the applied g-force, duration, and rotor geometry vary across laboratories, centrifugation provides a relative internal ranking rather than an absolute, universally comparable metric.

4.2 Microstructural characterisation

Confocal laser scanning microscopy (CLSM) is the standard method for visualizing bigel structures, providing simultaneous imaging of both phases via differential fluorescence staining. Nile Red partitions into the lipid phase, rendering the oleogel red, while fluorescein isothiocyanate (FITC) or calcofluor white stains the hydrogel phase green or blue, respectively (Liu et al., 2024; Luo et al., 2025; Wei et al., 2025). CLSM confirmed the structural classification, as described in Section 2 (Wei et al., 2025). demonstrated this transition in algal-oil/gellan-gum/GMS bigels, with O/H structures at 20%–50% oleogel content, bicontinuous at 60%–70%, and H/O at 80%–90%. The same progression was documented using CLSM in pea-protein/alginate/GMS bigel beads (Lin et al., 2024) and konjac-glucomannan/casein–WPI/ethylcellulose bigels, where phase inversion was observed with 2% casein addition (Han et al., 2025). CLSM also localizes encapsulated bioactives within specific phases; for example, notoginsenoside Rb1 was confirmed to be homogeneously distributed in the continuous hydrogel phase of optimized gellan/xanthan/monoglyceride bigels, preventing payload leakage (Luo et al., 2025). However, these limitations are equally significant (Table 4). The fundamental diffraction limit restricts the spatial resolution to approximately 200 nm, leaving nanoscale crystals and interfacial features completely obscured. Furthermore, fluorescent dyes can photobleach, migrate between phases, and actively disrupt interfaces. Finally, because FITC fluorescence is highly sensitive to pH, the staining protocols must be meticulously documented to allow valid cross-study comparisons.

Polarized light microscopy is particularly informative for crystalline oleogel networks, where birefringent wax or monoglyceride crystals appear as bright structures against a dark background, allowing the assessment of crystal morphology, density, and spatial distribution within the bigel matrix. This technique is rapid, requires minimal sample preparation, and provides direct evidence of the contribution of crystalline structuring to the bigel network (Zhu et al., 2021). Polarized microscopy can resolve wax crystal aggregation states, identify regions of incomplete crystallization, and characterize the spatial relationship between crystalline oleogel domains and the surrounding hydrogel matrix, complementing the CLSM view of aqueous lipid distribution. However, this technique only detects birefringent materials, leaving the amorphous hydrogel network and non-crystalline regions invisible. Furthermore, it does not provide structural or chemical identification of the imaged phases. Cryo-SEM is the technique of choice for high-resolution imaging of bigel internal structures without the drawbacks of room-temperature drying. Cryo-SEM was used to reveal the interfacial morphology of Pickering bigels, including the spatial localization of whey protein microgel particles at the oil–water interface and their corresponding effect on droplet packing (Liu et al., 2024). Its results depend heavily on cryo-fixation quality, as slow freezing forms ice crystals that damage the network, and the technique is costly and low-throughput. Conventional SEM of freeze-dried bigels is useful for examining the surface morphology of bigel beads and characterizing wax-crystal networks in the dried oleogel phase (Cai and Lu, 2026; Huang et al., 2026). However, freeze-drying inevitably collapses the hydrated hydrogel matrix; therefore, such images should not be considered as faithful representations of the native structure. Raman microspectroscopy provides spatial chemical mapping without staining and is a promising emerging method capable of localizing bioactives and characterizing molecular interactions in situ. However, its application to plant-based bigels remains limited compared to CLSM (Curtis et al., 2016; Gómez-Mascaraque and Pinho, 2021; Van der Vennet et al., 2025).

4.3 Rheological characterisation

Rheological characterization is important in bigel research because it provides quantitative metrics of mechanical performance that correlate directly with both processability (mixing, extrusion, and 3D printing) and product performance (texture, spreadability, mouthfeel, and in vivo behavior). Five standard rheological tests have emerged as core to bigel characterization: strain (or stress) amplitude sweep, frequency sweep, flow (shear rate) sweep, temperature sweep, creep recovery, and thixotropy (Table 2).

In the strain amplitude sweep conducted at a fixed frequency, the storage modulus G′ and loss modulus G″ were recorded at logarithmically incremented strains. The plateau region defines the linear viscoelastic region (LVR), and the strain at which G′ first deviates from the plateau identifies the yield point of the sample. Comparative studies of polysaccharide architecture in bigels indicate that branched-polysaccharide systems can withstand substantially higher critical stresses before structural breakdown than linear-polysaccharide systems, providing evidence that branched polymer architecture confers superior elastic properties (Behera et al., 2014). For all the gels examined, G′ exceeded G″ within the LVR, confirming predominantly elastic behavior (Akcicek et al., 2025; Martins et al., 2023). Frequency sweeps within the LVR show that G′ is largely independent of frequency and substantially exceeds G″ across the spectrum, confirming the gel character. Pea-protein and rice-bran-wax bigels cross-linked with transglutaminase (TGase) showed significant increases in storage modulus and structural recovery with increasing TGase concentration, with at least 0.2% TGase required for freeze–thaw stability (Yang et al., 2024b), illustrating how interfacial crosslinking modulates LVR boundaries.

Flow sweeps measured the apparent viscosity across shear rates of 0.01–300 s^-1. Bigels almost universally exhibit shear-thinning (pseudoplastic) behavior, fitted to a power-law model with flow consistency K and flow behavior index n < 1 (Cai and Lu, 2026; Fernandes et al., 2026). The shear-thinning characteristic is functionally important; it allows bigels to flow during extrusion or injection but recovers the structure once the shear is removed, a property essential for 3D printing, pumping, and injection-based applications. Temperature sweep maps of G′ and G″ across processing-relevant ranges. A characteristic gel-to-sol transition is observed at the melting point of the oleogel crystalline network, typically 50 °C–65 °C for wax- and monoglyceride-based bigels, with G′ collapsing as the crystals dissolve (Akcicek et al., 2025; Martins et al., 2023). This transition can be reversed upon cooling, with G′ recovering to values that frequently exceed those measured before heating, reflecting the reorganization of the crystalline network during recrystallization (Martins et al., 2023).

Creep recovery tests apply constant stress and monitor strain recovery over time. The ratio of the elastic recovery strain to the viscous strain quantifies the recovery capacity of the bigel, providing direct insight into the proportion of network elasticity to viscous dissipation. Three-interval thixotropy tests apply alternating low and high shear rates to assess time-dependent restructuring, which is critical for 3D-printable bigel inks, where the ink must flow through the nozzle, regain its structure on the print bed, and support subsequent layers. Konjac glucomannan/gelatin-based bigels have been shown to recover a substantial fraction of their initial viscosity in three-step shear rate tests, indicating a structural recovery behavior suitable for additive manufacturing applications (Tian et al., 2024). Corn silk particle Pickering bigels maintained yield stress and shear recovery suitable for 3D printing at 30%–50% oleogel content, whereas higher oleogel content showed poor shear recovery owing to crystal slip at the interface (Cai and Lu, 2026). Large-amplitude oscillatory shear (LAOS) has recently been applied to bigel and emulsion–gel systems, providing access to nonlinear viscoelastic behavior through Fourier transform analysis of the harmonic content of the stress response. LAOS has proven valuable for characterizing the deformation of dysphagia-targeted emulsion gels under realistic oral processing strains (Wang et al., 2025a).

Although rheological characterization yields powerful structural insights, the data obtained are highly sensitive to the experimental conditions. In oil-rich and Pickering systems, factors such as wall slip and edge fracture at high strain, compounded by sample loading techniques, pre-shear history, and gap settings can profoundly distort the measured modulus. Furthermore, recrystallization observed during cooling is strictly dependent on the specific thermal protocol used. Because these critical variables are frequently underreported, comparing the absolute modulus across different studies remains challenging. Consequently, internal trends within a single study offer a more reliable basis for analysis. It is also worth noting that conventional bulk rheology primarily probes intact materials rather than their structural breakdown properties. Although large-amplitude oscillatory shear (LAOS) and tribology offer closer approximations of oral processing, real-time rheological tracking during simulated digestion remains exceedingly rare.

4.4 Textural and mechanical characterisation

Texture profile analysis (TPA) quantifies the bulk mechanical response under conditions that closely simulate the oral processing. A texture analyzer fitted with a cylindrical, conical, or spherical probe compresses the bigel sample (typically twice, with a defined inter-cycle interval), and the force–distance curves are processed to extract hardness, cohesiveness, springiness, chewiness, gumminess, and adhesiveness (Akcicek et al., 2025; Qian et al., 2025). These parameters are empirical and depend on the probe geometry, compression depth, and rate; therefore, they are comparable only when the protocol is matched.

The TPA is inconsistently correlated with rheology, and this correlation depends on the architecture and chemistry of the bigel. Martins et al. showed that in carrageenan/locust-bean-gum/GMS bigels, increasing the GMS concentration from 5 to 20 wt% increased the storage modulus from 18,758 to 40,184 Pa (an increase of 2.14-fold); however, no proportional increase in bigel hardness was observed, which was attributed to inadequate interfacial stabilization above 5 wt% GMS (Martins et al., 2023). In contrast, in psyllium-husk-powder/beeswax bigels, the hardness increased monotonically with the hydrogel content because the PHP hydrogel was significantly harder than the beeswax oleogel (Akcicek et al., 2025). In agar/κ-carrageenan/sunflower-wax/monoglyceride bigels, hardness increased with the hydrogel proportion, and most formulations were self-supporting with low swelling (0.3%–3.4%) (Dimakopoulou-Papazoglou et al., 2025). These contrasting examples illustrate that the direction of the hardness and composition relationship depends on the harder phase. The implication for the formulator is that both rheology and TPA must be reported, as neither alone provides complete mechanical characterization.

For bigels intended for dysphagia applications, tribology and the study of friction and lubrication are increasingly used to analyze properties relevant to mouthfeel and oral coating (Liu et al., 2024; Tian et al., 2023; Tian et al., 2024). Tribological measurements typically report the friction coefficient as a function of the entrainment speed, generating a Stribeck-like curve from which the boundary, mixed, and hydrodynamic lubrication regimes can be identified. Whey protein microgel-stabilized Pickering bigels evolve sensorially from a lubricious texture at low particle concentrations to a stable plateau at higher concentrations, indicating that sensory performance is a tunable design dimension (Liu et al., 2024). Standardized dysphagia testing through the International Dysphagia Diet Standardization Initiative (IDDSI) framework, such as fork drop, fork pressure, and spoon tilt tests, has been integrated into bigel characterization across multiple recent applications, including 3D-printable astaxanthin-loaded emulsion gels that meet dysphagia-appropriate texture targets (Wang et al., 2025a), guar-gum/GMS/β-sitosterol bigels meeting IDDSI Levels 4 and 5 with bicontinuous H6:O4 structures (Jin et al., 2026), gelatin bigels meeting IDDSI Level 6 (Yang et al., 2026), and pea-protein/alginate bigel beads for dysphagic populations (Lin et al., 2024).

4.5 Thermal analysis

Differential scanning calorimetry (DSC) is the principal thermal analysis method for bigels, providing information on melting transitions, crystallization behavior, and energy of phase changes that are critical for both processing design and triggered-release applications. Bigel samples were sealed in aluminum pans and heated at 2 °C–5 °C/min over a relevant temperature range (typically −40 °C–100 °C) under inert nitrogen.

This has been consistently reported in previous studies (Table 2). First, the oleogel melting endotherm was preserved within the bigel, but with diminished enthalpy proportional to the oleogel mass fraction. In carrageenan/locust-bean-gum/GMS bigels, the melting enthalpy of the GMS crystals decreased progressively from the pure oleogel value as the hydrogel content increased, with the lowest oleogel formulations producing very small endothermic peaks (Martins et al., 2023). Second, pure hydrogels do not exhibit melting endotherms in the bigel-relevant temperature window because polysaccharide and protein hydrogels are predominantly amorphous at these temperatures. Third, a higher oleogel content produced sharper and more well-defined endotherms that approached the trace of the pure oleogel, reflecting a more complete crystalline structure (Akcicek et al., 2025; Lu et al., 2022).

In notoginsenoside Rb1-loaded gellan/xanthan/monoglyceride bigels, differential scanning calorimetry (DSC) identified an endothermic transition at 55 °C with ΔH = 6.25 J/g, indicating a thermal-buffering capacity for food-processing tolerance (Luo et al., 2025). Beeswax-based bigels melted at 60 °C–65 °C, whereas candelilla-wax-based bigels had higher transitions, matching the bulk wax melting profiles (Habibullah et al., 2025; Zhu et al., 2021). Flaxseed oil/beeswax/pectin-xanthan-gum bigels showed increased gel strength, hardness, and thermal stability as the oleogel fraction increased above 30% (Kavya et al., 2025), establishing a compositional threshold for thermal robustness. Thermogravimetric analysis (TGA) complements DSC by measuring mass loss with temperature, providing water content, organic decomposition temperatures, and thermal stability, with bigel decomposition profiles between those of the pure hydrogel and oleogel constituents (Rehman et al., 2014).

DSC outputs represent kinetic rather than equilibrium measurements because the transition temperatures and enthalpies vary with the scan rate and thermal history. In addition, polymorphic transitions may have occurred during the scan. Consequently, consistent protocols are essential for accurate comparison. Furthermore, water evaporation from the hydrogel can distort the high-temperature baseline unless the pans are adequately sealed. An advantageous yet underutilized application involves conducting DSC near body temperature, as the state of the oleogel, whether solid or molten at 37 °C, significantly influences its resistance to lipolysis.

4.6 Spectroscopic and crystallographic methods

Fourier-transform infrared (FTIR) spectroscopy with attenuated total reflectance (ATR) can be used to characterize molecular interactions and analyze chemical bond formation between bigel components. The analysis regions included 3,200–3,600 cm⁻¹ (O–H stretching from polysaccharide hydroxyls and water; broadening and shifts indicate hydrogen bonding), 2,800–3,000 cm⁻¹ (C–H stretching from oil and wax aliphatic chains), 1,740–1,745 cm⁻¹ (ester C=O stretching from triglycerides and waxes), 1,600–1,650 cm⁻¹ (water bending and protein amide bands), and 800–1,500 cm⁻¹ (polysaccharide fingerprint region) (Akcicek et al., 2025; Habibullah et al., 2025; Liu G. et al., 2025).

The most common analytical conclusion drawn from FTIR is that the bigel components retain their characteristic spectra in the composite, indicating that no covalent bonds are formed between the hydrogel and oleogel components, which justifies treating bigels as physically associated rather than chemically crosslinked composites. FTIR analysis confirmed the physical and non-covalent nature of the bigel assembly. The observed structural shifts and intensity variations in the characteristic absorption bands indicate that hydrogen bonding, electrostatic interactions, and minor conformational adjustments drive phase integration rather than chemical crosslinking (Ebrahimi et al., 2026; Xue et al., 2024; Xue et al., 2026). However, careful consideration is required when drawing such inferences. ATR spectroscopy samples only the outermost micrometres, and significant overlap occurs among the bands in these multicomponent systems. Furthermore, the absence of new covalent peaks does not confirm a lack of specific interactions. Subtle shifts, frequently cited as evidence of hydrogen bonding, can be easily overinterpreted in the absence of corroborative data.

X-ray diffraction (XRD) characterizes the crystalline organization of the oleogel network within the bigel. Wide-angle XRD (2θ scan range 5°–50°, Cu-Kα radiation, λ = 1.54056 Å) revealed diffraction peaks corresponding to the characteristic subcell spacings of the fat crystals. Prominent peaks near 2θ ≈ 19.3° (4.59 Å) signal the highly ordered triclinc subcell of the β polymorph, while reflections around 23.7° (3.75 Å) track the presence of the orthorhombic β′ polymorph matrix within the structured triacylglycerol or monoglyceride networks, confirming the polymorphism and lateral packing modes of the structured networks (Gao et al., 2023; Ye et al., 2019). Hydrogel-rich bigels predominantly display amorphous patterns, whereas oleogel-rich bigels exhibit sharper and more intense peaks, indicating denser crystalline structures (Akcicek et al., 2025; Lu et al., 2022). XRD is limited to detecting crystalline materials; thus, it characterizes the oleogel phase instead of the entire bigel. However, the polymorph identified by XRD is significant for digestion, as the β′ and β networks exhibit distinct melting behaviors and resistance to lipolysis.

Low-field NMR (LF-NMR) measurements of transverse (T₂) relaxation can be used to characterize the mobility and distribution of water and oil protons within bigels. Distinct relaxation peaks correspond to bound, immobilized, and free water and oil populations, with peak positions and integrated areas reflecting the network density, water-holding capacity, and oil-holding capacity (Lv et al., 2026; Zhong et al., 2022). The assignment of relaxation peaks to specific populations depends on the model.

4.7 Stability and functional performance tests

In addition to structural characterization, several functional metrics quantify the performance of bigels relevant to storage, processing, and usage (Table 4). The oil-binding capacity (OBC) measures the fraction of oil retained in the bigels after centrifugation. OBC values near 100% indicate strong bigel networks, whereas values below 80% indicate inadequate structuring or interfacial weakness. Carboxymethyl cellulose/monoglyceride bigels with coconut, avocado, or pomegranate oil achieved an OBC of 72.64%–98.66% across compositions (Machado et al., 2023), representing the practical upper range in well-formulated plant-based bigels. The optimized gellan/xanthan/monoglyceride bigel for Rb1 delivery achieved high water- and oil-holding capacities, with peak Rb1 encapsulation efficiency retention after freeze–thaw cycles (Luo et al., 2025). Pickering particle-stabilized bigels exhibit strong freeze–thaw performance, retaining oil through repeated cycles (Liu et al., 2024). Pea-protein/rice-bran-wax bigels resist freeze–thaw stress, maintaining structural recovery after multiple cycles (Yang et al., 2024b). Gelatin-pectin hydrogel with 6% beeswax oleogel offers optimal thermal, pH, and ionic stability, excelling as a vehicle for volatile amine freshness detection and controlled gastrointestinal delivery (Wang et al., 2026). The water-holding capacity (WHC) measures the ability of the hydrogel phase to retain water under a centrifugal force. Swelling behavior in simulated gastric or intestinal fluids is critical for delivery applications and is typically reported as a percentage mass increase over time (Habibullah et al., 2025; Huang et al., 2026).

Encapsulation efficiency measures the fraction of bioactive compounds entrapped in the bigel and is calculated by measuring the concentration of free compounds in the supernatant. Plant-based bigels often exceed 90% efficiency, reaching 99.91% for optimized systems, as shown for notoginsenoside Rb1 in a gellan/xanthan/monoglyceride bigel (Luo et al., 2025). Soy protein isolate/κ-carrageenan gels achieved 98%–99% curcumin encapsulation, retaining 78% after 21 days at 25 °C with 15–20 wt% GMS (Gray et al., 2025). Capsaicin and gallic acid in soybean oil/lecithin/κ-carrageenan/locust bean gum bigels retained 49.3% after 12 days at 28 °C, enhancing Caco-2 bioavailability by 1.77-fold (Wang et al., 2025c). Cytotoxicity tests on Caco-2 and HT29-MTX cell lines confirmed that plant-based bigels for medium-chain fatty acid delivery met ISO 10993-5 limits, which are crucial for food-grade applications (Machado et al., 2024).

5 Digestive behaviour and controlled release mechanisms

5.1 Fundamental mechanisms of nutrient release

The release of an encapsulated compound from a bigel is regulated by four primary mechanisms: diffusion, swelling, erosion, and digestion-triggered breakdown (Figure 3). The initial three mechanisms are common to gelled and polymeric delivery matrices and are well documented in controlled-release theory (Siepmann and Siepmann, 2008; Siepmann and Siepmann, 2012). The fourth mechanism is specific to the gastrointestinal tract, where digestive enzymes and bile salts act as biochemical triggers. These mechanisms rarely function independently within a single bigel system. Instead, they operate concurrently across the two gelled phases, overlap temporally, and vary in dominance depending on composition, phase architecture, bioactive partitioning, and stage of digestion. Nevertheless, distinguishing these mechanisms is beneficial, as each responds to a distinct design parameter, and the kinetic models employed to identify them describe the overall release behavior rather than confirming the operation of a single pathway.

5.1.1 Diffusion-controlled release

In the simplest case, the bioactive compound migrates along its concentration gradient through the continuous gel phase, whereas the matrix itself remains structurally intact. The rate of this migration is influenced by factors such as the diffusion path length, network mesh size, tortuosity, molecular size of the bioactive compound, and its partitioning between the phases and the surrounding fluid. When diffusion is the rate-limiting step, the cumulative release is proportional to the square root of time, as described by the Higuchi model (Higuchi, 1963), with the Korsmeyer–Peppas release exponent being approximately 0.45, which is indicative of Fickian transport (Korsmeyer et al., 1983; Peppas and Ritger, 1987). Diffusion generally predominates during the early release of soluble compounds from dense, non-eroding networks. For instance, the release of lutein from Idesia-polysaccharide beads conformed to Higuchi diffusion (R ² > 0.91) in conjunction with first-order kinetics (Huang et al., 2026), and starch and non-starch bigels exhibited exponents below 0.5, indicating diffusion control (Behera et al., 2014). A denser hydrogel matrix, narrower mesh size, extended diffusion path, such as that observed in larger beads, and partitioning equilibrium that favors the retaining phase collectively slow down solute diffusion.

5.1.2 Swelling-mediated release

When a glassy or tightly crosslinked hydrogel absorbs digestive fluid, it transitions to a rubbery and expanded state. This transformation widens the mesh and increases chain mobility, facilitating the release of previously trapped payloads. This mechanism is typical of hydrophilic polyelectrolyte hydrogels, such as alginate and pectin, whose ionization depends on pH. These hydrogels contract under acidic gastric conditions and swell at neutral-to-alkaline intestinal pH, enabling effective pH-triggered release. The concurrent processes of solvent uptake and solute diffusion exhibited anomalous kinetics (non-Fickian), characterized by a Korsmeyer–Peppas exponent ranging from approximately 0.45–0.89. The Peppas–Sahlin model distinguishes between the Fickian and swelling-relaxation contributions, as demonstrated in agar/stearyl alcohol bigels (Kodela et al., 2017). At intestinal pH, swelling facilitates the release from alginate and low-methoxyl pectin beads (Huang et al., 2026; Liu G. et al., 2025), while the swelling and rupture of a hydrogel-continuous matrix release notoginsenoside Rb1 in hydrogel-rich formulations (Luo et al., 2025). The degree of esterification, crosslink density, and selection of polyelectrolytes collectively influence the extent of swelling and pH at which they are activated.

5.1.3 Erosion-driven release

The structured network undergoes progressive degradation through dissolution, fragmentation, or crosslink stripping, resulting in payload release as the matrix disintegrates. Erosion may occur from the surface inward, potentially achieving a constant, near-zero-order release, or throughout the bulk, leading to more complex and often accelerated release profiles (Siepmann and Göpferich, 2001). An exponent approaching unity signifies erosion-dominated transport (Case II) (Peppas and Ritger, 1987). In bigels, erosion seldom resembles the slow physical dissolution observed in pharmaceutical tablet formulations. Instead, it is typically induced by the gastrointestinal environment, which includes peristaltic shear, ionic exchange displacing the Ca²⁺ ions that maintain the alginate network, and enzymatic hydrolysis; thus, it significantly overlaps with the digestion-triggered mechanism described below. The disruption of the intestinal network that facilitates Rb1 release (Luo et al., 2025) and the quantified diffusion-versus-erosion partition for agar/stearyl-alcohol bigels (Kodela et al., 2017) exemplify erosion occurring alongside diffusion. The rate of matrix erosion is determined by the crosslink density and susceptibility of the structurant to enzymes and ions.

5.1.4 Digestion-triggered release

The primary mechanism distinguishing bigels from inert polymer matrices is the biochemical degradation of structural networks by digestive agents such as enzymes. In hydrogels, pepsin targets protein networks, amylase targets starch, and bile salts emulsify the exposed lipids. In oleogels, pancreatic lipase hydrolyzes the structuring and carrier triglycerides into free fatty acids and monoglycerides. This dual process dissolves the crystalline lipid barrier and concurrently generates mixed micelles that solubilize the lipophilic bioactives. Consequently, digestion-triggered release is directly linked to the bioaccessibility. The rate of lipolysis is determined by the crystalline density of the wax or monoglyceride network and interfacial coverage, as these factors regulate lipase access to the oil. Denser oleogels and irreversibly adsorbed Pickering interfaces decelerate lipolysis and release (Liu et al., 2024; Qian et al., 2025; Wei et al., 2025). This mechanism also dictates the selection of the phase architecture of what is released and when. An oleogel-in-hydrogel structure exposes lipids to lipolysis while safeguarding the aqueous payload, whereas a hydrogel-in-oleogel structure protects the lipids (Wei et al., 2025). This mechanism also explains why probiotic carriers may fail when intestinal bile salts emulsify the protective lipid barrier (Li et al., 2025).

In practice, these four mechanisms function concurrently, with their balance aligning with the digestive sequence in the gastrointestinal tract. In the stomach, an intact bigel releases minimal content because its behavior is constrained by barriers and diffusion barriers. In contrast, in the small intestine, swelling, erosion, and enzyme-induced breakdown occur simultaneously and become the predominant processes. Consequently, composite kinetic models, such as first-order combined with Higuchi or Peppas–Sahlin, accurately represent bigel data, as multiple mechanisms genuinely contribute concurrently to the release of bioactive compounds from bigels. Therefore, such model fits should be interpreted with caution. A high coefficient of determination indicates that the model accurately describes the release curve but does not confirm that the assigned mechanism is unique. Because nearly all these methods rely on fitting models to static in vitro data, determining the dominant mechanism ultimately requires time-resolved structural measurements during digestion, such as in situ microscopy and rheology. This methodological gap is highlighted in the Characterization and Future Directions sections.

5.2 Oral phase

Digestion initiates in the mouth; however, the oral phase remains the least examined among the three phases of bigels, with numerous digestion studies excluding it from their scope. This phase is brief, lasting from seconds to approximately a minute, and involves mechanical fragmentation through mastication, along with dilution and wetting by saliva. Saliva provides salivary α-amylase, mucins, and salts at a near-neutral pH. The primary outcome of this phase is not the release of bioactive compounds, which is minimal because of the short residence time, but rather the formation of a bolus. The particle size and surface area of this bolus establish the initial conditions for subsequent gastric and intestinal breakdown processes.

The chemical composition of the hydrogel influences the reactivity of the bigel in the oral cavity. Salivary α-amylase hydrolyzes the α-1,4 linkages of starch, allowing starch and maltodextrin-based hydrogels to commence breakdown orally. In contrast, non-starch hydrocolloids prevalent in plant-based bigels, such as alginate, gellan, carrageenan, and seed gums, are largely inert to amylase and undergo only mechanical alterations in the mouth. Consequently, for most plant-based bigels, the oral phase serves as a brief structuring step rather than a phase for the release of bioactive compounds.

Direct examination of the oral phase typically focuses on texture and swallowing rather than on delivery. Dysphagia-oriented bigels are specifically designed to address oral behavior, with assessments of bolus cohesion and flow conducted according to the International Dysphagia Diet Standardization Initiative (IDDSI) criteria, and further analyzed through oral-phase rheology and tribology (Section 4.4). In studies incorporating an oral step, the static INFOGEST protocol is generally employed, involving simulated salivary fluid with α-amylase for approximately 2 minutes (Brodkorb et al., 2019). However, this protocol does not account for the variable mastication and residence time characteristics of actual eating, nor does it address the relationship between oral fragmentation and the surface area that subsequently influences gastric and intestinal digestion. This relationship remains largely unquantified and represents a significant area for future studies.

5.3 Gastric phase protection

The gastric environment (pH 1.5–3.0), pepsin-mediated proteolysis, and mechanical agitation are the principal sites of premature degradation of many encapsulated bioactive compounds. Pre-intestinal release is undesirable for two reasons: chemically labile compounds, such as ascorbic acid, polyphenols, and certain saponins, degrade rapidly under gastric acidity, and lipophilic bioactive compounds released into the aqueous gastric lumen in the absence of bile salts and lipases fail to micellarize and are lost upon absorption (Figure 1). Bioactives are protected in gastric phase mainly by the following mechanisms (Figure 4).

5.3.1 Physical barrier of the gel network

The physical barrier of the gel network, which slows the diffusion of water, acid, and pepsin diffusion into the interior, thereby slowing the network erosion and bioactive release kinetics. The protective effect improves with mechanical strength (storage modulus and yield stress), and bigels with higher G′ resist gastric stress for longer. The optimized gellan gum/xanthan gum/monoglyceride/Tween-80 bigel with 40% oleogel (H:O 60:40) achieved a G′ of 290.64 Pa and less than 5% gastric release of notoginsenoside Rb1 over 2 h (Luo et al., 2025). In whey protein isolate/κ-carrageenan/beeswax–phytosterol bigels, the 20% oleogel formulation retained 92.4% of curcumin after 28 days and limited premature gastric release due to the structural integrity of the WPI/κ-carrageenan hydrogel matrix (Qian et al., 2025).

5.3.2 Acid-resistant polysaccharide chemistry

Acid-resistant polysaccharide chemistry provides protection against acid damage. Chitosan, in alginate-blended bigel beads, acquires a positive charge at acidic pH, promoting electrostatic crosslinking with negatively charged alginate, tightening the network, and reducing shrinkage and swelling at gastric pH; chitosan-modified alginate bigel beads showed the most restricted gastric behavior among five polysaccharide modifiers (pectin, carrageenan, chitosan, xanthan, and konjac) (Liu G. et al., 2025). Xanthan gum and gellan gum are similarly stable under acidic conditions and provide gastric protection by resisting network disintegration (Luo et al., 2025). Conversely, κ-carrageenan and pectin are more vulnerable to acid hydrolysis and pepsin-accessible swelling, and bigels relying on these polysaccharides as primary structures exhibit accelerated gastric release unless reinforced through secondary protective polymers or ionic crosslinking (Liu G. et al., 2025).

5.3.3 Hydrophobic lipid barrier

The oleogel phase contributes to gastric protection by creating a hydrophobic compartment that excludes acidic fluid. Lipophilic bioactive compounds in oleogels are isolated from the gastric lumen, and their release depends on lipase-mediated lipolysis in the small intestine. Protection increases with oleogel mass fraction, as in algal-oil bigels structured with gellan gum hydrogel and monoglyceride oleogel. Ascorbic acid release was retarded at high oleogel content compared to high hydrogel content because the continuous oleogel phase limited gastric fluid penetration into the hydrogel droplets, where AA was sequestered (Wei et al., 2025).

The reported gastric release values vary significantly. Notoginsenoside Rb1 in optimized gellan/xanthan/monoglyceride bigels at a 60:40 (H:O) ratio showed less than 5% gastric release (Luo et al., 2025). Lutein in alginate/low-methoxyl pectin/Idesia polycarpa polysaccharide bigel beads with Ca²⁺ crosslinking had gastric lutein release below 10% and followed first-order kinetics with R ² > 0.95 (Huang et al., 2026). In high-acyl gellan gum/GMS–beeswax bigels, lycopene exhibited gastric retention that inversely scaled with oleogel content, with high-concentration oleogel networks demonstrating superior resistance to swelling compared to low-concentration oleogels (Zhu et al., 2021). Gastric protection can be achieved by designing a bigel structure that considers hydrogel chemistry, oleogel mass fraction, network density, and bioactive partitioning between the phases. This variation is attributable to differences in formulation and the heterogeneity of digestion protocols. While some studies adhere to the standardized INFOGEST method, others employ simplified simulated gastric fluid. For example, Luo et al. utilized pepsin alone, omitting the lipase and bile salts present in a comprehensive model. Consequently, absolute values are not directly comparable across studies, and gastric protection inferred from static testing may exaggerate performance in the dynamic, peristaltic, and continuously emptying stomach. Despite these limitations, the consistent finding is that gastric protection can be engineered through hydrogel chemistry, oleogel fraction, network density, and bioactive partitioning. However, the actual extent of this protection requires validation in dynamic and in vivo models.

5.4 Intestinal release kinetics

The transition from acidic gastric conditions to neutral-to-alkaline intestinal pH (6.5–7.5), combined with the influx of pancreatic lipase, protease, amylase, and bile salts, as standardized in the INFOGEST protocol (Brodkorb et al., 2019), triggers structural transformations that release encapsulated bioactives (Liu et al., 2024; Osullivan et al., 2017). For most plant-based bigels, the intestinal phase is the dominant release window, accounting for 60%–95% of the cumulative bioactive liberation in the reported studies (Huang et al., 2026; Qian et al., 2025; Zhu et al., 2021).

In the small intestine (Figure 4), the rate and extent of lipid digestion are governed less by the intrinsic reactivity of the oil than by how readily pancreatic lipase can reach it; therefore, enzyme accessibility becomes the controlling variable. Lipolysis proceeds only after the lipase–colipase complex adsorbs at the oil–water interface, and the interface itself sets the pace: dense interfacial coverage and irreversibly bound Pickering particles restrict adsorption, while a tightly crosslinked oleogel crystal network limits the diffusion of enzymes and bile salts to the lipid it structures (Qian et al., 2025; Wei et al., 2025). Once adsorbed, lipase sequentially hydrolyzes the structuring and carrier triglycerides to diglycerides, monoglycerides, and free fatty acids, which co-assemble with bile salts into mixed micelles that solubilize co-located lipophilic bioactives, thereby determining their bioaccessibility (Liu et al., 2024). This biochemical attack occurs simultaneously with the physical disassembly of the matrix. Together, the progressive erosion of the hydrogel network, the disruption of the stabilizing interface, and the coalescence of the released oil phase dictate the timing of lipid exposure. This consequence is mechanistically consistent across the literature, in that systems engineered for denser networks or more robust interfaces show both slower free fatty acid release and attenuated bioactive liberation (Li et al., 2025; Luo et al., 2025). Consequently, interfacial design and phase architecture, rather than oil identity alone, are critical parameters for modulating digestive performance.

5.4.1 Release kinetic models

Intestinal release from bigels involves diffusion, swelling, erosion, fragmentation, dissolution, and stimulus-controlled mechanisms. Release of bioactives proceeds by four overlapping mechanisms such as diffusion of the bioactive through the matrix, swelling of the hydrogel, erosion or fragmentation of the network, and digestion-triggered breakdown by enzymes. Their relative contributions are usually inferred by fitting release data to kinetic models. Plant-based bigels typically follow composite kinetic models. The first-order model (release rate proportional to remaining payload) and the Higuchi model (release proportional to t½, indicating Fickian diffusion through an eroding matrix) fit bigel release data well: lutein release from Idesia-polycarpa-polysaccharide-based bigel beads followed first-order kinetics with R ² > 0.95 and Higuchi diffusion with R ² > 0.91, confirming dual structural breakdown and diffusion mechanisms (Huang et al., 2026). The Korsmeyer–Peppas model (Mt / M ∞ = k · t n) discerns release mechanisms via exponent n, with n < 0.45 indicating Fickian diffusion, 0.45–0.89 indicating anomalous transport, and n ≈ 1 indicating erosion or zero-order release. In starch and non-starch hydrogel-based bigels for drug and probiotic delivery, Korsmeyer–Peppas exponents below 0.5 indicate Fickian diffusion dominance (Behera et al., 2014). The Peppas–Sahlin model distinguished diffusion from erosion in agar–stearyl-alcohol bigels, with R ² > 0.99 for drug release fits (Kodela et al., 2017). These fits are useful, but a high R ² only confirms that a model describes the data, not that the assigned mechanism is correct. Because the models are applied to static in vitro release curves, the mechanisms they imply are best read as descriptive rather than proven and may not hold under the variable transit and continuous absorption of the real intestine.

5.4.2 Lipolysis-governed release of lipophilic bioactives

For lipophilic bioactives in the oleogel phase, intestinal release is mainly governed by lipolysis, which is the pancreatic lipase-mediated hydrolysis of triglycerides into free fatty acids (FFA) and monoglycerides. Lipolysis was quantified in vitro using the pH-stat method, in which sodium hydroxide was titrated to maintain a pH of 7.2 during digestion. The cumulative NaOH consumption was converted to FFA release using stoichiometric equations. A consistent finding in the literature is the inverse correlation between oleogel content and FFA release; bigels with higher oleogel fractions showed slower and more incomplete lipolysis (Qian et al., 2025), with a 41.7% reduction across the series of formulations. Crystalline wax and monoglyceride networks act as barriers that limit lipase access to triglycerides, and denser networks reduce the available oil surface for enzymatic attack (Qian et al., 2025). In Lactobacillus plantarum-encapsulated bigels based on soy lecithin/beeswax oleogel and flaxseed gum hydrogel, increasing the oleogel fraction reduced FFA release while improving probiotic encapsulation efficiency until phase inversion above 70% oleogel collapsed the system (Li et al., 2025).

5.4.3 Interfacial composition as a release modulator

Beyond the bulk oleogel network density, the interfacial composition independently modulates lipase accessibility. Pickering bigels stabilized by whey protein microgels at the oil–water interface showed slower FFA release than equivalent bigels stabilized by molecular surfactants, owing to the irreversible adsorption of microgel particles, which form a barrier to lipase binding (Liu et al., 2024). Adjusting the concentration of glyceryl monostearate (GMS) in the O/H κ-carrageenan/GMS bigels had different effects on the two co-encapsulated bioactives. Increasing GMS significantly slowed curcumin release, with the gastric release dropping from 20% at 4% GMS to 10% at 8% GMS. However, this had a minimal impact on EGCG release. This is because lipophilic curcumin was retained within the densifying oleogel network, whereas hydrophilic EGCG was distributed in the external hydrogel and released through gelatin hydrolysis instead of lipolysis. Notably, the direction of GMS-curcumin reported by (Lu et al., 2022) and this gelatin-hydrogel system contrasts with the findings of (Gray et al., 2025) for soy protein isolate/κ-carrageenan bigels. In their study, increasing the GMS content from 0% to 20% enhanced curcumin intestinal release by over 18%, indicating that the directional effect of monoglyceride content on release depends on hydrogel chemistry and cannot be generalized across bigel systems without specific verification.

Notoginsenoside Rb1 in the 40:60 H:O gellan/xanthan/monoglyceride bigel achieved over 90% intestinal release at 4 h with under 5% gastric pre-release, demonstrating exceptional dual performance (Luo et al., 2025). Curcumin in WPI/κ-carrageenan/beeswax–phytosterol bigels achieved 87.6% intestinal release (Qian et al., 2025). Lycopene encapsulation in HA gellan gum/GMS-beeswax bigels showed that low oleogel (10%–20%) formulations released more lycopene than oleogel-rich (60%) ones (Zhu et al., 2021), opposite to β-carotene release, where higher oleogel fractions promoted higher release and faster rates (Zheng et al., 2020), indicating bioactive-specific interactions not predicted by bulk properties. Capsaicin and gallic acid in soybean oil/lecithin/κ-carrageenan/locust bean gum bigels achieved controlled gastric and intestinal release with independently controllable release kinetics through phase localization (Wang et al., 2025c). The primary architectural advantage of bigel matrices is that individual bioactive compounds can be selectively partitioned into a specific phase, and their distinct degradation kinetics provide a targeted release profile.

5.5 Bioaccessibility and bioavailability enhancement

Bioaccessibility is the fraction of ingested bioactive compounds released from the food matrix that is incorporated into the mixed micellar phase of the small intestine for enterocyte uptake (Figure 4). Bioavailability includes transformation and absorption, which is the fraction of surviving chemical conversion in the GI lumen that is absorbed across the intestinal epithelium. Most studies on bigels assess bioaccessibility through the micellization step of an in vitro digestion model. In contrast, true bioavailability requires data from cellular, animal, or human studies, which are rarely conducted for bigels. Consequently, reported gains in “bioavailability” should be interpreted cautiously, as they predominantly rely on in vitro micellization rather than actual absorption measurements (Table 1).

Plant-based bigels generally exhibit enhanced bioaccessibility compared to free bioactive compounds or simple oil controls, with improvements depending on the bioactive compound class. Lutein and esters from Idesia polycarpa oil were delivered via three polysaccharide-based bigel-bead systems (sodium alginate, low-methoxyl pectin, and IPVP oil cake polysaccharide). The sodium alginate-based bigel achieved the highest lutein bioaccessibility at 58.5% ± 3.9%, while the LMP-based system showed a Ca²⁺-modulated increase from 40.7% ± 3.1% to 47.1% ± 3.8% with increased crosslinking density, indicating that both hydrogelator selection and crosslinking density affect lipophilic bioactive bioaccessibility (Huang et al., 2026). The curcumin bioaccessibility in potato protein/candelilla wax/corn oil bigels was 16.3% ± 3.9%, similar to that of free curcumin (15.2% ± 2.3%) but lower than that of the pure oleogel control (49.1% ± 4.9%). However, bigels had the highest effective bioaccessibility at 7.11% ± 1.52%, due to better curcumin stability (43.8%) during digestion (Gonçalves et al., 2024). These findings suggest that potato protein bigels act as slow-release, colon-targeted delivery vehicles rather than bioaccessibility enhancers. Astaxanthin and ascorbic acid co-encapsulated in algal-oil bigels showed varied bioaccessibility based on architecture: O/H bigels enhanced astaxanthin bioaccessibility via increased lipolysis, whereas H/O bigels increased ascorbic-acid bioaccessibility (37.0%) by protecting AA from gastric degradation (Wei et al., 2025). This relationship indicates that no single bigel architecture optimizes the co-delivery of different bioactives, with the bicontinuous option being practical.

A small but growing number of studies have moved beyond INFOGEST-based bioaccessibility to evaluate intestinal absorption and biological activity. Caco-2 cell-monolayer transport studies of capsaicin and gallic acid co-encapsulated in soybean oil/lecithin/κ-carrageenan/locust bean gum bigels showed a 1.77-fold increase in bioavailability compared with free bioactive compounds (Wang et al., 2025c). κ-Carrageenan/GMS bigels stabilized with Tween-20 or PGPR delivered co-encapsulated curcumin and EGCG with enhanced cytotoxicity against HCT116 colon adenocarcinoma cells compared to free-bioactive controls, with combination-index analysis (DPPH and ABTS assays) confirming synergistic antioxidant activity and enhanced biological response, providing indirect but specific evidence of cellular bioavailability gains (Yang et al., 2023). Plant-based bigels developed for medium-chain fatty acid delivery demonstrated Caco-2/HT29-MTX cell transport with cytotoxicity within ISO 10993-5 acceptance limits, confirming that the absorption-enhancement effect does not arise from epithelial damage (Machado et al., 2024). For pharmaceutical applications, HPMC/sodium-alginate fish-oil bigels at 10:90 H:O composition produced significantly enhanced transdermal permeation of imiquimod compared to hydrogel controls, with omega-3 fatty acids acting as skin permeation enhancers, demonstrating bigel bioavailability gains across various delivery routes (Rehman et al., 2014).

Mechanistic principles for maximizing bioaccessibility were identified. First, the lipolysis–bioaccessibility relation: For lipophilic bioactives, FFA release correlates with bioaccessibility as FFAs and monoglycerides from lipolysis form mixed micelles (Qian et al., 2025; Wei et al., 2025). The second principle involves micellar zeta potential and stability: bigels that produce digesta with a strongly negative zeta potential can maintain stable micelles for the solubilization of bioactives in the intestinal lumen. The third principle concerns the particle size of intestinal digesta: smaller and more uniform digesta-particle distributions are associated with higher bioaccessibility due to larger surface areas for lipolysis and increased micelle formation (Huang et al., 2026). The fourth principle is synergy in co-delivery systems. Co-encapsulated bioactives enhance bioaccessibility and stability through synergy with hydrophilic antioxidants, which protect lipophilic compounds from degradation and increase the incorporation of intact bioactive compounds into micelles. Synergism index analyses of co-encapsulated curcumin/NeuAc bigels showed Q-values of 1.11–1.92, indicating antioxidant synergy (Qian et al., 2025), with Q-values of 1.07–1.31 for co-encapsulated ascorbic acid + astaxanthin in algal-oil bigels (Wei et al., 2025).

5.6 Structure–digestion relationships: toward predictive design

The emergence of structure–digestion relationships advances predictive design beyond empirical optimization. Five primary relationships were identified in the literature as shown in Figure 5. The storage modulus and gastric protection are closely linked to each other. Higher G′ values reduce premature gastric release in bigel systems. This relationship is mechanistic because denser networks hinder gastric fluid penetration and is therefore quantitative. Specifically, the peak storage modulus of G′ = 290 Pa at a 40:60 H:O ratio in gellan/xanthan/monoglyceride bigels correlates with less than 5% gastric release of notoginsenoside Rb1 (Luo et al., 2025).

Polysaccharide enzyme susceptibility and intestinal release rate are influenced by their composition. Polysaccharides that resist acids and enzymes, such as chitosan blends, xanthan gum, and gellan gum, facilitate sustained intestinal release. In contrast, enzyme-susceptible polysaccharides, such as κ-carrageenan, pectin, and starches, exhibit a faster release. This was demonstrated with modified alginate-based bigel beads, where chitosan-modified beads showed restricted gastric release, whereas pectin- or carrageenan-modified beads showed the highest cumulative intestinal release (Liu G. et al., 2025).

The crystalline network density in the oleogel phase is inversely related to the FFA release rate, while FFA release is positively associated with lipophilic bioaccessibility via the FFA-to-micelle pathway (Qian et al., 2025; Wei et al., 2025). Thus, the ideal oleogel network density is intermediate and optimizes both gastric protection and intestinal lipolysis. This poses a non-monotonic optimization challenge, depending on the specific bioactive and intended digestive pathways of bigel.

The interfacial composition, lipase accessibility, and release rate were interconnected. The interface composition independently influences lipase access to the oleogel network. Interfaces with Pickering particles, such as whey protein microgels and corn silk particles, create adsorbed barriers that resist lipase binding more than molecular surfactant films (Cai and Lu, 2026; Liu et al., 2024). Amphiphilic gelator interfaces (monoglycerides and lecithin) provide intermediate barrier behavior, with the effect of gelator concentration on release being hydrogel chemistry-dependent (Gray et al., 2025; Lu et al., 2022). Thus, interfacial engineering separates the release rate from the bulk composition, thereby offering an independent design dimension.

The phase architecture and release-mechanism balance are crucial factors (Figure 4). O/H architectures promote lipolysis-limited release of lipophilic bioactive molecules and the swelling-controlled release of hydrophilic bioactive molecules. H/O architectures offer gastric protection for hydrophilic bioactives and slow lipolysis-controlled release of lipophilic bioactives. Bicontinuous architectures facilitate simultaneous lipolysis and bioactive release with a strong correlation, allowing synchronized micellar incorporation. This often represents a practical compromise for the co-delivery of chemically dissimilar bioactive compounds (Wei et al., 2025). Thus, structural architecture is not merely a descriptor but also a selector of release mechanisms.

These relationships serve as provisional design principles that should be considered evidence-informed starting points rather than definitive rules. To enhance the bioaccessibility of lipophilic bioactives such as curcumin, lutein, lycopene, and astaxanthin, an oleogel-in-hydrogel or bicontinuous architecture with an intermediate ratio (H:O 5:5 to 7:3) is recommended. This includes a hydrogel composed of enzyme-susceptible and enzyme-resistant polymers, an amphiphilic gelator or surfactant interface, and a monoglyceride or low-melting-wax oleogel, all of which promote moderate lipolysis conducive to bioaccessibility. For sustained release over an extended transit, an oleogel-rich hydrogel-in-oleogel architecture featuring a dense wax crystal network and a Pickering or protein microgel interface is effective in slowing down lipase access. To protect and deliver acid-labile hydrophilic compounds such as ascorbic acid, EGCG, or other polyphenols, a hydrogel-in-oleogel bigel or an alginate bead reinforced with chitosan, with a dense dispersed hydrogel and an oleogel-continuous outer phase, is most effective in excluding gastric fluid. For the co-delivery of compatible hydrophilic–lipophilic pairs, an oleogel-in-hydrogel or bicontinuous bigel with intentional phase localization allows for the release of each compound to be adjusted by the phase ratio. As these principles are derived from heterogeneous, predominantly static in vitro studies, several of which are in non-plant systems, they necessitate systematic validation, preferably in dynamic and in vivo models, before they can be considered predictive design rules.

6 Bioactive compound delivery applications

6.1 Polyphenols

Polyphenols dominate the literature on plant-based bigel delivery because of their well-documented bioactivities and formulation challenges (Table 3). Curcumin is the most extensively studied bioactive compound for plant-based bigel delivery. Its pharmacological properties (antioxidant, anti-inflammatory, and anticancer) are offset by poor oral bioavailability arising from its low aqueous solubility, rapid hepatic and intestinal metabolism, and chemical instability under alkaline and oxidative conditions.

A wide range of bigel architectures has been used for curcumin delivery. Potato protein/candelilla-wax bigels (O/H) achieved 16.3% curcumin bioaccessibility and 43.8% post-digestion stability, thus acting as slow-release, colon-targeted vehicles (Gonçalves et al., 2018). Candelilla-wax/xanthan-gum bigels delayed gastric phase curcumin release by 24.9%–32.0%, with wax providing protection and xanthan hydrogel enabling intestinal disintegration (Habibullah et al., 2025). WPI/κ-carrageenan/beeswax–phytosterol bigels achieved 92.4% curcumin retention over 28 days and 87.6% intestinal release, indicating a protective hydrophilic–lipophilic architecture (Qian et al., 2025). The role of monoglycerides in curcumin release was studied in two hydrogel-phase systems with different outcomes. In gelatin/GMS O/H bigels, Lu et al. showed that higher GMS slowed curcumin release (gastric release dropped from 20% at 4 wt% GMS to 10% at 8 wt% GMS), while EGCG release was unaffected (Lu et al., 2022). Conversely, Gray et al. found that increasing GMS from 0 to 20 wt% accelerated curcumin intestinal release by over 18%, with increases in gastric delay and intestinal lipolysis (Gray et al., 2025). This contradiction highlights the dependence of GMS-mediated release control on hydrogel chemistry. SPI/κ-CG emulsion gels and bigels achieved 98%–99% curcumin encapsulation efficiency, retaining ∼78% curcumin after 21 days at 25 °C with 15–20 wt% GMS.

Co-encapsulation of EGCG with curcumin in bigels showed synergistic protection. κ-Carrageenan/GMS bigels with Tween-20 (O/H structures) or PGPR (H/O structures) enabled curcumin/EGCG delivery with synergistic antioxidant activity, enhanced HCT116 colon cell cytotoxicity, and EGCG-mediated oxidized curcumin regeneration (Yang et al., 2023). Beeswax/low-acyl gellan-gum bigels with a bicontinuous structure achieved the highest curcumin (68.2%) and catechin (56.2%) bioaccessibility, governed by delayed lipolysis and preservation of matrix integrity during early digestion (Chao et al., 2025). Pea-protein/sodium-alginate/GMS bigel beads were developed for dysphagia-targeted curcumin + EGCG delivery, with alginate concentration affecting bead strength and pea protein acting as an emulsifier (Lin et al., 2024). Alginate-based bigel beads with secondary polysaccharide layers (pectin, carrageenan, chitosan, xanthan, and konjac) adjusted curcumin/EGCG release, with chitosan-modified beads limiting gastric release and pectin- or carrageenan-modified beads enhancing intestinal release (Liu G. et al., 2025). Tea saponin modification of the oleogel–hydrogel interface in alginate/monoglyceride bigel beads increased mechanical strength, reduced oxidation, and prolonged curcumin release (Zhang et al., 2025).

3D printable bigels using candelilla wax/fish gelatin were employed for the co-delivery of quercetin (lipophilic) and catechin (hydrophilic), with the H:O ratio and emulsifier choice (monoglyceride vs. lecithin) independently controlling release. Monoglyceride-based bigels at 70% oleogel showed differential release in the gastric and intestinal phases, with monoglycerides restricting molecular mobility more than lecithin (Xie et al., 2023). Whey-protein-microgel (WPM)-stabilized Pickering bigels delivered limonene, a model lipophilic flavor compound, with the WPM Pickering interface improving oil retention through freeze–thaw cycling, enhancing oral lubrication, and slowing lipolysis compared to whey-protein-isolate controls (Liu et al., 2024). Soybean-oil/lecithin/κ-carrageenan/locust-bean-gum bigels achieved controlled release of co-encapsulated capsaicin and gallic acid in the gastric and intestinal phases, enhancing Caco-2 bioavailability by 1.77-fold (Wang et al., 2025c).

6.2 Carotenoids

Carotenoids, such as β-carotene, lycopene, lutein, astaxanthin, fucoxanthin, and microalgal extracts, are lipophilic, oxidation-sensitive, and low-bioaccessibility molecules that can be improved by bigel encapsulation. Carotenoids showed varying release relationships with the oleogel content. β-Carotene delivery from κ-carrageenan/monoglyceride bigels shows higher oleogel content, enhanced UV and thermal stability, and an increased intestinal release rate (Zheng et al., 2020). HA gellan-gum/GMS-beeswax bigels for lycopene delivery showed decreased intestinal release, with a compact crystalline network slowing release (Zhu et al., 2021).

Lutein is delivered through multiple platforms. Ethyl-cellulose oleogels with xanthan/guar gum hydrogels in sunflower oil achieved 83.2% intestinal release at 25% oleogel content, which is higher than the 46.6% for pure hydrogels (Kaimal and Singhal, 2023). Recently, alginate-based bigel beads with Idesia polycarpa polysaccharide (IPVP) achieved a lutein bioaccessibility of 58.5% ± 3.9%, with formulations limiting gastric release below 10% and following first-order intestinal release kinetics (R ² > 0.95) (Huang et al., 2026). Astaxanthin has been delivered using several platforms. Algal-oil-based H/O bigels retained 79.1% of astaxanthin after 35 days at 25 °C, with co-encapsulated ascorbic acid (97.2% EE) providing antioxidant stabilization (Wei et al., 2025). 3D printable oleogel-in-water emulsion gels with glycerol monolaurate and gellan gum delivered astaxanthin with twice the bioaccessibility of free astaxanthin, meeting dysphagia-modified texture requirements (Wang et al., 2025a). Sericin/sodium-alginate hydrogel + GMS/soybean-oil oleogel bigel beads loaded with β-carotene demonstrated a H/O architecture, providing high water-holding capacity and freeze–thaw resistance (Aung et al., 2025).

The latest advances show that complex carotenoid extracts from microalgae can be delivered using agar/carnauba-wax bigels, with the H:O ratio and 3D-printing controlling their bioaccessibility. 3D printing increases carotenoid bioaccessibility by up to 12% in oleogel-rich (80:20 O:H) bigels (Fernandes et al., 2026). This study marks a shift from standard delivery to complex botanical and microbial extract delivery systems.

6.3 Saponins

Saponins are amphiphilic glycosidic bioactive molecules whose delivery is constrained by gastric instability, poor membrane permeability, and rapid first-pass metabolism. Notoginsenoside Rb1, a cardioprotective saponin from Panax notoginseng, was delivered using a gellan gum/xanthan gum (12:1) hydrogel and 8% monoglyceride/soybean oil oleogel bigels with Tween-80 as the interfacial stabilizer (Luo et al., 2025). The optimized 4:6 H:O ratio achieved 99.91% encapsulation efficiency, less than 5% gastric premature release, and over 90% cumulative intestinal release at 4 h, which is the highest for any plant-based bigel system reported to date. The system also exhibited a distinct endothermic transition at 55 °C (ΔH = 6.25 J/g), providing thermal-buffering capacity. This application extends bigel technology beyond the historically dominant polyphenols and carotenoids to glycosidic bioactive compounds with established pharmacological benefits.

6.4 Bioactive fatty acids

The protection and delivery of bioactive fatty acids, such as LC-PUFAs, MCTs, and fatty-acid profiles of functional plant and marine oils, represent both delivery and fat-replacement opportunities. Carboxymethylcellulose/monoglyceride bigels with coconut, avocado, or pomegranate oil reduced post-digestion degradation of key fatty acids by factors of 3, 2, and 1.7, respectively, compared with free-oil controls, with oil-binding capacities of 87%–98% (Machado et al., 2023; Machado et al., 2024). Polymer/fish-oil bigels using EPA- and DHA-rich fish oil structured by HPMC or sodium-alginate hydrogels showed enhanced transdermal drug permeation for imiquimod compared with hydrogel controls, with omega-3 fatty acids as permeation enhancers and retaining EPA and DHA for over 6 months (Rehman et al., 2014). Bigels for medium-chain fatty acid delivery showed different FA recovery profiles compared to free oil during in vitro digestion, with cytotoxicity within ISO 10993–5 limits across Caco-2 and HT29-MTX cell lines (Machado et al., 2024).

6.5 Vitamins

Vitamin delivery via plant-based bigels is less efficient than that via polyphenols and carotenoids. Ascorbic acid (vitamin C) has been delivered through ethyl-cellulose/xanthan-guar bigels with sunflower oil, with bioaccessibility reaching 87% at 25% oleogel content. The formulation is controllable for gastric retention or controlled gastric release, with the encapsulated form showing improved thermal stability and antioxidant activity compared to free AA (Kaimal and Singhal, 2023). In a separate co-delivery study, H/O algal-oil bigels achieved 37.0% bioaccessibility of ascorbic acid using the oleogel-continuous architecture as a barrier against gastric degradation while delivering co-encapsulated astaxanthin (Wei et al., 2025). Other fat-soluble vitamins (D, E, K) have been incorporated into oleogel and emulsion-gel platforms, but rarely into rigorously characterized plant-based bigels and remain vastly underexplored, representing a substantial opportunity.

6.6 Capsaicinoids

Capsaicin, the principal pungent alkaloid of chilli peppers, is highly lipophilic and exhibits analgesic, anti-inflammatory, and metabolic-modulating bioactivities. The co-delivery of capsaicin and gallic acid in soybean oil/lecithin/κ-carrageenan/locust bean gum bigels achieved controlled release of both compounds in the gastric and intestinal phases, with the biphasic structure enabling independent kinetic control through phase localization and producing a 1.77-fold increase in Caco-2 bioavailability compared with free bioactives (Wang et al., 2025c). The system also produced measurable masking of capsaicin pungency, which is an important consumer-acceptance benefit for sensorially challenging bioactive compounds.

6.7 Probiotics

Probiotic delivery differs from small-molecule delivery in that the payload is a living microorganism whose viability must be preserved during processing, storage, gastric transit, and intestinal delivery. Starch and non-starch hydrogel-based bigels with Span-40/sunflower oil oleogels delivering Lactobacillus plantarum showed that biopolymer chemistry strongly affected probiotic protection. Bigels with maltodextrin achieved the highest viability of Lactobacillus plantarum (Lp299v) under most conditions compared to other polysaccharides (soluble starch, sodium alginate, and sodium carboxymethyl cellulose) and free cells. Maltodextrin–Span-40 bigels retained 9 × 10⁸ cfu/g viability after 60 days at 4 °C, while alginate-, sodium-CMC-, and soluble starch-based bigels showed complete viability loss, attributable to maltodextrin’s osmotic regulation preventing congestion-induced deformation (Behera et al., 2014).

A fully plant-based probiotic system was developed using a soy-lecithin/beeswax oleogel with a flaxseed-gum hydrogel for L. plantarum encapsulation, showing synergistic effects between SL and BW in enhancing structural stability and encapsulation efficiency. As the oleogel fraction increased from 10% to 50%, EE rose to 90.75%, and viability was maintained at 7.33 log CFU/g after simulated GI digestion. Critically, when the oleogel proportion reached 70%, the bigel underwent O/H to H/O phase inversion, which is detrimental to probiotic encapsulation and protection (Li et al., 2025). The main limitation, recognized in the literature, is that bile salts in the intestinal phase can emulsify the lipid-rich oleogel barrier, and enzymatic disruption of the hydrogel can expose probiotics to intestinal stresses; therefore, optimal bigel probiotic carriers require bile-resistant lipid structures and slowly hydrolyzed hydrogel components.

In examining these classes, the evidence is inconsistent and, at times, contradictory. Most studies focused on polyphenols and carotenoids, while capsaicinoids, saponins, and bioactive fatty acids were the subjects of only a few investigations (Tables 1, 3). In contrast, vitamins, peptides, and essential oils have been scarcely explored, with peptides being particularly under-represented. Comparative analyses reveal that findings are specific to both class and molecule; for instance, monoglyceride content, oleogel fraction, and structural architecture influence chemically similar compounds in divergent ways. Consequently, the results obtained for one bioactive compound should be cautiously applied to another. The most reliable outcomes have been achieved in the protection and controlled release of individual lipophilic polyphenols and saponins. However, significant gaps remain in the validated delivery of peptides and fat-soluble vitamins, as well as in translating in vitro bioaccessibility into proven in vivo benefits.

7 Stability, safety, and regulatory aspects

7.1 Oxidative stability

The key advantages of plant-based bigel applications, such as the encapsulation of polyunsaturated lipids and oxidation-sensitive bioactives, also limit system stability (Tables 2, 3). The biphasic architecture creates a significant oil–water interface that can promote lipid oxidation, despite the gelled bulk phases preventing oxygen diffusion and trace metal contact. Oxidative degradation occurs via primary and secondary processes, such as hydroperoxide formation during initiation and propagation (measured by the peroxide value, PV) and decomposition into aldehydes, ketones, and other secondary products (measured by TBARS). PV typically increases during storage, whereas TBARS shows negligible initial accumulation, which then accelerates (Dimakopoulou-Papazoglou et al., 2025). The Codex Alimentarius PV limit of 15 meq O₂/kg oil for cold-pressed and virgin oils (Codex Alimentarius Commission, 1981) was the benchmark for the bigel formulations. The literature shows oxidative stability in algal-oil systems: H/O bigel > bicontinuous > O/H, with H/O and co-encapsulated antioxidants reducing PV and TBARS over 35 days at 25 °C compared to O/H controls (Wei et al., 2025). However, the agar/κ-carrageenan/sunflower-wax/monoglyceride bigels exhibited higher PV than the pure oleogels, exceeding the Codex limit at multiple H:O ratios over 35 days. This indicates that the structure and oxidation relationships depend on the gelator chemistry and antioxidants. β-Carotene was degraded by 90%–97% at 55 °C in sericin/SA/GMS bigel beads, highlighting the need for cold chain handling of thermally sensitive carotenoids (Aung et al., 2025). Co-encapsulated antioxidant pairs provide synergistic protection. Interaction-index (Q) values of 1.07–1.31 for AA + AST in algal-oil bigels (Wei et al., 2025) and 1.11–1.92 for curcumin + NeuAc in WPI/κ-CG bigels (Qian et al., 2025) establish co-encapsulation as a practical strategy for oxidation-prone bioactives.

7.2 Microbial stability

The water content of plant-based bigels (50%–80% aqueous phase) makes them more susceptible to microbial growth than lipid-rich systems. Bigels stored at 25 °C exhibited fungal contamination after 14 days, whereas refrigeration (4 °C) extended their stability to at least 1 month (Dimakopoulou-Papazoglou et al., 2025). Probiotic-encapsulating bigels face the challenge of maintaining organism viability (7.33 log CFU/g after simulated GI digestion) (Li et al., 2025). For commercial use, refrigerated distribution is required unless thermal pasteurization, hurdle technology, or natural antimicrobials are used. Water activity reduction strategies include the use of low-molecular-weight humectants, oleogel systems, increased biopolymer cross-linking, and freeze-drying to inhibit microbial growth (Siachou et al., 2023; Walendziak et al., 2025). Natural antimicrobials such as essential oils, nisin, lysozyme, and plant phenolic extracts are underexplored, especially bigels with antimicrobial phenolics (gallic acid, EGCG, curcumin, and capsaicin) (Wang et al., 2025c).

7.3 Physical and thermal stability

Long-term stability without phase separation is supported by 6-month storage studies of polymer/fish-oil bigels (Rehman et al., 2014), 28 days at ambient and elevated temperatures for plant-protein/κ-carrageenan bigels with over 90% curcumin retention (Gray et al., 2025; Qian et al., 2025), and 6 months at room temperature for liquid-paraffin/poloxamer-407 systems in early pharmaceutical formulations (Mazurkeviciute et al., 2018). Quantitative stability measures include the liquid-release percentage (0.3%–3.4% for stable bigels) (Dimakopoulou-Papazoglou et al., 2025), oil-binding capacity (87%–98%) (Machado et al., 2024), and freeze–thaw stability. Pickering particles and high-protein bigels exhibit strong freeze–thaw performance (Aung et al., 2025; Cai and Lu, 2026; Liu et al., 2024; Yang et al., 2024c).

The thermal stability is governed by the oleogel melting endotherm (50 °C–65 °C for wax- and monoglyceride-based bigels), above which the bigel softens into a less-structured emulsion (Kavya et al., 2025; Luo et al., 2025). This information is useful for hot processing, such as cake-baking applications. PHP/beeswax bigels at hydrogel-rich H:O ratios have successfully replaced palm oil (Akcicek et al., 2025), and ethylcellulose/wheat-starch bigels at H25:O75 have sustained beef-burger cooking with up to 50% animal-fat replacement (Ghiasi and Golmakani, 2022). As a design principle, oleogel-rich bigels (25H:75O) exhibit superior thermal stability and higher phase-transition limits, making them suitable for high-temperature processing. Hydrogel-rich formulations (75H:25O) exhibit highly porous hydrophilic networks that favor accelerated bioactive diffusion and lower thermal resistance (Kaimal and Singhal, 2023).

7.4 Toxicological and allergenicity considerations

The safety profile of plant-based bigels (Table 5) is attributed to the safety of their component ingredients, as the assembly of these ingredients into a biphasic gel does not introduce novel chemical species. FTIR analyses consistently demonstrated that the bigel components retained their characteristic spectra in the composite, with no evidence of new covalent bonds (Akcicek et al., 2025; Habibullah et al., 2025; Liu G. et al., 2025).

TABLE 5. Regulatory status of gelators commonly used in plant-based bigels/bigels (oleogel and hydrogel structurants).

Ethylcellulose is authorised for food use in both the EU, and US; its limitation is technological (dissolution above ∼130 °C can degrade unsaturated oils and heat-labile bioactives), not regulatory.

Rice bran wax, sunflower wax, and stearyl alcohol are permitted for food use but do not carry dedicated EU E-numbers.

Carrageenan (E 407) carries a provisional/temporary group ADI, pending ongoing EFSA, re-evaluation.

Plant polysaccharides (alginate, κ-carrageenan, gellan, xanthan, agar, pectin, starches, and modified celluloses), plant proteins (pea, soy, potato, fava, and rice bran), natural waxes (candelilla, carnauba, rice bran, sunflower, and beeswax), monoglycerides, β-sitosterol/γ-oryzanol, and vegetable oils are GRAS or food-additive approved, with soy and pea protein allergenicity being the major labelling concerns (Gray et al., 2025; Lin et al., 2024). Some ingredients of bigels are controversial. The potential gastrointestinal effects of κ-carrageenan have been debated, with degraded carrageenan (poligeenan) classified as a possible carcinogen, although food-grade undegraded κ-carrageenan has been approved by EFSA, FDA, and JECFA. PGPR is allowed at restricted levels (up to 4 g/kg in chocolate) with specific labelling (Sanpedro-Díaz et al., 2025; Yang et al., 2023). Effect of Tween-80 on intestinal microbiota have been scrutinized, although it remains food grade (Luo et al., 2025; Xie et al., 2024). Span-40 (sorbitan monopalmitate) is permitted under E495/21 CFR 172.842, and its plant-derived feedstock and synthetic emulsifier classification may complicate clean-label positioning (Behera et al., 2014). Polysaccharide derivatives such as ethylcellulose offer excellent organogelation, but food-grade approval varies by jurisdiction (Osullivan et al., 2017). Using animal-derived hydrogelators, such as gelatin (Lu et al., 2022; Xie et al., 2023), whey protein isolate (Gray et al., 2025; Liu et al., 2024; Qian et al., 2025), or sericin (Aung et al., 2025), excludes bigel platforms from strict plant-based markets (Table 5).

The most substantial weakness of the safety literature is the near-complete absence of in vivo tolerance and clinical data for whole-bigel formulations. Cell-culture cytotoxicity data are emerging, such as ISO 10993-5 compliance for MCFA bigels in Caco-2 and HT29-MTX (Machado et al., 2024), non-cytotoxic Caco-2 (Wang et al., 2025c) acceptable hemolysis for CW/XG bigels (Habibullah et al., 2025) animal pharmacokinetic and human clinical studies remain rare.

7.5 Regulatory pathways and labelling

For food applications, plant-based bigels with GRAS or approved food additive ingredients can be marketed without novel-food approval if each ingredient is approved for a specific category, health claims are authorized, allergens are declared, and the product is substantially equivalent to existing products (Table 5). In the EU, the Novel Food Regulation (EU 2015/2283) applies to ingredients without a significant pre 1997 consumption history. Most plant polysaccharides and waxes are exempt, but byproduct-derived polysaccharides such as IPVP (Huang et al., 2026) would likely require novel food authorization. In the US, GRAS notification or self-affirmation is an option for food additives. In pharmaceutical applications, bigels are regulated as drug products that require full CMC, efficacy, and safety documentation (Rehman et al., 2014). Bigels used in cosmetics are subject to less stringent cosmetic product regulations. Claims such as “plant-based,” “natural,” “clean-label,” and “vegan” depend more on consumer expectations and competitor litigation than on formal regulations. All ingredients should be plant-derived, beeswax clearly labelled, and animal-derived hydrogelators replaced with plant-protein analogs for strict plant-based claims. The difference between “plant-based” as an academic label (often including beeswax, gelatin, and whey protein in early formulations) and as a commercial claim is under-discussed.

7.6 Sensory and consumer-acceptance considerations

Although sensory data on plant-based bigels are limited, they are emerging in various applications. PHP/beeswax bigels at hydrogel-rich H:O ratios achieved comparable hardness, color, and crumb structure to palm-oil controls in cakes (Akcicek et al., 2025). Ethylcellulose/wheat-starch bigels achieved acceptable sensory results in beef burgers with up to 50% animal-fat replacement (Ghiasi and Golmakani, 2022). Nanocellulose/monoglyceride bigels at 50% hydrogel ratio replaced butter in cookies (Hou et al., 2024). Corn-starch/candelilla-wax bigels successfully replaced shortening in cookies (Barragán-Martínez et al., 2022). Gelatin/rice-bran-wax bigels replaced pork-back fat in sausages with acceptable sensory profiles (Kibler et al., 2022). Capsaicin-loaded bigels masked capsaicin pungency, showing that bigel encapsulation can transform challenging organoleptic profiles (Wang et al., 2025c). Pickering bigels evolve from a lubricious texture at low particle concentrations to a stable plateau at higher concentrations (Liu et al., 2024). Dysphagia-targeted bigels have been validated against IDDSI criteria across multiple bigel formulations (Chao et al., 2024; Fernandes et al., 2025; Liu W. et al., 2025; Yang et al., 2026). Comprehensive sensory validation across diverse food matrices and consumer demographics remains a significant pre-commercial requirement.

7.7 Commercial and industrial translation

Despite the rapid expansion of bigel research, the translation of plant-based systems from laboratory demonstrations to commercial food products remains in its early stages, and several practical barriers temper the optimistic projections made in the literature. The regulatory groundwork is largely favorable, as shown in Table 5. The gelators most widely used in plant-based bigels already hold food additive approvals in the European Union and the United States. Therefore, the principal obstacles to commercialization lie not in regulation but in manufacturing, economics, and evidence-based sustainability.

Manufacturing scalability is a significant constraint. Most reported bigels are produced using a two-step hot-emulsification route, in which preformed oleogels and hydrogels are sheared and cooled under controlled conditions to form the bigel. The resulting microstructure, phase continuity, and mechanical properties are strongly governed by the oleogel-to-hydrogel ratio and gelator type, with phase inversion from an oleogel-in-hydrogel to a hydrogel-in-oleogel arrangement occurring beyond the threshold oleogel fraction (Tian et al., 2024). This sensitivity, which is advantageous for fine-tuning in the laboratory, complicates process standardization and reproducibility at an industrial scale, where thermal and shear gradients are inherently less uniform and a small compositional drift can shift the structure across a phase-inversion boundary. The near absence of pilot- or production-scale studies is a clear gap, and claims of manufacturing readiness should be regarded as provisional until continuous or semi-continuous processing is demonstrated. Emerging approaches, such as three-dimensional food printing, have shown that bigels can be processed into structured, value-added formats (Fernandes et al., 2026), although this does not resolve bulk-scale production issues.

Economic and sustainability arguments are frequently invoked but rarely substantiated in the literature. Plant-derived gelators, such as monoglycerides and candelilla wax, are relatively inexpensive and abundant. However, the additional processing steps, energy demand for controlled heating and cooling, and cost of protein or polysaccharide hydrogel components must be weighed against the functional value delivered by these products. No published techno-economic analyses of food-grade bigels are available. Likewise, although the plant-based composition of these systems is presumed to confer sustainability benefits over animal fat or fully synthetic alternatives, this advantage has not been quantified, and no life-cycle assessment specific to bigel manufacture has, to the best of our knowledge, been reported. Sustainability claims should be framed as plausible expectations grounded in feedstock origins rather than as demonstrated outcomes. Another technological consideration is that some structurants impose processing constraints that are independent of their regulations. For instance, functional ethylcellulose oleogels require dissolution temperatures above approximately 130 °C, which can degrade unsaturated oils and heat-labile bioactive compounds (Davidovich-Pinhas et al., 2015). This limits their use, even though ethyl cellulose is an approved additive.

Overall, these considerations indicate that plant-based bigels are technologically promising and regulatorily feasible, but commercially unproven. Bridging this gap will require pilot-scale processing studies, systematic shelf-life and sensory evaluations (Sections 7.2, 7.3, and 7.6), and quantitative techno-economic and life-cycle analyses.

8 Research gaps and future directions

Despite advances in plant-based bigel technology, critical gaps hinder the transition from laboratory innovations to commercial products. A major limitation is the reliance on static in vitro digestion models, mainly INFOGEST, to assess bioactive release and bioaccessibility. These models provide standardized insights but cannot replicate the dynamic mixing, peristaltic shear, mucus barrier, microbiota interactions, and individual variability of human digestion, risking the misrepresentation of true bioavailability. Existing Caco-2 monolayer studies (Machado et al., 2024; Wang et al., 2025c; Yang et al., 2023) are closest to intestinal absorption, but animal pharmacokinetic studies and human trials are largely unreported for plant-based bigel formulations. Future studies should systematically move from static to dynamic in vitro models (TIM-1, SHIME), then to ex vivo tissue preparations, animal pharmacokinetics, and ultimately human trials with measurable bioavailability and efficacy endpoints.

The second gap is the absence of a unified predictive framework. The Bigel formulation relies on empirical experimentation and lacks mathematical or computational tools for predicting morphology, rheological behavior, or release kinetics from compositional inputs. The contrasting GMS–curcumin release behavior in gelatin versus soy-protein-isolate hydrogels (Gray et al., 2025; Lu et al., 2022) and contrasting oleogel content release behavior for lycopene versus β-carotene (Zheng et al., 2020; Zhu et al., 2021) indicate that the empirical design rules of bigels are not uniform. Multi-scale computational modeling, integrating molecular dynamics simulation, finite-element analysis, and machine learning applied to the growing in vitro digestion dataset, offers a route to rational in silico bigel design.

The third gap pertains to a lack of methodological standardization. Characterization protocols, including digestion conditions, rheological geometries, centrifugation parameters, and release model fitting, exhibit significant variability across studies. This variability undermines the reliability of quantitative cross-study comparisons, as demonstrated in Sections 4, 5. Establishing a community-agreed minimum reporting standard for bigel characterization could facilitate the consolidation of structure–function relationships, similar to the impact of standardized reporting guidelines in other experimental fields (Poutanen et al., 2018; Bernstein et al., 2026). Additionally, there is a notable deficiency in long-term data, as most bigels are characterized shortly after fabrication, while systematic aging studies under realistic storage conditions and within actual food matrices, which are crucial for commercial applications, are limited. The fourth gap concerns scale-up processes. Plant-based bigel research is mostly conducted at the bench scale (50–500 g batches) using rotor-stator mixers. The translation of these findings to industrial production through continuous-flow processing or twin-screw extrusion poses unresolved challenges in heat-transfer management, cooling-rate control, and shear distribution, which are variables that govern crystalline-network formation and the final architecture. Finally, although bigels have been proposed as fat replacers, engagement with sensory panels, untrained consumer cohorts, and acceptability evaluations across demographics are critical missing links between formulation science and market readiness of bigels.

Emerging design directions include multi-stimuli-responsive systems for transforming bigels into programmable carriers. Although heat-triggered release (Lee et al., 2024) and pH-responsive alginate bead architectures (Huang et al., 2026; Liu G. et al., 2025) have been developed, fully programmable designs that combine intestinal pH-triggered swelling with colon-targeted enzyme-mediated degradation through dynamic covalent chemistries, such as reversible Schiff base linkages, remain aspirational. Microbiome-targeted systems are promising; non-digestible hydrogel components can serve as prebiotic substrates for colonic fermentation, while bigels deliver probiotics or bioactives, building on plant-based probiotic-delivery work (Behera et al., 2014; Li et al., 2025). Personalized nutrition via 3D and 4D printing is advancing, with 3D-printable bigel inks for dysphagia and customized functional foods (Cai and Lu, 2026; Fernandes et al., 2026; Wang et al., 2025c; Xie et al., 2023) and a trajectory toward stimuli-responsive 4D constructs. Sustainability in formulation is growing, with byproduct-derived polysaccharides and underutilized plant resources such as oil-cake polysaccharides, corn silk, olive pomace, and seed-derived gums used in formulation (Álvarez et al., 2025; Cai and Lu, 2026; Huang et al., 2026; Li et al., 2025). However, as mentioned in Section 7.7, life cycle quantification of the assumed environmental benefit is still lacking.

9 Future perspectives

Beyond addressing the gaps identified above, the next phase of plant-based bigel research is likely to be shaped less by new formulations and more by a shift from descriptive optimization to predictive, data-driven, and personalized design. Six interconnected directions can be distinguished: several are already underway, while others are still aspirational.

9.1 Predictive modeling and in silico design

The near-term opportunity is to convert the empirical design rules currently used into predictive models. Currently, the release behavior is described retrospectively by kinetic fits such as the Korsmeyer–Peppas and Peppas–Sahlin models (Section 5.3), which classify a mechanism after the fact but cannot predict release from the composition. Mechanistic and multi-scale modelling offers a route forward: molecular dynamics to probe gelator–oil and interfacial interactions, finite element analysis to relate microstructure to bulk mechanics, and population balance or phase-field approaches to predict phase morphology and inversion thresholds. An integrated composition-to-structure-to-digestion model has not yet been demonstrated for bigels and remains a target for future research. However, each component method has been established in adjacent soft matter and lipid crystallization research (Acevedo-Fani and Singh, 2022; Giang et al., 2016; Grahn et al., 2025), making assembly a realistic medium-term goal.

9.2 Artificial-intelligence-assisted formulation design

Machine learning is the most discussed and least realized direction for the development of bigels. In principle, supervised models can predict rheology, encapsulation efficiency, or release from compositional and processing inputs. In addition, inverse design and Bayesian optimization strategies can propose formulations that meet target release profiles, and active learning can direct experiments efficiently across a large compositional space. Machine learning has begun to be applied to food structuring and emulsion design problems (Meng et al., 2025; Wong et al., 2025), but no bigel-specific implementation has yet been reported, and this should be presented as an emerging rather than an established capability. Critically, the constraint is evidential rather than algorithmic. Predictive models require large, consistently reported datasets, which is a requirement currently hindered by the absence of standardized characterization protocols (Section 8). Therefore, progress in AI-assisted design requires resolving both the standardization and dynamic digestion gaps; because these agendas are coupled, advancing either in isolation will yield limited returns.

9.3 Digital and advanced food manufacturing

3D food printing has already moved bigels from passive matrices toward structured, customizable formats, with printable inks demonstrated for dysphagia-oriented and functional foods (Cai and Lu, 2026; Fernandes et al., 2026; Wang et al., 2025c; Xie et al., 2023). The prospective extension to four-dimensional printing, in which a printed construct changes shape, texture, or release behavior over time in response to a stimulus, is conceptually attractive but remains speculative for bigels and is proposed here as a direction rather than as a demonstrated result. More immediately, the scale-up obstacles identified in Sections 7.7, 8 point toward continuous and data-driven manufacturing, in which in-line process analytical technology monitors crystallization and phase structures during production. Such digital process control is well established in lipid and emulsion processing but has not yet been applied to bigel manufacturing (Glader et al., 2025; Grumbach and Czermak, 2022; Qwist et al., 2020).

9.4 Personalized nutrition

Digital manufacturing enables personalized delivery, in which the dose, release kinetics, and texture are tailored to individual physiological needs. The most concrete example is already in practice: texture-modified, IDDSI-classified bigels matched to a patient’s swallowing capacity (Sections 4.4, 5.1). Printing extends this logic to individualized bioactive dosing and, prospectively, to formulations adapted to a person’s digestive physiology or microbiome composition, although closed-loop personalized production remains a long-term prospect.

9.5 Smart, responsive, and microbiome-targeted delivery

Stimuli-responsive systems aim to convert bigels into programmable carriers. Although heat-triggered release (Lee et al., 2024) and pH-responsive alginate-bead architectures (Huang et al., 2026; Liu G. et al., 2025) have been developed, fully programmable carriers that combine intestinal pH-triggered swelling with colon-targeted, enzyme-mediated degradation through dynamic covalent chemistries, such as reversible Schiff-base linkages, remain aspirational. A natural extension is microbiome-targeted design, in which non-digestible hydrogel components act as prebiotic substrates for colonic fermentation, while the bigel simultaneously delivers probiotics or bioactives, building on early plant-based probiotic work (Behera et al., 2014; Li et al., 2025).

9.6 Sustainability-driven formulation

Finally, sustainability is increasingly shaping ingredient selection, with by-product-derived polysaccharides and underused plant resources, such as oil-cake polysaccharides, corn silk, olive pomace, and seed gums, being incorporated into bigel formulations (Álvarez et al., 2025; Cai and Lu, 2026; Huang et al., 2026; Li et al., 2025). As noted in Section 7.7, however, the presumed environmental advantage of these feedstocks still awaits life cycle quantification before it can be claimed as an outcome rather than as an expectation.

These directions are mutually dependent rather than independent. Predictive modelling and AI-assisted design rest on the standardized, dynamic-digestion datasets identified as gaps in Section 8; digital manufacturing depends on resolving scale-up; and personalized, responsive delivery ultimately requires in vivo validation that the field still lacks. Therefore, the next-generation of plant-based bigel research will be defined as much by data infrastructure and validation rigor as by chemistry, transforming empirical formulation into a predictive, personalized, and demonstrably effective delivery strategy.

10 Conclusion

This review emphasizes that the functional performance of bigels is critically dependent on the rational design of their structural parameters, including the oleogel-to-hydrogel mass ratio, biopolymer selection, and interfacial engineering. Advanced stabilization strategies, such as Pickering interfacial stabilization using plant-based nanoparticles and the development of semi-interpenetrating polymer networks, have significantly expanded the structural design space of bigels. These microstructural modifications directly dictate the rheological properties, thermodynamic stability, and precise gastrointestinal digestive behavior of the matrices. Well-formulated bigels can achieve targeted release kinetics, higher gastric protection, and significant improvements in the bioaccessibility of sensitive nutraceuticals. Beyond nutrient delivery, the highly controllable viscoelasticity of these systems has paved the way for pioneering applications in extrusion-based 3D food printing, personalized dysphagia diets, and intelligent sustainable food packaging.

Plant-based bigels intersect with food structure research, nutritional biochemistry, and sustainable food technology. As the field advances in stimuli-responsive mechanisms, microbiome-targeted designs, and printed architectures. This review highlights that bigel structures dictate digestive and biological outcomes, making them a compelling tool for precision nutrition and sustainable food engineering.

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Footnotes

Edited by: Vanessa Naciuk Castelo-Branco, Fluminense Federal University, Brazil

Reviewed by: Héctor Emmanuel Cortés-Ferré, Monterrey Institute of Technology and Higher Education (ITESM), Mexico

Priya Patel, Saurashtra University, India