Bio-based alginate films reinforced with bacterial nanocellulose and MAA-rich extracts for food packaging

The growing demand for sustainable food packaging has encouraged the development of biodegradable materials with improved protective and functional properties. This study developed sodium alginate (ALG) composite films containing bacterial nanocellulose (BNC) and mycosporine-like amino acid (MAA)-rich extracts. The novelty lies in combining nanocellulose reinforcement with the valorization of marine-derived by-products in a multifunctional biodegradable packaging system. Films were produced by solution casting and evaluated for their structural, mechanical, surface, moisture-related, and barrier properties. Their effectiveness in preserving fresh blueberries was also assessed during four weeks of storage. BNC incorporation strengthened the alginate matrix, increasing tensile strength and reducing film porosity. The addition of MAA-rich extracts improved film flexibility and contributed functional properties. However, higher MAA concentrations increased film hydrophilicity, resulting in greater moisture content, water solubility, swelling, and rehydration capacity. Among the tested formulations, the ALG–BNC film containing 3% MAA showed the most balanced performance, combining low porosity, adequate tensile strength, improved flexibility, and controlled moisture behaviour. This formulation also provided the best preservation of blueberries, with the lowest weight loss of 3.6% after four weeks. The results demonstrate that film performance depends on balancing mechanical stability, flexibility, and water sensitivity. The synergistic combination of BNC and MAA-rich extracts offers a promising circular-economy strategy for producing sustainable food-packaging materials with improved functionality and potential for extending fresh-produce shelf life.

The global reliance on petroleum-based plastics has caused severe environmental challenges, particularly due to their persistence, low recycling efficiency, and accumulation in terrestrial and marine ecosystems. Consequently, developing biodegradable and renewable packaging materials has become a key priority in sustainable materials science and the circular bioeconomy framework (Metha et al., 2024; Wang F. et al., 2024). At the same time, modern food packaging research is already shifting from passive containment systems to multifunctional active materials that extend shelf life, improve food safety and reduce food waste through antioxidant, antimicrobial, and UV-protective functionalities (Feng et al., 2025; Lin and Dufresne, 2014; Jacob et al., 2024; Wang et al., 2025; Zhang et al., 2025). Among bio-based polymers, sodium alginate has attracted considerable attention due to its biodegradability, non-toxicity, film-forming ability, and availability from abundant brown seaweed resources (Amir et al., 2025). Alginate-based films are generally transparent and provide good oxygen barrier properties, making them suitable for food-packaging applications (Metha et al., 2024). However, their industrial use is limited by poor mechanical strength, high hydrophilicity, and insufficient resistance to ultraviolet radiation, which accelerates photooxidative degradation of packaged foods (Infante-Neta et al., 2024; Chan et al., 2025). Therefore, incorporating reinforcing and functional additives is essential to improve their structural and protective performance. On the other hand, nanocellulose has emerged as one of the most promising reinforcement materials for biodegradable packaging systems due to its high crystallinity, nanoscale dimensions, mechanical strength, and renewable origin (Vrabič-Brodnjak, Ružič, 2025; Komal et. at., 2026; Musa and Ab’lah, 2026). In particular, bacterial nanocellulose (BNC) has gained increasing attention because it is produced in a highly pure form without lignin or hemicellulose impurities, resulting in superior compatibility with polysaccharide matrices (Vrabič-Brodnjak and Ružič, 2025). Incorporating BNC into biopolymer films has been shown to significantly improve tensile strength, barrier properties, and structural integrity through the formation of a dense hydrogen-bonded network (Wang F. et al., 2024; Infante-Neta et al., 2024; Jacob et al., 2024). These properties make BNC an ideal candidate for high-performance sustainable packaging applications. In addition to mechanical reinforcement, protection against ultraviolet radiation is a critical requirement for advanced food packaging. UV exposure promotes lipid oxidation, pigment degradation, vitamin loss, and off-flavor formation in sensitive food products, reducing quality and shelf life. While synthetic UV absorbers are effective, concerns about toxicity, migration, and environmental persistence have increased interest in natural alternatives (Gan et al., 2022; Zhang et al., 2023). Among these, marine-derived mycosporine-like amino acids (MAAs) have emerged as promising UV-protective compounds due to their strong absorption in the UV-A and UV-B range, photostability, and antioxidant activity (Peng et al., 2023). MAAs are water-soluble secondary metabolites naturally produced by marine organisms, particularly red algae of the genus Porphyra (nori), as a protective mechanism against solar radiation. Key compounds such as porphyra-334 and shinorine exhibit strong UV absorption between 310 and 360 nm, making them highly effective natural UV filters (Peng et al., 2023; Gan et al., 2022). Recent studies have shown that incorporating MAAs into biopolymer films enhances UV-shielding capacity while also improving antioxidant properties, making them suitable for active food-packaging applications (Zhang et al., 2023; Li et al., 2024).

Industrial processing of nori generates significant quantities of by-products that remain underutilized despite being rich in valuable bioactive compounds, including MAAs, proteins, and polysaccharides. Recovering and valorizing these residues aligns with circular economy principles by reducing waste and creating additional value from marine biomass streams (Kim et al., 2009; Xu et al., 2024). However, despite growing research on nanocellulose-based packaging and MAA-functional films, there is still limited knowledge on the combined use of bacterial nanocellulose and MAA-rich extracts derived specifically from nori-processing by-products within a single alginate-based matrix. Furthermore, the potential synergistic effects between structural reinforcement (BNC) and functional UV-protective additives (MAAs) on the mechanical, barrier, optical, and antioxidant properties of alginate films remain largely unexplored. Addressing this gap could enable the development of multifunctional packaging materials that simultaneously improve mechanical durability, shelf protection, and sustainability through waste valorization.

Therefore, the aim of this study was to develop biodegradable alginate-based films reinforced with bacterial nanocellulose and enriched with MAA-rich extracts for use as multifunctional, active food packaging materials.

The objectives of our research were to (i) fabricate alginate/BNC/MAA composite films by solution casting, and (ii) evaluate their structural, mechanical and barrier properties (iii) evaluate shelf-life properties of blueberries.

Materials

Sodium alginate (biological source: brown algae; white powder, viscosity ≥2000 cP, 2% (25 °C)) was obtained from Sigma-Aldrich (St. Louis, MO, United States) the same as glycerol. Bacterial nanocellulose suspension (1% w / w) was produced at Pulp and Paper institute in Ljubljana, Slovenia via fermentation using Komagataeibacter xylinus following established protocols from our previous research (Vrabič-Brodnjak, Ružič, 2025). MAA-extract (brownish-yellow powder; pH 4.8; sulfated ash (800 °C): ≤3.0%) as fine powder was obtained from Sigma-Aldrich (St. Louis, MO, United States).

Film preparation

The film formulations were selected through sequential preliminary screening rather than formal statistical optimization. A 2% (w/w) sodium alginate concentration was selected because it provided an appropriate balance between solution viscosity, processability, and continuous film formation. The 2% BNC concentration was selected based on previous experiments with alginate–BNC films and preliminary screening of BNC incorporation levels. Glycerol was maintained at 10% relative to the dry weight of alginate because this was the lowest concentration that enabled satisfactory film flexibility and handling without visible cracking. MAA-rich extract concentrations of 1%, 3%, and 5% were selected as low, intermediate, and high levels to investigate concentration-dependent effects while maintaining homogeneous casting solutions and continuous film formation. Preliminary alginate–BNC–MAA combinations were prepared before the main experiment to confirm the processability and compatibility of the selected concentration range.

Alginate-based films were prepared using the solvent-casting method. Sodium alginate (2%, w/w) was dissolved in distilled water under magnetic stirring at room temperature (23 °C ± 2 °C) for 1 h until a homogeneous solution was obtained. Bacterial nanocellulose (BNC) suspensions were then incorporated into the alginate solution at concentrations of 2% (w/w), based on the dry weight of alginate. The alginate concentration (2%, w/w) was selected based on our previous study, which demonstrated that this concentration provides films with suitable film-forming properties, mechanical performance, and structural stability for food-packaging applications (Vrabič-Brodnjak and Ružič, 2025). To ensure uniform dispersion of the nanocellulose fibrils within the polymer matrix, the mixtures were treated using an ultrasonic processor (Sonorex; Bandelin GmbH, Germany) operating at 20 kHz for 5 min. Glycerol was subsequently added as a plasticizer at 10% (w/w) relative to the dry weight of alginate and homogenized under continuous stirring. For formulations containing marine bioactive compounds, the MAA-rich extract was incorporated at a concentration of 1%, 3% and 5% (w/w) based on the dry weight of alginate. The film-forming solutions were further stirred for 30 min to ensure complete homogenization and uniform distribution of all components. All chemicals and reagents used in this study were of analytical grade. Fixed volume of 80 mL of each film-forming solution was cast onto leveled polystyrene plates and dried in a laboratory oven at 40 °C for 24 h. After drying, the films were carefully peeled from the casting surface and conditioned at 25 °C and 50% ± 5% relative humidity for at least 48 h prior to further characterization. Eight film formulations were prepared as presented in Table 1.

Characterization of prepared packaging films

Thickness and porosity

Thickness of the films were measured at ten random points per film using a digital micrometer Mitutoyo, resolution 0.01 mm (Mitutotyo Corp., Kanagawa, Japan). Porosity was carried out using a Bendtsen instrument (Alat UJI, Jakarta, Indonesia) and the measurement range was 0–3,000 mL/min, with air flowing through a flat metal ring and the test sample. The pressure difference was recorded using the rotameter tube provided. Ten replicates were measured on each sample.

Tensile properties

Tensile strength and elongation at break were determined according to ASTM D882, by a dynamometer, testing machine (Instron 5567, Instron, Norwood, MA, United States). Rectangular strips (10 mm × 60 mm) were mounted and subjected to tensile testing at a crosshead speed of 5 mm/min. Data was collected to derive tensile strength and elongation at break and Bluehill® software (Instron, Norwood, MA, United States) was used to record the measurements.

FTIR analysis

Fourier-transform infrared (FTIR) spectroscopy was used to identify the characteristic functional groups of the films and to investigate possible molecular interactions among alginate, BNC and MAA. The spectra were recorded using a Spectrum 3 ATR-FTIR spectrometer (PerkinElmer Inc., Waltham, MA, United States) equipped with an attenuated total reflectance accessory. Before analysis, the film samples were conditioned under controlled laboratory conditions (temperature: 23 °C ± 2 °C and relative humidity: 50% ± 5%) cut into pieces of suitable dimensions to ensure direct and uniform contact with the ATR crystal. Measurements were performed at room temperature over the wavenumber range of 4,000–500 cm −1, and 64 scans were averaged for each spectrum to improve the signal-to-noise ratio. A background spectrum was collected before each measurement and automatically subtracted from the corresponding sample spectrum. The ATR crystal was carefully cleaned between measurements to prevent cross-contamination. The obtained spectra were baseline-corrected and normalized to facilitate comparison among the different formulations. Characteristic absorption bands were identified, and changes in their position, shape, and intensity were evaluated to assess hydrogen bonding and other potential interactions within the ALG–BNC–MAA film network.

Water vapour permeability and surface wettability

Water vapor permeability was measured using standard gravimetric method, following standard ASTM E96-16. Film samples were sealed over cups containing water and placed in a controlled humidity chamber for 24 h, at 25 °C ± 1 °C and 75% ± 3% relative humidity. The weight gain was recorded over 24 h, and WVP was calculated using the standard procedure as described in the above-mentioned standard.

The surface wettability of the packaging films was evaluated by determining the water contact angle using a Krüss Drop Shape Analyzer (Krüss GmbH, Hamburg, Germany) employing the sessile drop method, following the principles described in ASTM D7334. Prior to surface wettability analysis, the packaging film samples were conditioned in a controlled atmosphere at 23 °C ± 2 °C and 50% ± 5% relative humidity for 40 h, following the recommendations of ISO 291. A droplet of ultrapure or distilled water (5 μL) was carefully deposited onto the film surface using an automated dosing system. The droplet profile was immediately captured and the contact angle was determined using the instrument software based on the Young–Laplace fitting method. For each film formulation, measurements were performed at several different locations on the sample surface to account for possible surface heterogeneity. A contact angle lower than 90° indicated a hydrophilic surface with a higher affinity toward water, whereas values higher than 90° indicated a more hydrophobic character and reduced surface wettability.

Solubility and water interaction properties of packaging films

The moisture content (MC), water solubility (WS), swelling ratio index (SR), and rehydration ratio (Rr) of the developed packaging films were determined to evaluate their water interaction behaviour, which significantly influences the structural integrity, barrier performance, and potential applicability of biodegradable packaging materials. Moisture content (MC) was determined according to the gravimetric method described in the standard ASTM E104 and commonly adapted procedures for biopolymer films. Film specimens were initially weighed to obtain their initial mass (W 1), followed by drying in a hot-air oven at 105 °C until reaching a constant weight. The dried samples were subsequently weighed (W 2), and the moisture content was calculated as follows:

Water solubility (WS) was evaluated following the methodology used for biodegradable and edible films based on the principles of ASTM D570. Initially, the film samples were dried at 105 °C to obtain their initial dry mass (W 1). The samples were then immersed in distilled water and maintained under continuous gentle agitation at room temperature for 24 h. Following immersion, the remaining insoluble fractions were recovered, dried again at 105 °C to constant weight, and the final dry mass (W 2) was recorded. The water solubility was calculated using the following equation:

The swelling ratio index (SR) was measured based on a modified gravimetric method adapted from ASTM D570 to determine the water uptake capacity of the films. Pre-dried specimens with an initial dry weight (W 1) were immersed in distilled water at ambient temperature for a predetermined time interval. After immersion, the samples were removed, excess surface water was carefully eliminated using filter paper, and the swollen weight (W 2) was immediately measured. The swelling ratio was calculated according to the following equation:

The rehydration ratio (Rr) was determined to assess the ability of the dried films to regain water after dehydration. Dried film samples with a known dry mass (Wd) were immersed in distilled water under controlled conditions. After the designated rehydration period, the samples were removed, gently blotted to remove surface moisture, and weighed to obtain the rehydrated mass (Wr). The rehydration ratio was calculated using the following equation:

All measurements were performed in triplicate, and the results were expressed as mean values ± standard deviations. The experimental conditions, including immersion time, water volume-to-sample ratio, and temperature, were maintained constant throughout all analyses to ensure reproducibility and reliable comparison between different film formulations.

Storage of packed blueberries in sample films

The films were cut into appropriately sized sheets and formed into pouches 7 × 7 cm. The blueberries were placed inside the pouches, and the open edges were sealed using an industrial heat sealer (Keiser Kraft GmbH; Stuttgart, Germany) to obtain closed packages for the preservation experiment. The packed samples were then stored in refrigerator at 4 °C, for 4 weeks. The number of blueberries was limited to three, as adding more would have exceeded the pouch capacity and risked opening during storage.

Weekly evaluations included the following: visual Quality Assessment: Inspection for color changes, mold growth, and other visual spoilage indicators.

weight Loss: Weighed before and after storage; expressed as a percentage of initial weight.

Statistical analysis

Film thickness, porosity, and mechanical property measurements were performed in 10 probes at each sample. WVP was carried out in parallel. Data was analyzed using one-way ANOVA at a 95% confidence level (p < 0.05) in Microsoft® Excel 2016 with the Data analysis ToolPak. Experiments were performed following applicable standards, and the results are presented as mean ± standard deviation.

Structural, tensile and molecular characterization of the films

The incorporation of BNC and mycosporine-like amino acid extracts significantly influenced the structural characteristics of alginate-based films, particularly their thickness (Table 2). The control alginate film (ALG) exhibited a thickness of 0.072 ± 0.003 mm, reflecting the formation of a relatively homogeneous polysaccharide matrix with moderate polymer chain entanglement and water evaporation-driven consolidation. The addition of 2% BNC led to a slight but consistent increase in thickness (0.081 ± 0.004 mm), which can be attributed to the high aspect ratio and nanofibrillar morphology of BNC. These nanofibers are known to form an interconnected three-dimensional network within polysaccharide matrices, increasing solid fraction and restricting chain packing efficiency (Habibi et al., 2010; Klemm et al., 2011). Similar thickness increases in alginate–nanocellulose systems have been previously reported and are generally associated with enhanced structural reinforcement rather than simple mass addition. A more pronounced increase in thickness was observed in MAA-containing films, ranging from 0.076 ± 0.003 mm (ALG-1-MAA) to 0.089 ± 0.004 mm (ALG-5-MAA). This trend suggests that MAA-rich extracts interact with alginate chains through hydrogen bonding and secondary intermolecular interactions, reducing chain mobility and promoting a more compact yet slightly expanded matrix architecture. Polyphenolic and UV-absorbing compounds in natural extracts are widely reported to influence polymer packing by introducing steric hindrance and increasing intermolecular spacing. The highest thickness values were recorded in composite systems containing both BNC and MAA (ALG-BNC-5-MAA: 0.094 ± 0.005 mm), indicating a synergistic effect between rigid nanofibers and bioactive small molecules. In such systems, BNC provides a structural scaffold while MAA compounds modify polymer interactions, leading to a densified but mechanically stabilized matrix.

Porosity strongly influenced the structural integrity and mechanical performance of the developed alginate-based films. The neat ALG film exhibited the highest porosity, at 32.5% ± 1.6%, indicating a relatively open polymer network containing microvoids formed during solvent evaporation. Its FTIR spectrum showed the characteristic broad O–H stretching band at approximately 3,420 cm −1, C–H stretching at around 2,890 cm −1, and bands near 1,610 and 1,370 cm −1 associated with the asymmetric and symmetric vibrations of alginate carboxylate groups (Figure 1). Additional bands at approximately 890 and 590 cm −1 were attributed to C–O–C and C–O-related vibrations, respectively (Huq et al., 2012).

The incorporation of BNC reduced the porosity to 27.1% ± 1.3%, suggesting that the cellulose nanofibrils occupied void spaces and promoted the formation of a more compact and continuous polymer matrix. Changes in the broad O–H band at approximately 3,420 cm −1 in the ALG–BNC spectrum support the presence of intermolecular hydrogen bonding between the hydroxyl groups of BNC and alginate chains (Siqueira et al., 2019). These interactions likely improved interfacial adhesion and reduced the number of structural defects within the film. The improved matrix continuity was also reflected in the mechanical properties.

The improved matrix continuity was also reflected in the mechanical properties. Tensile strength increased from 32.5 ± 1.6 MPa for ALG to 40.8 ± 2.0 MPa for ALG–BNC. This enhancement may be attributed to the high intrinsic stiffness of cellulose nanofibrils and efficient stress transfer across the BNC–alginate interface through hydrogen-bonding interactions (Habibi et al., 2010). However, the reinforcing effect was accompanied by a slight reduction in elongation at break, from 12.4% ± 1.1% to 12.3% ± 0.8%, indicating restricted polymer-chain mobility. This behaviour is consistent with the typical stiffness–flexibility trade-off reported for cellulose-reinforced polysaccharide matrices, in which strong interfacial interactions improve stress-transfer efficiency but limit the mobility and rearrangement of polymer chains during deformation (Sánchez-García et al., 2008). The incorporation of the MAA-rich extract into the alginate matrix progressively reduced porosity from 25.4 ± 1.2 mL/min for ALG–1–MAA to 18.4 ± 0.9 mL/min for ALG–5–MAA. At the same time, tensile strength decreased from 28.6 ± 1.4 to 21.3 ± 1.0 MPa, whereas elongation at break increased from 18.2% ± 1.5% to 38.5% ± 2.4%. These results indicate that the extract increased film flexibility but reduced resistance to tensile loading. Low-molecular-weight constituents of the extract may have become positioned between adjacent alginate chains, increasing chain mobility and partially disrupting alginate–alginate interactions. The MAA-rich extract therefore appears to have exerted a concentration-dependent plasticizing effect. The FTIR spectra of the MAA-containing films showed additional bands at approximately 1720 cm −1, attributed to C=O stretching vibrations of extract-derived compounds, and at approximately 1,510 cm −1, associated with aromatic C=C vibrations (Zhang D. et al., 2026). The presence of these bands supports the incorporation of extract components into the alginate matrix. Changes in the O–H, carboxylate, and carbonyl regions further suggest secondary interactions between the hydroxyl and carbonyl groups of the extract constituents and the functional groups of alginate. Similar interactions have been reported for phenolic-rich compounds incorporated into polysaccharide films, where the extract components altered chain organization, film compactness, and mechanical behaviour (Gómez-Estaca et al., 2014). Nevertheless, these interactions did not increase tensile strength in the present films, indicating that the plasticizing or matrix-disrupting effects of the extract predominated over any potential reinforcing effect. A similar concentration-dependent trend was observed in the ALG–BNC–MAA films. Increasing the MAA-rich extract concentration reduced porosity from 20.6 ± 1.0 mL/min for ALG–BNC–1–MAA to 14.2 ± 0.7 mL/min for ALG–BNC–5–MAA. However, tensile strength decreased from 35.2 ± 1.8 to 27.9 ± 1.3 MPa, while elongation at break increased from 14.0% ± 1.2% to 31.2% ± 2.1%. Thus, although the combined incorporation of BNC and MAA produced films with lower porosity and greater flexibility, increasing the extract concentration progressively reduced the reinforcing effect of BNC.

Among all formulations, ALG–BNC exhibited the highest tensile strength of 40.8 ± 2.0 MPa, whereas ALG–BNC–5–MAA exhibited the lowest porosity of 14.2 ± 0.7 mL/min. These findings demonstrate the distinct and complementary roles of the two additives. BNC primarily acted as a rigid nanofibrillar reinforcement and contributed to improved stress transfer, whereas the MAA-rich extract reduced porosity and increased flexibility but simultaneously lowered tensile strength. The combined formulations therefore provided a balance among compactness, mechanical resistance, and flexibility rather than a simultaneous maximization of all properties.

Barrier properties (water vapor permeability and contact angle)

The evaluation of surface wettability through static water contact angle measurements, combined with water vapor permeability (WVP) analysis, enabled a detailed assessment of how the incorporation of BNC and MAA-rich extracts affected the surface properties, hydrophilic–hydrophobic balance, and moisture barrier performance of the alginate-based films. These two parameters are strongly interrelated, as surface hydrophilicity/hydrophobicity governs initial water affinity, while internal microstructure and polymer network tortuosity control long-term vapour transport. Results of WVP and contact angle are shown in Figure 2.

The neat alginate film exhibited a contact angle of 38°, confirming its intrinsically hydrophilic nature derived from abundant hydroxyl and carboxyl functional groups and a relatively open polymer network. This high surface wettability is consistent with the results of WVP, which reflects the ease of water molecule sorption and diffusion through loosely packed polysaccharide chains. Such behavior is typical for sodium alginate films and has been widely associated with weak interchain interactions and high free volume within the biopolymer matrix. Incorporation of 2% bacterial nanocellulose (BNC) significantly increased the contact angle to 46°, indicating a reduction in surface wettability. This effect can be attributed to the formation of a more compact and hydrogen-bonded network, where nanocellulose fibrils partially shield hydrophilic groups and promote surface densification. Concurrently, WVP decreased to 2.78 g mm/m 2. h. kPa, demonstrating improved barrier properties. This reduction is consistent with the well-established tortuosity mechanism described by the Nielsen model, where high-aspect-ratio nanofillers create elongated diffusion pathways that hinder water vapour transport (Azerado et al., 2017). Recent studies on polysaccharide–nanocellulose systems have similarly reported that even low nanocellulose loadings can substantially enhance barrier performance due to the formation of percolated fibrillar networks that increase diffusion path length and reduce effective permeability (e.g., Klemm et al., 2011; Siqueira et al., 2010; Hubbe et al., 2017). The introduction of MAA (MAA-modified phase) induced a more complex trend in surface wettability. At low loading (ALG-1-MAA), the contact angle slightly increased to 41°, which may indicate partial surface rearrangement and local enrichment of less polar moieties. However, further increases in MAA content led to a progressive decrease in contact angle, reaching 38° (ALG-3-MAA) and 36° (ALG-5-MAA). This trend suggests that at higher concentrations, MAA introduces additional polar functionalities or disrupts polymer packing, increasing surface hydrophilicity. A similar phenomenon has been reported in recent biopolymer modification studies, where excessive incorporation of hydrophilic modifiers leads to increased water affinity due to enhanced availability of polar groups at the surface (e.g., recent polysaccharide functionalization studies, 2022–2024).

Interestingly, despite the decrease in contact angle at higher MAA content, WVP decreased markedly from the control to 2.11 g mm/m 2. h. kPa for ALG-5-MAA. This apparent decoupling between surface wettability and vapour permeability suggests that internal structural densification and intermolecular interactions dominate transport behaviour. In other words, although the surface becomes more hydrophilic, the bulk network becomes more resistant to vapour diffusion, likely due to enhanced hydrogen bonding and reduced free volume. The synergistic effect of combining BNC and MAA was most evident in the ALG-BNC-5-MAA system, which exhibited the lowest WVP value of 1.62 g mm/m 2. h. kPa, corresponding to an approximate 53% reduction relative to neat ALG. The contact angle of this sample (36°) indicates a hydrophilic surface; however, its superior barrier performance highlights that water vapour transport is primarily governed by internal structural architecture rather than surface wettability alone. The simultaneous presence of nanocellulose fibrils and MAA-induced network modifications likely promotes a highly tortuous and tightly packed microstructure, reducing diffusion pathways and limiting water mobility. This synergistic enhancement is consistent with recent findings in bio-nanocomposite films, where combined reinforcement and chemical modification strategies outperform single-component systems in terms of barrier efficiency. The results demonstrate that while surface wettability varies within a relatively narrow hydrophilic range (36°–46°), the water vapour barrier properties are significantly more sensitive to internal structural modifications. The integration of BNC and MAA produces a hierarchical barrier mechanism combining nanofibrillar tortuosity, enhanced intermolecular hydrogen bonding, and reduced effective free volume, leading to substantially improved resistance to water vapour transmission.

Solubility and water interaction properties of packaging films

Considering the provided structural and mechanical properties, the expected water interaction behaviour of the films can be rationally estimated (Table 3). Generally, the incorporation of BNC increases the density of the polymer network and reduces water penetration, resulting in lower water solubility and swelling. In contrast, increasing the MAA content increases film thickness, flexibility, and polymer chain mobility, which can enhance water uptake and swelling while reducing mechanical strength. Therefore, a gradual increase in MC, WS, SR, and Rr is expected with increasing MAA concentration, especially for the ALG-MAA series. The presence of BNC partially compensates for this effect due to its reinforcing and network-forming ability.

The moisture-related properties of the developed films were strongly influenced by the incorporation of BNC and the concentration of MAA, as shown in Table 3. The neat ALG film exhibited a moisture content of 13.2%, water solubility of 34.8%, and a swelling ratio of 185%, reflecting the inherently hydrophilic character of alginate-based materials. The addition of BNC reduced the moisture content, water solubility, and swelling ratio, with the ALG–BNC film showing the highest resistance to water among all formulations. This sample exhibited the lowest moisture content (11.5%), water solubility (27.6%), and swelling ratio (145%). The improved water stability of the BNC-containing films can be related to the formation of a denser and more interconnected polymer network. As illustrated in Figure 3, the hydroxyl groups of BNC can form multiple hydrogen bonds with the hydroxyl and carboxylate groups of alginate. Physical entanglement of the nanocellulose fibrils within the alginate matrix may further stabilize the network and reduce the free volume available for water penetration. Although BNC itself contains numerous hydrophilic hydroxyl groups, its reinforcing and network-forming effect appears to dominate, limiting polymer-chain mobility and slowing the diffusion of water molecules into the film structure. In contrast, the incorporation of MAA increased the affinity of the films for water in a concentration-dependent manner. As the MAA concentration increased from 1 to 5 wt%, the films showed progressively higher moisture content, water solubility, swelling ratio, and rehydration ratio. The ALG–5–MAA film exhibited the highest values, with a moisture content of 18.9%, water solubility of 55.3%, swelling ratio of 420%, and rehydration ratio of 5.20. This behaviour may be associated with the presence of polar hydroxyl, carboxylate, and amino groups in MAA, which can interact readily with water molecules. In addition, MAA incorporation may increase film thickness, polymer-chain mobility, and free volume, thereby facilitating water diffusion, retention, and network expansion during hydration.

The interaction mechanism proposed in Figure 3 may explain the behaviour of the ternary ALG–BNC–MAA films. Polar functional groups in the MAA-rich extract may form hydrogen bonds with both alginate and BNC, while pH-dependent electrostatic interactions may also occur between ionizable groups of the extract constituents and the negatively charged carboxylate groups of alginates. These interactions may facilitate the incorporation of the extract into the polymer network. At the same time, the highly polar and water-soluble nature of MAAs can increase the number of hydrophilic sites and promote water uptake, particularly at higher extract concentrations (Claverie et al., 2020; Pangestuti et al., 2021; Gan et al., 2022). This effect was less pronounced in the BNC-containing formulations. The lower water solubility and swelling ratios of the ALG–BNC–MAA films compared with the corresponding ALG–MAA films indicate that BNC partly compensated for the increased hydrophilicity introduced by the extract. Hydrogen bonding and physical entanglement between BNC fibrils and alginate chains likely formed a more cohesive network, thereby restricting polymer-chain mobility, swelling, and dissolution (Huq et al., 2012; Siqueira et al., 2019; Rahman et al., 2024). Among the tested formulations, ALG–BNC–1–MAA and ALG–BNC–3–MAA provided the most favourable balance between moisture resistance, structural integrity, flexibility, and potential functional properties. BNC primarily strengthened and stabilized the polymer matrix, whereas the MAA-rich extract increased water affinity and flexibility, allowing the moisture-related properties of the films to be adjusted according to the intended packaging application.

Shelf-life packed blueberries

Blueberries were selected as a representative high-moisture food model due to their high-water content, delicate epidermal tissue, intense respiratory activity, and rapid susceptibility to dehydration and quality deterioration during storage. The developed ALG-, BNC-, and MAA-based biodegradable films were evaluated for their ability to reduce moisture migration, thereby preserving the freshness, firmness, and visual quality of the fruit. Similar protective effects may be expected for other highly perishable fruits and vegetables, including strawberries, raspberries, grapes, cherries, tomatoes, and fresh-cut products, where effective control of moisture transfer is essential for extending shelf life. The use of biodegradable polysaccharide-based packaging materials has gained increasing attention due to their potential to maintain postharvest quality and reduce dependence on petroleum-based plastics (He et al., 2023; Ncube et al., 2020). The weight loss of blueberries gradually increased during the 4-week storage period for all investigated film formulations (Figure 4). After 4 weeks, the weight loss ranged from 3.6% to 6.5%, indicating differences in the moisture barrier performance of the developed packaging materials. The neat ALG film showed the highest weight loss (6.5%), which can be associated with its relatively high porosity (32.5 mL/min) and lower resistance to water vapor transfer.

The incorporation of BNC significantly improved the barrier efficiency of the alginate matrix. The ALG-BNC film reduced the final blueberry weight loss to 4.7%, which can be attributed to the formation of a more compact and mechanically reinforced polymer network. This behaviour was supported by the lower porosity (27.1 mL/min) and higher tensile strength (40.8 MPa) of the ALG-BNC film compared with the pure ALG formulation. The reinforcing effect of nanocellulose on the structural integrity, mechanical properties, and water vapor barrier performance of biodegradable films has been widely reported due to the formation of strong hydrogen-bonding interactions between nanocellulose and polymer matrices (Morais et al., 2023; Zhang et al., 2024).

The incorporation of MAA further modified the structural characteristics of the films by increasing thickness and flexibility. However, increasing MAA concentration also enhanced the water affinity of the films, as demonstrated by the increased moisture content, water solubility, swelling ratio, and rehydration ratio. Consequently, although higher MAA concentrations contributed to lower porosity, excessive hydrophilicity could partially reduce the moisture barrier performance. The water resistance of biodegradable films is strongly dependent on polymer interactions, cross-linking density, molecular mobility, and the availability of hydrophilic functional groups that regulate water diffusion and moisture sorption behavior (Wang X. et al., 2024; Yang et al., 2024). Among the ALG-MAA formulations, ALG-3-MAA and ALG-5-MAA exhibited improved preservation compared with pure ALG, showing final weight losses of 5.1% and 5.4%, respectively, due to their reduced porosity and enhanced flexibility. The best preservation performance was achieved with the BNC-containing MAA-modified films. In particular, ALG-BNC-3-MAA exhibited the lowest weight loss throughout the entire storage period, reaching only 0.9%, 2.3%, 3.0%, and 3.6% after the first, second, third, and fourth weeks, respectively. This superior behavior can be attributed to the synergistic effect of BNC reinforcement and an optimal MAA concentration, which resulted in a dense and mechanically stable polymer network. The ALG-BNC-3-MAA film exhibited a favorable combination of reduced porosity (16.8 mL/min), adequate tensile strength (30.7 MPa), and improved elongation at break (20.8%), allowing it to maintain structural integrity and conform to the irregular blueberry surface, thereby reducing moisture loss.

Although ALG-BNC-5-MAA exhibited the lowest porosity (14.2 mL/min), its slightly higher weight loss (3.8%) compared with ALG-BNC-3-MAA may be explained by its increased moisture affinity, reflected by higher moisture content, water solubility, swelling ratio, and rehydration capacity. These findings demonstrate that the preservation efficiency of biodegradable films is not determined solely by low porosity or increased thickness but rather by an optimized balance between film compactness, mechanical strength, flexibility, and moisture interaction behavior. Similar structure–property relationships have been reported for advanced polysaccharide-based biodegradable films, where optimized polymer networks provide improved barrier and preservation performance (Zhang et al., 2024; Huie et al., 2023).

The visual evaluation of the packaged blueberries supported the gravimetric measurements as presented in Figure 5.

Blueberries packaged in BNC-containing films, especially ALG-BNC-3-MAA and ALG-BNC-5-MAA, maintained a more uniform appearance, darker characteristic colour, and fewer signs of surface dehydration compared with blueberries packed in pure ALG or lower-modified films. In contrast, samples packed in ALG and ALG-MAA films showed more noticeable surface wrinkling and a less fresh appearance after storage, corresponding to their higher weight-loss values. The improved visual quality observed for BNC-containing films confirms the beneficial role of nanocellulose reinforcement in enhancing the protective properties of alginate-based packaging materials (Morais et al., 2023; Zhang et al., 2024). The combined gravimetric and visual analyses demonstrate that the synergistic incorporation of BNC and an appropriate concentration of MAA significantly enhanced the performance of alginate-based biodegradable films. Among all tested formulations, ALG-BNC-3-MAA provided the most balanced combination of barrier properties, mechanical performance, flexibility, and controlled water interaction, making it a promising candidate for sustainable packaging applications of high-moisture fruits.

The present study successfully demonstrated the development of novel sodium alginate-based biodegradable films reinforced with bacterial nanocellulose and enriched with mycosporine-like amino acid (MAA)-rich extracts obtained from nori processing by-products. The combination of a bio-based nanomaterial and a marine-derived bioactive extract represents a sustainable strategy for producing multifunctional packaging materials while simultaneously promoting the valorization of underutilized marine waste streams. The incorporation of BNC significantly enhanced the structural integrity of the alginate matrix, resulting in improved mechanical properties, reduced porosity, and enhanced moisture barrier performance due to the formation of a compact and interconnected polymer network. The addition of MAA-rich extracts effectively modified the physicochemical properties of the films by increasing flexibility and providing functional characteristics associated with natural marine compounds. However, the results demonstrated that film performance depended on achieving an appropriate balance between flexibility, mechanical stability, and water interaction properties, as excessive MAA concentrations increased moisture affinity, reflected by higher moisture content, water solubility, swelling, and rehydration capacity. Among the investigated formulations, the ALG-BNC-3-MAA film exhibited the most favorable overall performance, combining low porosity (16.8 mL/min), adequate tensile strength (30.7 MPa), high elongation at break (20.8%), controlled surface wettability, and optimized moisture-related properties. These characteristics translated into superior practical performance during the blueberry storage study, where ALG-BNC-3-MAA effectively reduced fruit weight loss to 3.6% after 4 weeks and preserved the visual quality, firmness, and freshness of the fruit compared with other developed formulations. The findings emphasize that the efficiency of biodegradable food-packaging films is governed not only by individual parameters, such as thickness or porosity, but by the synergistic interaction between microstructural organization, mechanical behavior, interfacial properties, and moisture transport.

The novelty of this work lies in the integration of BNC reinforcement with MAA-rich extracts recovered from nori processing residues into an alginate matrix, creating a multifunctional biodegradable packaging system that combines improved mechanical performance, controlled water interactions, and enhanced food preservation ability. To the best of our knowledge, studies exploring the simultaneous application of bacterial nanocellulose and MAA-rich marine extracts from nori waste in alginate-based films remain limited, highlighting the originality and potential of this approach. Future research should focus on a deeper understanding of the interactions between alginate, BNC, and MAAs at the molecular level using advanced characterization techniques, as well as evaluating long-term stability, biodegradation behavior, migration safety, and scalability of the film production process. Furthermore, the application of these films should be extended to different food matrices, particularly highly perishable fruits and vegetables, and their potential active functions, including antioxidant and UV-protective properties, should be further investigated under real storage and commercial packaging conditions. This research also provides a promising circular-economy approach for the design of next-generation active biodegradable packaging materials, where marine by-products and nanocellulose-based reinforcement are utilized to improve the functionality, sustainability, and practical applicability of bio-based food-packaging systems.