Advances in retort-based preservation of meat products: modern approaches for shelf-life extension and quality retention

1 Introduction

The retort process is the gold standard for industrial sterilization of meat-based food products with low acidity, as it provides sufficient heat energy needed for microbial control and prevention of Clostridium botulinum spores (Jimenez et al., 2023).

Despite the wide use of retorting technology for many kinds of foods with low acidity (such as vegetables, seafood, soups, sauces, legumes, and dairy foods), meat deserves special consideration because of specific qualities of its structure, perishability, and technological difficulties. Meat is a biologically complex, highly perishable, yet nutritious product that consists of high moisture content, high protein content, variable fat content, haemoproteins, enzymes, and, in many cases, heterogeneous structure of meat particles. Taking into account the above-mentioned properties of meat products, they are more prone to microbial spoilage, oxidation of lipids and proteins, pigment deterioration, toughening of the meat texture, and non-uniform heating during sterilization. Moreover, the low-acid condition of many meat products makes them very important from the point of view of commercial sterility against spore-forming pathogens like C. botulinum. Therefore, meat products may be considered one of the most challenging categories of retorted foods from both safety and quality standpoints (Jimenez et al., 2023; Zhu et al., 2024).

The retort processing technology is generally divided into two categories based on static and agitated (dynamic) operations, with industrial examples being steam retorts, water immersion retorts, water spray retorts, steam-air retorts, rotary retorts, and continuous retorts (Tucker and Featherstone, 2021; Jimenez et al., 2023). Static retorts are more frequently used due to their simplicity, ease of operation, and cost-effectiveness; yet they typically involve long heating time and have a tendency to result in quality degradation due to poor heat penetration (Tucker and Featherstone, 2021; Jimenez et al., 2023). Water immersion and water spray retorts ensure better temperature distribution and are ideal for retort pouches and semi-rigid containers, though their water circulation process is more complex (Tucker and Featherstone, 2021). Steam-air retorts provide good pressure control and flexible packaging solutions, though they are complicated to operate (Jimenez et al., 2023). Rotary retorts and other agitated types increase heat transfer due to continuous movement of the product, decreasing the time required for processing and maintaining quality, though they are characterized by a higher price and certain package limitations (Tucker and Featherstone, 2021; Jimenez et al., 2023). Continuous retort systems allow producing high-quality products quickly in large quantities, though they are costly to install and maintain (Tucker and Featherstone, 2021; Jimenez et al., 2023). Therefore, despite the advantages of each retort type, traditional static retorts prevail in the meat industry due to economic feasibility and convenience of their usage.

However, despite the widespread application of this technology in the food industry, conventional static retort systems often result in undesirable quality changes, including oxidation of fats and proteins, pigment fading, protein denaturation, and increased firmness due to prolonged exposure to heat and slow heat-transfer characteristics (Zhu et al., 2024; Ou et al., 2026). The need to produce ready-to-eat (RTE) meat products without quality deterioration aligns with current consumer trends (Cui et al., 2024). However, the increasing requirement of shelf-stable, convenient, and high-quality meat products has imposed an obligation on food processors to design retort technology capable of achieving commercial sterility with reduced quality degradation and increased efficiency in the processing process. This is the primary challenge that should be met by retort technology nowadays, taking into account that besides the question of microbial safety, the problem of preservation of nutritional value, sensory attributes, and sustainability of the process is to be considered as well. To address such challenges, modern retort technology innovations have emphasized process optimization and improvement of heat transfer efficiency in conjunction with the use of digital control systems to ensure high process accuracy and efficiency (Tucker and Featherstone, 2021; Jimenez et al., 2023). In this review, “modern approaches” means new approaches used in retort processing, including variable temperature profile processing in retort, rotary/agitated retort processing, CFD-assisted processing optimization, hybrid processing technologies, intelligent retort systems, and innovative packaging/ingredient approaches in order to avoid over processing of the product while preserving its quality and commercial sterility. Simultaneously, advancements in packaging systems, including the development of advanced barrier materials, new generations of multilayer retort pouches, and innovative polymeric packaging structures, now play a crucial role in maintaining product quality not only by enhancing resistance to oxidation, but also by reducing moisture transfer, preserving color and flavor, improving seal integrity during retorting, resisting both mechanical and thermal stress in harsh retorting conditions, preventing any contamination from outside, and increasing product shelf life (Batista et al., 2022; Jimenez et al., 2023; Gupta, 2024; Sani et al., 2024; Tripathi et al., 2026). Additionally, ingredient-level interventions such as the use of natural antioxidants, stabilizers, and texture modifiers offer further opportunities to mitigate oxidative and thermal damage during processing.

In addition to innovations within the field of conventional retort technology, there have been developments of some new thermal sterilization techniques and methods for complementary preservation in the context of overcoming the quality problems of static retorting. They include the development of MATS, ohmic heating, super-heated steam and sous-vide/retort technology. Even though these innovative methods use a different principle of heating compared to traditional steam or water retorting, their key feature remains the same–to reach commercial sterility or at least comparable microbial safety along with quality improvement. That is why they are considered in this review as alternative thermal preservation technologies. Despite being observed in a variety of studies, however, much of the data concerning the mechanisms underlying these innovations, as well as their strengths, weaknesses, and applications to meat products produced via retort processing, is fragmentary. Hence, there arises an obvious need for critical analysis of the aforementioned innovations in order to understand the way in which the modern retort techniques can work together in order to compensate for the quality deficiencies inherent in conventional methods without compromising safety and shelf stability of the resultant products. In this respect, the current paper provides such a critical review of the modern retort processing innovations as variable retort temperature profile processing, rotary/agitated retort systems, computational fluid dynamics (CFD) based process optimization, advanced retort packaging, quality enhancement via use of ingredients, MATS, ohmic heating, and smart retort controls.

In order to enhance coherence of science and to gain an organised view about the topic, this literature review is divided into sections based on important factors influencing retort performance in meat products. Section 3 provides the basic principles of retorting including thermal sterilisation kinetics, heat transfer mechanisms, and types of retort systems. Section 4 compares traditional retort technology and new technologies, as well as indicates the necessity of innovation due to quality degradation linked to traditional static retorting. Section 5 describes process design and digitalisation issues, i.e., variable temperature regimes of retorts, computational fluid dynamics (CFD), and intelligent retort systems. Section 6 deals with thermal lethality, microbial inactivation, heat transfer behaviour, and packaging-process interaction aspects affecting food safety and efficiency of the process. Sections 7, 8 analyse physico-chemical, sensory, and microbiological changes taking place during retorting and storage of meat products from the perspective of quality maintenance, commercial sterility, and shelf life prolongation. Sections 9, 10, 11, 12 summarise the influence of packaging and retort innovations, as well as technical recommendations and future research needs.

2 Review methodology

This article was written as a narrative review to cover the recent developments in retort processing techniques for meat and meat products with a specific focus on advancements in processing techniques, packaging materials, preservation of food qualities, and innovative hybrid thermal techniques. This type of review differs from systematic reviews since the purpose of conducting this study was to critically analyze the available literature rather than performing any kind of quantitative analysis.

The scientific articles were collected by searching various databases that include Scopus, Web of Science, Science Direct, PubMed, and Google Scholar. The search criteria included the following keywords: “retort processing”, “retort pouch”, “thermal sterilization”, “ready-to-eat meat products”, “variable retort temperature processing”, “agitated retort”, “MATS”, “ohmic heating”, “computational fluid dynamics”, “food packaging”, and “shelf-life extension”. Preference was accorded to articles in peer-reviewed journals published during the period of 2005–2025 in order to capture recent advancements in technology. However, classical literature and references were also used when needed to illustrate the basic concept of thermal treatment and sterilization in industry. Articles that had relevance to the topic of retorting of meat and meat products, advancements in technology, packaging technology, and quality attributes were chosen for the review. Conference abstracts, non-peer-reviewed papers, duplicated papers, and articles that had no direct bearing on the topic were excluded from the review.

3 Fundamentals of retort processing

3.1 Principles of thermal sterilization

Thermal sterilization is based on the destruction of heat-resistant spores, particularly C. botulinum, using specific kinetic parameters such as D-values, Z-values, and F₀ values which define microbial heat resistance and process lethality (Holdsworth and Simpson, 2016; Singh et al., 2017a). The D-value (decimal reduction time) denotes the time taken at a particular temperature to kill 90% (log cycle reduction) of the microbial population, hence the heat resistance capacity of the microorganism at particular processing parameters. Z-value represents the change in temperature necessary to change the D-value by a log cycle (90%) and hence defines the sensitivity of the microbial population to processing temperatures. Both D- and Z-values help in predicting the thermal resistance pattern of target spores and the heat treatment intensity required for their deactivation. F₀ represents a lethality process value, the total sterilizing effect of the thermal treatment schedule represented by a reference heating time at 121.1 °C with z = 10 °C. The time–temperature history at the coldest point of the retort process is used to calculate F₀, which shows whether enough lethality has been achieved for obtaining commercial sterility. Therefore, while D represents the heating time required at a specific temperature for a certain organism to be killed, z represents the change in the heating time when temperature changes, and F₀ integrates all this information (Figure 1).

Such kinetic parameters are vital for obtaining a successful retort process since they provide the means by which the thermal schedule can be developed to effectively eliminate the spores of C. botulinum without causing more heat treatment than necessary. In case the F₀ value derived falls below the desired value, there may be difficulties in microbial survival and safety issues; on the other hand, in case the F₀ value is too high, there will be overcooking of food and thus degradation of nutrients, toughening of texture, discoloration, and loss of flavor.

3.2 Heat transfer phenomena and thermophysical characteristics in retort technology

Heat transfer in retorting operations depends not only on the retort operational parameters, but also on the thermophysical characteristics of the food matrix, affecting the rate and mode of heat diffusion within the packed product. Amongst these properties, the important ones are thermal conductivity, thermal diffusivity, and specific heat capacity. The property of thermal conductivity shows the ability of a food to transmit heat and hence foods having higher thermal conductivity would have more penetration of heat. Thermal diffusivity is related to the rate of transmission of temperature change in the food product and is affected by thermal conductivity, density, and specific heat; hence, it becomes an important factor in deciding heating rate and occurrence of cold spot. The specific heat capacity is the energy needed for a rise in temperature of a food product and consequently has an influence on the thermal load required for retort processing. These physical properties control the occurrence of cold spots, lethality time, and the probability of either over- or under-processing.

The technique of retorting does not only apply to meat; it is actually one of the most commonly applied heat preservation techniques in the food industry that deals with low acid shelf-stable foods such as vegetables, fish, soups, sauces, milk products, legumes, and composite RTE meals. On the contrary, this review aims specifically at meat and meat products because of their complexity in terms of retorting from the perspective of microbiology and quality. Meats are normally low acid, highly perishable and diverse in nature, making them difficult to be sterilized without altering their quality adversely.

Comparatively speaking, the thermodynamics of meat products is different from that of many other foods being retorted due to the combined effect of muscle proteins, connective tissues, fat, water and entrained particulates. In the case of vegetables or legume products, heat conduction and quality modification are dependent on starch, fibre and structural strength, while for dairy products or sauces, it may be similar to a homogeneous liquid system. On the contrary, meat systems normally consist of individual pieces of muscles, emulsified fat, connective tissues containing high amounts of collagen, and gravy/sauce ingredients within the same package, resulting in heterogeneous conductive-convection heating behavior and complicated cold spot behavior. In addition, meat proteins become denatured and aggregated in the course of sterilization process, whereas lipids are prone to oxidative spoilage and formation of warmed-over flavor, which makes the ratio between microbial killing and preservation of food quality particularly important in retorted meat systems. Thus, knowledge of meat composition and structure is necessary for retort processing scheduling and lethality delivery.

The values of thermal conductivity, diffusivity, and specific heat in foods are strongly influenced by composition, structure, and temperature. Water generally enhances thermal conductivity and diffusivity, whereas fat and entrapped air reduce heat transfer efficiency because of their lower thermal conductivity. Protein denaturation, starch gelatinization, fat melting, and structural breakdown during processing may further alter the thermal properties of the product, thereby changing heating behaviour as sterilization progresses. It therefore follows that correct characterization of these thermophysical properties is critical for the design of retort cycles and the prediction of heat penetration, location of cold spots, and lethality in packaged foods (Park and Yoon, 2018; Li et al., 2023; Polachini et al., 2023).

The mechanism for heat transfer in the case of retorting process is primarily dependent on the physical nature of the product, whereby the heat transfer can either be by conduction, convection, or both. In the case of retort-processed meat products, solid and semisolid meat products include products that have a well-defined structure and low flow of internal fluids upon heating. These consist of meat chunks, canned meat pieces, sausages, meat patties, meatballs, mincemeat, meat batters, meat emulsions, and dense curry bases having limited fluid. In the case of solid and semisolid meat products, heat transfer takes place through conduction, moving from the surface of the package into the geometric center of the food body. This makes the heating process take long time with cold spot situated at the center of the product.

On the other hand, the category of liquid meat products includes those that flow freely and can be pumped easily like broths, soups, gravy, sauces, meat stock, and liquid phase of curry-based retort meals. Convection heating is usually much faster compared to conduction heating as it results in fast and even distribution of temperatures in the food package. The strength of convection, however, relies on several factors including the viscosity, density, particle loading and the orientation of the food package during processing.

The majority of retort processed foods available commercially are heterogeneous foods with a liquid and solid particulates component. Examples of such heterogeneous foods include meat curry, stew, meat chunked soup, gravy-based RTE meat products and mixed meat and vegetable food items. For such foods, the heat transfer takes place by the conduction-convection mechanism whereby the liquid component is heated by convection while the conduction process transfers heat into the solid meat particles. The importance of such type of heat transfer becomes very clear for designing retort process as it is responsible for the heating rates, cold spots, and lethality distribution in the package. In some heterogeneous foods, the slowest component in terms of heating might be the solid particles not the liquid.

Recent studies have highlighted the importance of product formulation, rheology, and package geometry in determining heat-transfer behaviour during retorting. Viscosity-modifying ingredients such as starches, hydrocolloids, and stabilizers can reduce internal fluid circulation and shift heating behaviour toward conduction-dominant patterns. Similarly, the ratios of water, fat, protein, and trapped air significantly influence thermal conductivity and diffusivity. The shape of the package also matters, because thin pouches, trays, and cans of various shapes have different heating characteristics and cold spot locations (Park and Yoon, 2018; Cevoli et al., 2013; Li et al., 2023; Polachini et al., 2023).

The advancement of Computational Fluid Dynamics (CFD) and finite element models has helped greatly in making it possible to accurately predict the transfer of heat in retorted foods. The models make it possible to analyze the temperature distribution, cold-spot migration, and lethality in foods of complex structure, especially those that have particulates and viscous sauces. It has been observed from CFD analyses that the effects of the size of particles, viscosity, geometry of the package, and agitation of the retort on convection flow patterns and heat penetration are significant, while finite element analyses can be employed in cases where heat transfer is conducted predominantly through conduction along with other aspects like change in quality (Zhu et al., 2022; Li et al., 2023; Szpicer et al., 2025).

The design of the retort also influences the efficiency of heat transfer process. The agitation in retorts with rotary, reciprocating, and end-over-end mechanisms improves the efficiency of the forced convection process because of the disruption of the thermal boundary layer and greater mobility of the products in the process of heating. Thus, higher temperature distribution uniformity and reduced processing time required for achieving the required level of lethality can be achieved compared to stationary retorts. In the same way, innovative methods of food sterilization, including microwave assisted sterilization and in-container heating processes, change the conventional heat transfer pathways and the locations of the cold spots within the system.

3.3 Types of retort systems

The types of retort systems applied in the thermal processing differ in terms of the type of heating media, pressure control method, packaging material, and heat transfer properties. In the meat industry, the following systems are the most widespread: still retort, water immersion retort, water spray retort, steam-air retort, and continuous retort. Every type of retort has certain peculiarities that affect its applicability to a particular product and packaging material. There are certain types of retorts that are more suitable for metal cans and there are those that are specifically designed for retort pouches and trays. Thus, along with the classification of the retort systems in general, it is necessary to know the special features of each of them to choose the right thermal process for meat products. Still retorts are the most traditional designs and are commonly employed for canned meat preparations due to their simple design, cheap installation costs, and easy handling process. In these kinds of retorts, packages are kept static while the process takes place, and the heat transfer inside the food is conducted either through conduction or natural convection based on the consistency of the product. Despite all these advantages of still retorts, the process time is relatively long in this case. The basic design of the vertical batch retort involves a pressure vessel that has a provision for steam/water supply, tray that is perforated for keeping bottles, provision for measuring pressure and temperature and control valves (Figure 2).

In the case of water spray retorts, the heated water is sprayed on the product load with simultaneous application of pressure inside the retort chamber. Such retort systems are commonly used for pouches, trays, plastic containers, and glass jars since they offer better package protection, heat transfer, and water conservation than other types of systems. It must be emphasized, however, that the efficiency of these systems heavily depends on the spraying pattern, nozzles’ effectiveness, and recirculation.

Steam-air retorts work according to the steam and compressed air mixture principle and thus make it possible to provide both heating and controlling of the overpressure. They are especially helpful in the case of flexible and semi-rigid packaging since they provide protection from deformation or loss of seal integrity during the process of sterilization. Steam-air retorts provide a better control of overpressure compared to steam retorts but involve more complex control mechanisms.

Continuous retorts are ideal for applications involving high production rates where packages are passed continuously through heating, holding, and cooling cycles. Such an approach is very suitable for canning in terms of increased productivity and reduced handling. The only drawback of continuous retorts is that they are very costly both in terms of initial installation and maintenance costs.

A comparative summary of the major retort systems, the heating medium used, heat transfer properties, type of packaging material, and strengths and weaknesses is shown in Table 1 below.

TABLE 1. Different retort systems and their characteristics.

In general, the choice of retorting system will depend on the character of the meat product, the kind of package used, and the scale of production along with the need for the optimal compromise between the sterility of the food and maintenance of its quality. The still retort system is still relevant for traditionally packed goods due to its simplicity and economical nature, while the liquid and steam-air systems will fit well the new flexible packaging which needs better pressure management. Continuous retort system will work in case when high productivity is needed.

4 Retort technology in meat preservation: traditional vs. modern perspectives

The technique of retorting is considered to be among the most widely applied methods for the production of food products made from meat that have microbiological safety and shelf-life stability; however, the growing need for RTE food products of high quality has led to a change in approach towards sterilization by means of ensuring safety, retention of nutrients, sensory characteristics, and efficiency of process. Traditionally, retorting techniques have been used successfully for meat products which are packed into cans and packages; however, long-term heating cycles in conventional retorts can negatively impact flavor, color, texture, retention of nutrients, and oxidative stability. Therefore, current advancements in retorting technology involve minimizing thermal abuse and maintaining the appropriate lethal effect for commercial sterility. In such an aspect, it is necessary to compare traditional retort limitations and current advancements in retort technology.

4.1 Traditional challenges

The conventional static retorting process is mostly marked by high temperatures and extended times of heating and holding. This is especially so in conduction heated processes and highly viscous meat products where heat has to be slowly introduced from the outside of the package into the center of the product. While this method is useful in ensuring that products are free from microbes, excessive exposure to high temperatures may lead to various adverse effects on meat products. This includes lipid oxidation which is responsible for causing the rancid taste, protein oxidation and denaturation that may lower the ability of meat products to hold water and tenderize the meat and degradation of pigments hence making meat products lose their color. It may also cause hardening or drying of meat products due to loss of moisture and damage of muscle protein structures.

The degree of quality damage during conventional retorting is dependent on a combination of a number of interacting parameters such as product viscosity, particle size, package shape, oxygen concentration in the package headspace, temperature of the process and F₀ value. For instance, highly viscous products like gravies and meat emulsions take time to heat due to low thermal conductivity. Large particles or non-uniform distribution of particles inside the product will lead to changes in the location of the cold spot in the package and thus affect the heating pattern. Oxygen concentration in the package headspace will speed up lipid oxidation and colour damage while excess thermal load will promote nutrient loss and damage in the texture of the food. It is therefore clear that many of the quality issues arising out of conventional retorting are not only attributed to the static nature of the technology but also the interactions between product, packaging and the process.

In order to compensate for these shortcomings, some innovative technologies and techniques linked with retorts and process optimization have been developed to reduce heating time, temperature uniformity, and preservation of product quality. The new techniques are listed in Table 2.

TABLE 2. Emerging technologies in retort processing.

4.2 Scope of innovation

There has been a greater demand for shelf stable RTE meat products as a result of convenience, lifestyle changes, military and institutional food demands, and wide-scale retail availability, leading to a need for a retort process that not only assures safety but also maintains high-quality of the product. Although the conventional retort technique assures commercial sterility, the contemporary consumer demands that foods be retorted in such a way that they maintain quality characteristics similar to that of freshly prepared foods.

Need for innovation in the retort process is also necessary because there are many different types of meat products available in terms of composition and physical form, from solid meat pieces to emulsions to curry gravies to particulate food products, all with varying heat transfer properties. Consequently, a single approach to the conventional retort process may not be ideal for all food products. Variables like viscosity, particle size, pack thickness, oxygen levels in the head space, storage temperatures, and desired F₀ values have an impact on heat transfer efficiency and microbial kill rate, oxidation resistance, and shelf life.

Consequently, modern innovations in retort technology have become more holistic in nature through the combination of various technologies such as heating, process control and modelling, packaging and formulation based protection. These strategies seek to not only mitigate the effect of the thermal insult but also provide optimal heat distribution, lethality delivery, oxidation reduction, and stability of the product post-processing. Therefore, modern innovations in retort technology do not solely focus on the sterilization part of the process but also on the whole product-process-package system.

Despite the unique features of each novel technology, there is no technology that is superior to all other meat products. VRTP technology allows reducing the risk of thermal abuse due to optimization of the temperature profile, but nevertheless, it is based on the use of standard conductive heating method. Improved heat exchange occurs with agitated retorts, which are especially suitable for treating particulate and viscous foods. However, these advantages do not apply to rigid products and include increased equipment cost. The most significant reduction in treatment time and improved quality indicators (sensory and nutritive) occur with the MATS process that uses volumetric heating, but its use in industry is restricted by expensive equipment, special packaging, and regulatory validation. Thus, the current trend is not about the substitution of standard retorting technology with one novel technology, but rather an application of several complementary technologies depending on the food type.

The key elements affecting the efficiency of the retort process and their effects on the safety and quality of the products are shown in Table 3.

TABLE 3. Factors affecting retort process efficiency.

The change in retort technology from traditional to modern can be seen as the bigger change in meat preservation technology from “sterility-oriented processes” to “quality optimized sterilization.” Traditional retorts continue to play an important role in providing microbiologically safe low acid meat products; however, there are possibilities with new technologies such as MATS, ohmic heating, hybrid pre-treatment techniques, intelligent retort control, and CFD modeling. Therefore, the future of retort technology in the meat industry lies in the development of validated lethality in combination with intelligent process design and formulation/packaging.

5 Improvements in retort heating methods

5.1 Variable retort temperature processing (VRTP)

VRTP uses controlled dynamic temperatures for decreasing over processing without compromising the safety. It has been proved through modeling studies that significant decrease in processing time is possible without any compromise on lethality (Chen and Ramaswamy, 2002).

5.2 Agitating retorts

Agitated retort systems contribute to heat-transfer efficiency in thermally processed foods by means of increasing forced convection via constant product movement in a container. Induced mixing helps to reduce thermal gradients and decrease the formation of cold spots, which is a common phenomenon in non-agitated retort systems, especially in viscous and particulate meat products. Agitation results in increased heat-transfer efficiency and comes up with a rapid heat penetration process that leads to reduced come-up times, decreased overall processing times, but still provides a necessary level of microbial destruction (F₀). Studies show that reciprocating and rotating agitation modes provide high levels of heat transfer coefficients and improved temperature distributions in containers, which help to achieve a target value of sterilization quickly (Singh et al., 2015; Holdsworth and Simpson, 2016; Erdogdu et al., 2017). An important role is played by the product viscosity factor because the presence of high viscosity inhibits natural convection and makes agitation inevitable (Ramaswamy, 2010).

As such, agitated responses not only enhance the efficiency of the process but will also ensure improved product quality due to minimal heating, which ensures that there is no degradation of nutrients, colors, and textures, yet maintaining commercial sterility (Singh et al., 2015).

5.3 Heat-transfer aids and inserts

Attempts have been made to reduce non-uniformities in temperature distribution in meat-based products using conductive inserts and packaging designs optimized based on modeling techniques. In processes where conduction is dominant, the coldest point of heating, referred to as the cold spot, occurs at the geometrical center of the product (Datta, 2007). The introduction of conductive inserts and redesign of packaging geometry is important in improving thermal processing efficiency in the retort system (Datta, 2007). Modeling and experimental results have shown that re-designing of the container geometry and the addition of internal metallic or high thermal conductivity inserts is effective in reducing thermal gradients and heat diffusion within the product (Ketteringham and James, 2000; Singh et al., 2017b). Thermal conductivity inserts are useful in enhancing internal heat transfer rates by raising thermal conductivity in the food matrix (Ketteringham and James, 2000). According to Azar et al. (2020), the location of the slowest heating region was determined by using CFD modeling of heat transfer through conduction in canned foods during the retorting process. The model allowed for the optimization of the thermal process schedule so as to reduce the processing time without compromising the F₀ requirement (Azar et al., 2020). Like container geometry, changing the geometry of containers can be useful in shifting or reducing the effect of the coldest part of the food, which is normally the slowest heating region (Datta, 2007). By combining these two techniques, it is possible to lower the temperature gradient and make the temperature distribution more even in the products (Singh et al., 2015). This leads to efficient heat transfer and lowers processing time without reducing the required microbial lethality (F₀) (Stumbo, 1973).

5.4 Computational modelling and simulation

Modeling and CFD provide valuable means of predicting heat penetration, cold spot behavior, and suitable process schedule (Datta, 2007), and thus eliminating the trial and error approach in retort design. Modeling and simulations have become key elements in contemporary food engineering for optimization of thermal processing of foods such as retort sterilization, dehydration, and quality control (Simpson et al., 2008; Singh et al., 2017a; Erdogdu et al., 2017; Azar et al., 2020). Through modeling and simulations, it is possible to effectively predict coupled heat and mass transfer in food system and eliminate the need for many experiments (Chen and Ramaswamy, 2002; Singh et al., 2017b; Erdogdu et al., 2017; Azar et al., 2020).

Numerical techniques including computational fluid dynamics (CFD) and heat transfer models have been commonly used to predict temperature profile, locate the slowest-heating region, and compute F₀ value under different retort processing scenarios (Chen and Ramaswamy, 2002; Simpson et al., 2008; Singh et al., 2015; Azar et al., 2020). The models play an essential role especially in conduction-controlled meats where the temperature difference between surface and the core leads to over-cooking of the outer layers while under-cooking occurs at the geometric center (Singh et al., 2017a; Erdogdu et al., 2017; Azar et al., 2020; Fardella et al., 2021). Advanced simulation models enable the examination of process innovations such as agitation, reforming of package geometry, and optimization of heat profiles that increase heat transfer rate and shorten processing time without any effect on the required lethality (Simpson et al., 2008; Singh et al., 2015; Erdogdu et al., 2017; Fardella et al., 2021). Optimization of processing through model simulation has shown that thermal exposure was significantly reduced without affecting the sterility of foods (Chen and Ramaswamy, 2002; Simpson et al., 2008; Singh et al., 2015; Azar et al., 2020).

The recent developments in this field include linking CFD with artificial intelligence methods, which include neural networks and genetic algorithms, for optimizing variable retort temperature profiles (Chen and Ramaswamy, 2002; Simpson et al., 2008; Azar et al., 2020; Fardella et al., 2021). While there are many benefits of using these models, their accuracy depends heavily on the availability of accurate thermophysical property data and experimental validation (Singh et al., 2017b; Erdogdu et al., 2017; Azar et al., 2020; Fardella et al., 2021). Other than using CFD, the use of hybrid modeling methods, which incorporate artificial neural networks (ANN) and genetic algorithms and other optimization techniques has become common practice (Chen and Ramaswamy, 2002; Simpson et al., 2008; Azar et al., 2020; Fardella et al., 2021).

Such integrated approaches are highly relevant when it comes to large-scale meat production, as the optimization of balance between safety and sensory properties is crucial in this case. Recent literature states that multiscale and multiphysics approaches are essential for modeling of such complex food systems that contain a variety of complex structures and moisture variations (Singh et al., 2017a; Erdogdu et al., 2017; Park and Yoon, 2018; Szpicer et al., 2023). Further development in terms of computational capabilities and model integration is expected to make the application of such models more effective in case of industrial retorting. In general, computational modeling offers an excellent tool for designing efficient and safe thermal treatment processes, especially with regard to retorting of meat products.

Comparisons between recent research reveal significant differences in the approach to quality enhancement through optimization strategies. While VRTP primarily focuses on reducing the cumulative heat load by means of optimization of heating cycles, agitated retorts improve convective heat transfer processes and decrease cold spots. The role of CFD lies in the indirect prediction of temperatures prior to the process application, which helps avoid expensive validation tests. Conductive inserts and pack design modifications, unlike the previous engineering strategies, change the actual way of heat transfer inside the product. Thus, the highest results are obtained in combination of the three methods.

5.5 Digital and smart retort systems for advanced thermal processing of meat products

The advent of digital retorts and smart retort systems for thermal processing of meat and ready-to-eat (RTE) food products is an important breakthrough in processing techniques. Traditional retort processing techniques follow fixed temperature-time conditions, which lead to overprocessing in order to ensure lethal treatment at the coldest spot–usually present in the geometric center of meat products heated by conduction. Overprocessing results in poor quality due to factors such as toughness, dehydration, lipid oxidation, and nutrient losses. However, digital retort processing relies on computational techniques along with sensor-based controls to ensure the required lethality without affecting the quality of the products.

Traditionally, heat treatments are conducted according to established procedures that include cycles of processing based on the results of heat penetration studies and microbe destruction. Nevertheless, this approach does not take into account the variability in the properties of the food, the shape of packaging, and the thermal non-uniformity in the retort charge. The use of new computing technologies allows building predictive models based on heat transfer simulations in food packages by means of computational fluid dynamics (CFD) and finite element method (FEM). Many researchers used this approach for estimating the temperature distribution and F₀ value in packages containing meat products and particulates (Singh et al., 2017b; Park and Yoon, 2018; Azar et al., 2020).

Although significant technological advancements have been made, smart retort systems are still in the infancy stage of their application in the industry. Previous research on the topic has largely been carried out in laboratories or pilot plants while the main challenges to adoption in the industry include huge cost of implementation, retrofitting the retort systems with the new technology, cybersecurity needs due to the interconnectedness of the system, and validation of the technology by regulation.

5.6 Role of computational modelling in smart retort design

Modelling through CFD is fundamental to digital retorts because it facilitates visualization of the heat transfer processes within the complicated structure of the food. CFD modelling is able to predict the position of the cold spot and thermal gradients especially in the conductive heat transfer of meat products. Numerical modeling of conductive heat transfer in canned foods, according to Azar et al. (2020), will allow optimizing retorting without any risk of reducing the necessary lethality level. These models minimize processing time without endangering food security. Singh et al. (2017a) have pointed out that the thermal process in the system of suspended particles in liquids can be predicted with mathematical models and CFD modeling. Further, CFD in sterilization systems was identified by Park and Yoon (2018) due to multiphase flows, transient temperatures, and lethalities’ building up.

5.7 Integration of artificial intelligence and optimization algorithms

Another one of the prominent aspects of intelligent responses is the incorporation of AI, machine learning, and optimization algorithms with models for thermal treatment. According to the pioneering study of Chen and Ramaswamy (2002), the employment of neural networks and genetic algorithms was capable of enabling the optimal design of VRT processing parameters. The results of their research indicated that model-based optimization could help in reducing the time of processing without breaching the constraints of F₀. A subsequent study conducted by Simpson et al. (2008) further confirmed that adaptive optimization methods could prove helpful in designing variable retort temperature profiles where the processing temperatures changed during the sterilization process in order to reduce thermal loading while ensuring commercial sterility. In addition, according to a recent study by Fardella et al. (2021), VRTP in combination with intelligent packaging systems can ensure minimal thermal damage to heat-treated foods.

5.8 Sensor integration and real-time monitoring

Today’s digital retorts utilize advanced sensors to track temperature, pressure, and state of products in real time. With the advent of wireless sensors, fiber-optic probes, and infrared cameras, precise measurement of temperature distributions is possible within the retort vessels. Predictive modeling based on real-time sensor data allows for dynamic control of the process variables in real time. Closed loop control system reduces the uncertainties due to inter batch variations. The integration of real-time sensor data and predictive modeling not only improves the lethality calculations but also helps in meeting the regulatory standards like FDA thermal processing. According to Park and Yoon (2018), real-time data becomes an essential component for validation of the CFD model.

5.9 Application in meat and RTE products

The food products made from meat, due to their complex nature, provide the best opportunity for applying intelligent retorting technology. Generally, heat transfer in meat systems is primarily characterized as conductive, which leads to slow heat transfer and cold spots occurrence. The solution to this problem using digital retort consists of accurate heating control and process scheduling based on various physical parameters of the product. Several computational fluid dynamics (CFD) investigations have shown that such meat parameters as particle size, fat content, and packing configuration have an impact on heat transfer characteristics and lethality profiles during processing (Singh et al., 2017b; Erdogdu et al., 2017). These findings were taken into consideration while processing meat using intelligent retorts. This was confirmed by the research conducted by Azar et al. (2020), where process optimization using numerical simulations has greatly decreased processing time while ensuring required lethality.

5.10 Variable retort temperature profiles and smart control

The variable retort temperature profile system is one of the innovations in smart retort systems. Contrary to the traditional retorts which run at constant temperature, the VRTP processes temperatures based on responses from the product and models. According to Chen and Ramaswamy (2002), VRTP systems that have been optimized through neural network and genetic algorithm increase efficiency of the process while reducing heat damage. In the same way, Simpson et al. (2008) found out that adaptive process optimization helps in minimizing processing time without compromising the need for achieving microbial safety. The innovation is very suitable for processing meat because maintaining desirable texture and taste is important.

While there have been considerable advances in this area, certain difficulties persist in the realization of intelligent retorts. For example, there is an absolute need for accurate modeling and thermophysical properties of meat matrices which vary depending on their structure and composition. Moreover, there is a requirement to validate the models used whether AI or CFD models in order to meet regulatory requirements. The future developments of such intelligent systems should be focused on cyber-physical retorts using IoT technologies.

Among others, digital retort systems are the most remarkable innovation in the area of meat thermal processing due to the integration of CFD simulations, sensors, and AI into one system that increases process efficiency and the quality of end products. There are numerous advantages of the application of such systems which include effective heat transfer, no overprocessing, and increased food safety by optimizing F₀ values.

Though static and agitated retort systems continue to be prevalent industrial technologies because of their reliability and regulation approval, the emerging systems vary in terms of the degree of their commercialization. The use of CFD for optimization is becoming more common as an engineering design method while intelligent retorts are used primarily by well-automated processing plants. On the other hand, MATS and ohmic heating have not moved beyond pilot plant and some limited commercial use due to reasons such as equipment cost, packaging suitability and regulatory approval. Technology selection would, thus, depend not just on the properties of the product but also on other factors like the processing scale.

The comparison given in Table 4 clearly illustrates that there is no technology which is the best for each and every meat product. Technologies like VRTP, agitated retorts, and CFD are based on engineering principles and increase efficiency in processing and heat transfer, whereas techniques like MATS and ohmic heating reduce thermal load using a different approach to heating. Packaging innovations mostly prevent deterioration of food quality after processing. Thus, the contemporary tendency in industrial retorting is to use a combination of technologies rather than a single one.

TABLE 4. Critical analysis of current retort technologies for shelf stable meat products.

6 Principles of retort sterilization: basics, thermal destruction, and process optimization

Retort sterilization is the current technology in use that provides the most recognized and applied way of thermal sterilization for manufacturing of shelf-stable ready-to-eat meat products. The success of this method is based on a scientific combination of thermal destruction, heat penetration, and proper packaging of food products. Although considered a “traditional” processing technology, modern retort processing now incorporates advanced modelling techniques, improved heat-transfer mechanisms, and high-performance barrier packaging systems, thereby enhancing both food safety and product quality (Stumbo, 1973; Holdsworth and Simpson, 2016).

6.1 Thermal lethality (F₀ concept) and microbial inactivation in retort processing of meat products

Thermal lethality (F₀) is a fundamental parameter in retort sterilization used to quantify cumulative microbial destruction at a reference temperature of 121.1 °C. It represents the equivalent time in minutes required to achieve a defined level of microbial inactivation, forming the scientific basis for designing safe thermal processes for low-acid foods such as meat products. In commercial sterilization, a 12-log reduction of C. botulinum spores is widely accepted as the benchmark for ensuring commercial sterility and food safety in shelf-stable foods (Holdsworth and Simpson, 2016; Singh et al., 2017a).

In meat systems, achieving target F₀ values is challenging due to their heterogeneous structure and conduction-dominant heat-transfer behaviour, which leads to the development of a slowest heating zone (cold spot), typically located at the geometric centre of the product. This unequal temperature distribution calls for longer processing time to guarantee proper microbial deactivation in the entire mass of the product. Modelling research through computational fluid dynamics (CFD) has shown that temperature gradients and geometry of the product play an important role in lethality distribution in meat retort products (Singh et al., 2017b; Erdogdu et al., 2017; Azar et al., 2020).

F₀ parameter is directly connected with microbial thermal resistance characteristics, such as D-value–the time needed for one log reduction of microbes and z-value that shows the sensitivity of microbes to temperature change. In terms of process design, these three values correlate with each other and define the minimum heat treatment needed to make sure the food is safe from microbial contamination without overheating. D-value defines the necessary time needed for every log reduction at a selected temperature, whereas z-value is needed for modification of this necessary time depending on the change of temperature during the process. The F₀ value incorporates the effects of temperature change during the process in real time and transforms it into an accumulated lethal value. In this way, it becomes possible for processors to be sure that necessary reduction in the quantity of microbes was provided during the process, such as 12D value needed to kill C. botulinum spores in low acid canned foods. Integrating these parameters into predictive modeling allows estimating the kinetics of microbial death more accurately (Singh et al., 2015; Park and Yoon, 2018).

Factors that affect the penetration of heat and lethal dose distribution within the retorted pack include process variables such as come-up time (CUT), packaging style, and formulation of the products (Devadason et al., 2013; Singh et al., 2017a; Jimenez et al., 2023). Any mismanagement of these factors leads to either an under-process resulting in survival of microorganisms or over-process leading to moisture loss, toughening, and oxidative rancidity of the products (Wazir et al., 2021; Jimenez et al., 2023).

Advances in optimization based on model-driven and CFD simulation have greatly improved the efficiency of F₀ application. According to Azar et al. (2020), the use of numerical simulation of conductive heat transfer makes possible the optimization of the schedule of the process while ensuring microbial safety and, thus, making it much more efficient. In a similar way, Singh et al. (2017b) have pointed out the role of modelling of heat transfer in understanding non-homogeneous lethality in particulate foods.

Another technique which is aimed at enhancing the efficiency of the process through its variability in terms of temperature during processing is variable retort temperature profiles. Variable retort temperature profiles and dynamic control strategies guarantee that meat products will be treated thermally in such a way that it is safe from microorganisms but at the same time not overexposed (Chen and Ramaswamy, 2002; Simpson et al., 2008; Fardella et al., 2021).

Consequently, F₀ is crucial for the design of retort sterilization, while new methods made it more effective in meat industry applications.

6.2 Heat transfer behaviour in meat systems subjected to retorting

Heat transfer involved in retorting meat products is governed by several mechanisms, including conduction, natural and forced convection, and transient-state heat diffusion. It depends mainly on intrinsic characteristics of meat systems, including the size of particles, viscosity, fat content, and geometrical shape of the product. Such characteristics define the type of heat transfer either conductive or convective, and also influence the heat penetration speed into the core of the product, thus influencing F₀ value obtained in the process (Singh et al., 2017a; Erdogdu et al., 2017; Park and Yoon, 2018; Azar et al., 2020).

Heat transfer in meat–gravy systems characterized by anisotropy in terms of thermal conductivity and physical structure takes place in a non-uniform way and creates complicated temperature distributions in the course of retort processing. Hence, the application of numerical methods is required for the estimation of thermal behaviour in such systems in which meat products can be considered as heterogeneous porous media (Datta, 2007; Simpson et al., 2008; Singh et al., 2015; Azar et al., 2020).

Further research based on CFD has shown that there are several factors which can greatly impact the heat transfer rates during the retort process, such as the structure of the product, phase distribution, and packaging restrictions. This approach allows one to accurately predict temperature distribution and heat transfer in complex foods (Erdogdu et al., 2017; Singh et al., 2017b; Park and Yoon, 2018; Azar et al., 2020).

Thanks to the implementation of thermal physics laws and boundary conditions, CFD can be considered an effective approach for retort process optimization, especially for those products where heating is conduction-based and cannot be measured directly.

The packaging format is one of the factors that affect heat transfer in retorting operations. Retort pouches, due to their high surface area to volume ratio as opposed to metal cans, have a much lower thermal resistance. This allows pouches to enable fast heat penetration, shortened processing time, and lower required F₀ value while maintaining commercial sterility (Singh et al., 2015; Erdogdu et al., 2017; Park and Yoon, 2018; Azar et al., 2020). The low thermal load experienced by pouches makes them efficient at preserving sensory and nutritional qualities, which makes them very appropriate for meat ready-to-eat products.

The improvements in thermal processing have led to heat transfer control by utilizing model-based optimizations and variable retort temperature profiles. This enables dynamic changes in the heating profile to achieve the necessary lethality without any excessive heat exposure. This becomes especially important for meat systems due to the high susceptibility to protein denaturation, lipid oxidation, and water loss (Chen and Ramaswamy, 2002; Simpson et al., 2008; Singh et al., 2017a; Fardella et al., 2021).

In general, heat transfer in retorted meat products is determined by the structure of the products, their thermophysical characteristics, and the design of their packaging. Combining porous media approaches, computational fluid dynamics, and advanced packaging like retort pouches have improved the possibility of predicting and optimizing thermal performance. The relationships between thermal lethality, heat transfer, package properties, and process optimization technologies are outlined in Figure 3.

All the available researches suggest that the effect of package geometry and product rheology is often more influential in heat transfer than the heat transfer medium itself. Therefore, it is necessary to consider both product packaging and retort operating conditions at the same time. This new approach to the process of design of the retort differs significantly from conventional one, where only the value of F₀ was taken into consideration.

6.3 Packaging–process interactions in retort processing

The choice of packaging material is crucial for the success of the retort process. Multi-layer barrier films like PET/foil/CPP offer strength, oxygen and light barriers as well as satisfactory sealing capabilities during thermal treatment cycles. Enhanced barrier characteristics further allow for better regulation of lipid oxidation and color stability in thermally treated meat products.

Several variables like container upper temperature, package geometry, and composition of the product affect the distribution of heat and lethalities in retorted meat products. Such variables affect the generation of temperature gradient as well as identification of the slowest-heating point in retorted products and consequently the amount of F₀ applied to the product (Singh et al., 2017b; Erdogdu et al., 2017; Azar et al., 2020). Failure to regulate the above-mentioned parameters will result in either inadequate processing that poses safety problems or excessive processing leading to quality deterioration due to moisture, texture and lipid oxidation losses (Domínguez et al., 2019; Wazir et al., 2021).

Proper sealing strength and minimal residual oxygen are essential to avoid issues such as pouch collapse, paneling, or post-process contamination. Additionally, the cooling phase after retort processing must be carefully designed to maintain package integrity and prevent quality losses (Zhang et al., 1998).

6.4 Package failure during retort cooling process

There are many challenges that arise from package failure during the cooling process of the retort process. This issue affects the quality and safety of thermally processed meat products especially for retort flexible pouches. While sterilization occurs during the heating process, the cooling process exposes the package to mechanical and pressure-related stresses that may affect the structural stability of the package. The package failures are caused by vacuum, difference between internal and external pressure, oxygen level, and sealing strength of the package (Han, 2005).

6.5 Vacuum stress and pressure imbalance

In the retorting process, there is an expansion of gases and the product volume due to high temperature and pressure conditions. During the next step which is cooling, there is a sudden decrease in temperature which results in contraction of the product and the gases, thus causing a negative pressure inside the package (vacuum). However, if the pressure inside the package is not effectively managed, or if the package itself does not have enough rigidity, then there may be paneling caused by vacuum. Retort pouches are especially vulnerable due to their multi-layer thin construction and high surface-area-to-volume ratio (Robertson, 2016). The extent of stress caused by the vacuum is highly affected by the type of the gas in the package. A high amount of oxygen and non-condensable gas may increase pressure difference resulting in deformation of the walls and even delamination of the sealant layer (Han, 2005).

6.6 Seal strength and hermetical integrity

The sealing performance of any container is critical since it ensures prevention of microbial contamination and shelf-life stability of the products. Poor sealing may be a result of an insufficient sealing temperature, pressure, duration of sealing, or contamination of the sealing areas. Such defects may result in micro-leaking, leading to microbial contamination post-processing and failure of commercial sterility (Robertson, 2016). It has been demonstrated through several studies that sealing conditions optimization provides greater resistance to internal pressure variations during cooling process. Proper sealing creates uniform stress distribution throughout the package.

6.7 Paneling and deformation of structure

Paneling denotes deformation of pouch surfaces caused by the creation of high vacuums during the cooling process. This defect is frequently encountered in flexible retort packs that are inadequately rigid or have been subjected to faulty cooling procedures. Besides affecting the appearance, paneling may affect the sensory characteristics of products as well. The problem arises due to the imbalance between internal and atmospheric pressures during fast cooling cycles (Robertson, 2016). Paneling is occasionally associated with pouch collapse, which entails permanent deformation caused by the plasticization of polymers due to combined thermal and mechanical stresses.

6.8 Effect of cooling technique on packaging stability

The cooling rate and technique are essential factors influencing the stability of the packaging. The fast cooling process without controlled backpressure may lead to increased formation of vacuum, while slower cooling processes may extend the period of exposure to heat and affect the product quality. Cooling techniques optimized through balancing the issues of microbial safety and mechanical stability via controlling the pressure gradients during cooling have proven their effectiveness at reducing pouch distortion. This kind of techniques is especially significant for meat products due to their high moisture content and complex structure (Han, 2005; Robertson, 2016).

The occurrence of packaging failure in the cooling step of the retort cycle is based on a combination of vacuum stress, oxygen content, sealing strength, and cooling process. Pouch collapse, paneling, and sealing problems can be avoided using effective oxygen control, strong sealing process, and proper cooling methods. Knowledge about the above processes is vital to ensure safety of the meat products.

These results demonstrate that packaging should not be seen as a standalone preservation technique but as a part of the complete system for preserving food thermally. All the aspects including the product formulation, packaging barrier characteristics, retorting process, and environmental factors where the product will be stored together decide the final quality of the shelf stable meat product.

7 Changes in product quality during the retort process

The quality of meat products in retort depends on the interaction of many factors associated with the process, product, packaging, and storage environment. While the prior sections have discussed the technology innovations that make processes efficient, the upcoming section will concentrate on the biochemical aspects of quality degradation and their effect on new retort techniques. Major processing parameters and their influence on product quality attributes for meat products that have undergone retort processing are shown in Table 5.

TABLE 5. Physicochemical changes during retort processing.

The major biochemical reactions responsible for meat quality deterioration due to heat in retort processes include lipid oxidation, protein oxidation, Maillard reaction, collagen denaturation, and pigment degradation, all occurring simultaneously at very high temperatures. During lipid oxidation, hydroperoxides and secondary aldehydes like malonaldehydes are formed; these are responsible for generating the off-flavors and lower nutritional quality. Protein oxidation involves the development of carbonyl content, sulfhydryl destruction, and cross-linking, which results in lowered water retention capacity and higher cooking losses. Concurrently, pigment degradation and alteration caused by heat result in color loss, whereas collagen denaturation affects the connective tissues’ tenderness. These biochemical reactions depend on the process temperature, the time of holding, oxygen concentration, composition of the product, and nature of packaging. As such, the most recent developments in retort technology are focused on reducing thermal exposure. Comparative summary of selected studies on various retorted meat products is shown in Table 6.

TABLE 6. Comparison between effects of retort processing in different meat products.

7.1 Effect of retort processing on colour parameters of the retorted meat products

Color is one of the important quality factors in the retorted meat products because it is an indication of freshness, appearance, and shelf-life. Since the product undergoes high thermal stress in the process of sterilization, changes occur in the physical properties of meat products. Objective measurements of colour through instrumental analysis in terms of L, a, b*, and hue angle give information about changes in visual properties (Rajkumar et al., 2010; Shah et al., 2017; Nalini et al., 2018; Lee and Shin, 2023; Sugiarto and Widiaputri, 2024).

One common trend among many researches is that there is a drastic drop in lightness (L*) due to retorting process. The darkening is caused by non-enzymatic browning (Maillard reaction), protein denaturation, pigment decomposition, and dehydration during sterilization. Such foods as chicken curry mousse (Lee and Shin, 2023), Chettinad goat curry (Rajkumar et al., 2010), and pepper chicken (Nalini et al., 2018) have shown large drops in L* value due to the retorting process and after storage. In the case of retorted Chuncheon dakgalbi, there is a large drop in L* due to long cooking time.

Similarly, yellowness (b*) also tends to be low immediately following retorting. The breakdown of carotenoids, thermal denaturation of chromophores, and lipid-protein interaction lower b* values. Multiple studies have noted a steady decline in yellowness during storage, suggesting that oxidative damage is a factor contributing to color stability (Rajkumar et al., 2010; Lee and Shin, 2023). Nonetheless, there are some formulation-specific instances where the above observation does not hold true. For example, an increase was seen in the b* value of Gulai-Besar during storage, possibly due to browning reaction in coconut milk, a result of thermal caramelization and non-enzymatic browning (Tayeh et al., 2019).

The a* value and hue angles display maximum variability since these are highly influenced by the formulation and chemical characteristics of pigments. Certain products exhibit increased a* values temporarily right after processing, especially those with high carotenoid content. Pigments like lycopene and capsanthin may be extracted from plants through heat treatment, resulting in a temporary increase in the a* value (Pathak and Sagar, 2023). Nevertheless, this phenomenon lasts for only a brief period. Ambient storage causes a decrease in redness as a result of the oxidation of heat-sensitive pigments, such as carotenoids and other plant pigments used as food coloring. This trend is highly evident in Indian curry-based products like pepper chicken and Rogan josh.

Additional mechanisms have been studied using retorted Rembiga Satay; in which higher sterilization intensity led to an increase in the degradation rate of capsanthin, resulting in decreased hue value and deepening of the red colour (Ariyana et al., 2023). On the other hand, the red colour of Gulai-Besar was maintained, despite a relatively high total colour difference (ΔE), during accelerated storage at 55 °C. It is likely due to the presence of fat content in the curry matrix, which protects the pigments by preventing oxidative degradation and heat destruction (Tayeh et al., 2019). The effect of temperature on the color stability is another important factor. Thus, higher temperature (25 °C) resulted in a faster development of lightness and faster color differences when compared to storage at 4 °C in Chuncheon dakgalbi (Muhlisin et al., 2013).

Further research also indicates the presence of phase-dependent color changes in composite meat and gravy systems. Sugiarto and Widiaputri (2024) reported differences in color changes between meat and broth in the case of retorted mutton tongseng for 3 months. Meat showed increased values of ΔE caused by the transfer of colorants in the broth, leading to a darker color. On the other hand, broth showed decreased values of ΔE due to the transfer of colorants such as spices and Maillard reaction intermediates into the meat phase. This transfer of pigment is similar to that of pepper chicken and goat curry (Rajkumar et al., 2010; Nalini et al., 2018).

These results indicate that color change in retorted meat-based products is not static but rather depends on several interrelated factors such as pathways of pigment degradation, Maillard reactions, lipid oxidation, degree of heat treatment, composition of ingredients, permeability of tissues, and storage temperature. In addition, interactions between meat matrix and the surrounding gravy may affect color stability via pigment diffusion and structural changes in matrix. Understanding these processes is vital for establishing optimal process conditions and ingredient formulations. Improvement of pigment stability, prevention of oxidative degradation, and proper combination of ingredients may reduce undesired color changes in RTE retorted food products. Major changes in quality that take place during storage of retort-processed meat products are highlighted in Table 7.

TABLE 7. Quality changes during storage.

7.2 pH analysis

pH, which refers to the logarithmic expression of the concentration of hydrogen ions, is an important property that affects acidity, microbial stability, and overall quality of food systems. For composite foods like meat curry, measurement of pH is conducted for the meat component and the gravy component separately for comprehensive assessment of the stability of the entire system (Shah et al., 2017; Sugiarto and Widiaputri, 2024). Sterilization with thermal processing changes the pH value due to chemical reactions caused by heating, whereby high values of F₀ or retort cooking for long time causes an increase in pH of meat and gravy. The elevation in pH value is because of the thermal denaturation of proteins and Maillard reactions between amino groups and reducing sugars, resulting in alkaline products (Muhlisin et al., 2013).

Most of the curry-based retort pouch foods possess gravy having a lower pH than the meat. This phenomenon is attributed to the inclusion of chili filtrate and spices that contain organic acid and heat-stable phenols, which get dissolved in the aqueous solution. These acidic elements tend to dissolve easily in the gravy rather than the meat during cooking, resulting in the formation of a pH gradient in the pouch (Shah et al., 2017). The same trends have also been observed for other ethnic foods such as Rembiga satay, where pH is found to reduce with increasing sterilization severity owing to the solubilization of acidic chilies and proteins (Sugiarto and Widiaputri, 2024).

One consistent pattern observed in post-processing of retorted meat products is a decrease in pH over time. This phenomenon has been reported in buffalo meat blocks (Devadason et al., 2013), Chettinad goat curry (Rajkumar et al., 2010), and several poultry-based products. High storage temperature is known to have a pronounced effect on pH reduction. Pepper chicken (Nalini et al., 2018) and Gulai-Besar (Tayeh et al., 2019) exhibit more pronounced changes under elevated temperatures (45 °C–55 °C) because of protein, lipid, and other macromolecules degradation.

Several factors account for pH reduction in retorted meat products during storage. Protein hydrolysis results in free amino acids and peptide formation and subsequent decrease in pH level (Devadason et al., 2013; Shah et al., 2017; Girish et al., 2018). Residual oxygen in the pouch leads to lipid oxidation and formation of acidic peroxides and aldehydes. Also, carbohydrate and other heat-labile compounds degrade into acidic intermediates, which was seen in chicken curry mousse, when pH drop occurred right after sterilization as a result of hydrogen ion production, Maillard reactions, and protein degradation (Lee and Shin, 2023).

Overall, the gradual increase in acidity levels within the retorted foods is due to the cumulative effect of protein hydrolysis, lipid oxidation, and browning reactions. Although the development of acidity plays an important role in microbial stabilization, it could affect the sensory qualities of the food products. Hence, regular monitoring of the pH value is very necessary during storage to estimate the shelf life of foods (Muhlisin et al., 2013; Shah et al., 2017; Tayeh et al., 2019; Sugiarto and Widiaputri, 2024).

7.3 Lipid oxidation

The primary reason for fat deterioration in retort-processed meats is due to oxidation of lipids and hydrolysis of triglycerides by heat. The high temperatures utilized for sterilization in the retort process make the lipids prone to oxidative deterioration, leading to formation of hydroperoxides, aldehydes, and other volatile substances, which cause undesirable changes in flavor and shelf-life. At the same time, thermal hydrolysis causes liberation of free fatty acids (FFA) that not only signal fat deterioration but also increase the oxidative potential of fats. Both these processes act as indicators of the poor quality of retort-processed meats stored at room temperature.

Increased levels of FFA were observed in retort-processed Rogan josh with increasing intensity of the thermal processing and throughout 12 months of storage. The higher level of FFA in the gravy was consistent throughout the period, probably owing to its relatively higher content of fats. Increased levels of FFA have been reported in retort-processed fish peera and Chettinad chicken (Bindu et al., 2010; Rajan et al., 2014; Shah et al., 2017).

The intensity of processing is important in promoting hydrolysis and oxidation of lipids. Processing for longer time periods (10–30 min) significantly increased the rate of lipid oxidation at the beginning of the process in retort-packaged Chuncheon dakgalbi. The progress of oxidation was noted during storage at 4 °C and 25 °C for 1–3 weeks; however, the further progress was registered in the 10-min processed samples stored at 25 °C only (Muhlisin et al., 2013). Such thermal effects are explained by the disruption of membranes and inactivation of the enzymes responsible for antioxidation (Ahn and Min, 2005). The increase in storage temperature promotes degradation via hydrolysis and oxidation. For example, in Gulai-Besar retort-processed product the content of FFA (palmitic acid) increased from 3.40% to 8.14% during 6 months under accelerated conditions of storage at 55 °C ± 2 °C, while at normal conditions of storage (at 30 °C ± 2 °C), the content of FFA made 5.08%. At the same time, peroxide value (PV) increased similarly to FFAs; this tendency was more pronounced in the samples containing coconut milk, which is a lipid substrate having high sensitivity to oxidation (Tayeh et al., 2019).

The correlation between lipid hydrolysis and oxidation can be seen when storing food at freezing temperature. For example, in the case of frozen sardines stored for 24 months at −18 °C, FFA increased from 1.1 to 11.8 g/100 g lipids and TBA index from 0.5 to 29.9 mg MDA/kg. At −10 °C, both parameters changed more rapidly, showing strong positive correlation between FFA and TBA index and, therefore, concomitant progression of hydrolysis and oxidation during long-term freezing (Tayeh et al., 2019). Retort-pouched rings of squid in curry showed significant increase of FFA after 90 days of storage because of the continuous lipid hydrolysis despite good sensory attributes. While PV was not analyzed, the TVBN and TMA slightly increased and, thus, gradual chemical changes occurred. Despite the high fat content (4.38%), which was a result of coconut inclusion in formulation, the retorting provided the sensory acceptability of the product (Dileep et al., 2012).

For Rogan josh, high sterilization intensities (F₀ 7–11 min) led to increased levels of TBARS right after preparation and during 12 months of storage (Shah et al., 2017). Linear increases in TBARS (0.21–0.52 mg MDA/kg) during storage were recorded in case of retort-processed pork curry (Girish et al., 2018). Also, the TBARS in chicken curry increased during the first 15 days and then decreased up to 90 days due to migration of secondary oxidation products into the gravy or interaction with amino compounds (Nalini et al., 2021). The effects of processing intensity, storage temperature, and oxygen in packaging on the results of oxidative processes should be mentioned. Despite the possibility of increasing TBARS in case of retort processing in the presence of non-vacuum packaging, their concentration still stays within the acceptable sensory limits (0.5–1.0 mg MDA/kg) (Girish et al., 2018).

7.4 Proximate composition analysis

The proximate composition of meat products—including moisture, protein, fat, and ash content—serves as a fundamental indicator of nutritional quality and product integrity. The retorting process, involving high heat treatment in presence of pressure, might cause considerable modification of the compositional factors due to the reasons of thermal degradation, loss of moisture, and reactions with packaging material. Knowledge of these alterations is required in order to maintain desirable nutritional and physical characteristics of the processed meat products.

7.4.1 Changes in moisture content of retorted meat products

One of the most common changes in retorted meat products is the decrease in moisture content, caused mainly by the effect of high temperatures and protein denaturation, which results in the loss of bound water. The decrease in moisture has been observed in various products: in pork curry, moisture has been decreased from 75.98% in raw meat to 67.89% after retorting (Girish et al., 2018), and in squid rings–after blanching and thermal treatment (Dileep et al., 2012). Similar observations have been made in beef curry (Gulai-Besar); the decrease of moisture is due to the universal effect of thermal processing of muscle proteins (Girish et al., 2018; Tayeh et al., 2019). In case of complex dishes, there may be differences in the distribution of moisture; thus, in Rogan josh, there was more moisture in gravy than in meat chunks (Shah et al., 2017): 82.1 g/100 g vs. 64.2 g/100 g respectively.

7.4.2 Modifications of protein content in retorted meat products

The protein content in meat products that have undergone retorting is typically high in a wet weight basis, mainly owing to dehydration that occurs during the process of thermal sterilization. This modification is evident from the protein increase by up to 16.74%–24.75% in pork curry (Girish et al., 2018) and increase of protein content from 15.75% to 17.50% in squid rings in curry (Dileep et al., 2012). It is also evident in the case of the high protein content in meat-based composite products, for instance, the meat chunk of Rogan josh having a content of 28.5 g/100 g compared to the protein in its gravy that was 4.7 g/100 g (Shah et al., 2017). However, trends in protein content storage were found to differ in different meat products, whereby Chettinad chicken maintained steady protein content during storage (Rajan et al., 2014), whereas the protein content of Gulai-Besar declined (Tayeh et al., 2019).

7.4.3 Variation in fat content of retorted meat products

The fat content of retorted meat products may vary greatly based on formulation and behaviour of lipids during thermal treatment. The addition of oils or oily substances will increase fat content in the product, as was shown for the gravy of Rogan josh (9.6 g/100 g) and meat chunks (3.3 g/100 g) (Shah et al., 2017), and for the squid rings in curry, when additional coconut increased total fat content (Dileep et al., 2012). On the contrary, heat-induced transfer of lipids from meat to gravy will lower fat content in solid part, which was shown in the case of pork curry, when fat content in meat part lowered from 6.69% to 4.26% after retorting (Girish et al., 2018). These variations highlight the interaction between recipe.

7.4.4 Ash and mineral content

The total mineral contents, as reflected by ash contents, normally become elevated in retort-produced meat items due to the inclusion of various salts and spices. In pork curry, there was a substantial rise in ash content, which went up from 0.28% in fresh meat to 3.08% in retorted form (Girish et al., 2018). In the case of Rogan josh, there was a higher ash content in the gravy (2.8 g/100 g) compared to the meat content (2.1 g/100 g), which could be explained by the fact that more minerals are present in the liquid part (Shah et al., 2017). The use of coconut milk and spices in the preparation of Gulai-Besar also helped increase the ash content of the food during storage (Tayeh et al., 2019).

7.5 Storage stability of proximate components

Compositional stability in the course of ambient storage of the products treated via the retort process is diverse and is highly dependent on the formulation and thermal load. For example, Chettinad chicken had high compositional stability, which is characterized by the absence of changes in such parameters as moisture, protein, fat, and ash after storage for 6 months (Rajan et al., 2014). At the same time, Gulai-Besar had changes in the direction of decreasing moisture and protein contents and increasing fat and ash (Tayeh et al., 2019). It should be mentioned that even processing parameters sometimes do not change the compositions; thus, when the processing time of Chuncheon dakgalbi increased from 10 to 30 min, there were no significant differences in the proximate composition (Muhlisin et al., 2013).

7.6 Texture profile analysis

The texture profile analysis technique is an important instrumental technique used in measuring the mechanical properties like hardness, cohesiveness, springiness, and chewiness, which are very essential in evaluating the sensory characteristics of the retorted meat products (Rajan et al., 2014; Girish et al., 2018). Retorting exposes the products to harsh temperatures, which change the structural organization of the meat by denaturing myofibrillar proteins, dissolution of collagen into gelatin and thermal aggregation of muscle fibers (Girish et al., 2018; Nalini et al., 2021). The structural changes that take place often make the meat firmer or tough and, therefore, less tender (Jang and Lee, 2012; Girish et al., 2018).

Considering all the above-discussed impacts, it becomes clear that textural profiling is essential for product quality control, process optimization, and predicting the stability during storage (Shah et al., 2017; Girish et al., 2018). However, even despite the common trend of increasing textural properties under heating, the extent of changes still strongly depends on the type of products and formulations used, as well as the intensity of processing and conditions of storage (Jang and Lee, 2012; Rajan et al., 2014). So, for instance, retorted pork curry demonstrated significant softening effect during storage; hardness was lowered from 61.91 N to 4.92 N, while cohesiveness was decreased from 0.49 to 0.27 (Girish et al., 2018). Similar changes in hardness, cohesiveness and chewiness were observed in Chettinad goat meat curry as well (Rajkumar et al., 2010).

There were no substantial changes in the texture profile of retorted Chettinad chicken over a period of 6 months storage, implying that the product matrix had structural stability (Rajan et al., 2014). But textural changes in retorted products are not consistent and can develop slowly over time in the course of storage, depending on formulations and matrices (Jang and Lee, 2012; Dileep et al., 2012; Muhlisin et al., 2013). For instance, Ginseng chicken porridge was characterized by an increase in hardness from 1.6 N to 2.7 N within 9 weeks of storage at 25 °C, which is mainly attributed to starch retrogradation in the rice portion but not due to meat (Jang and Lee, 2012). Therefore, in retorted composite products, changes in texture could be influenced by other ingredients apart from the meat.

On the other hand, the use of gravy/sauce element is useful in overcoming negative changes in texture because it serves as a protective layer that prevents dehydration of the surface and improves lubrication for mastication. Such effect improves the perception of tenderness and juiciness despite increased hardness, as shown in the case of pork curry and squid rings retorted in curry medium (Dileep et al., 2012; Girish et al., 2018). Generally, changes in texture due to retort cooking of meat products are due to protein denaturation, formulation, and storage (Jang and Lee, 2012; Rajan et al., 2014; Girish et al., 2018). Thus, successful product development requires proper consideration of ingredient interactions and heating process.

In general, previous literature reveals that loss in terms of color, texture, lipid stability, and nutritive value is regulated not just through sterilization but also through interactions between various packaging factors, formulations, and processing designs along with storage environments. Although new techniques like MATS and VRTP have been found to be relatively better in terms of quality attribute retention than the traditional static retorts, the extent of improvement depends on the nature of food ingredients and the geometry of the packages used. Thus, further researches should focus on comparative studies under similar conditions.

8 Microbiological safety, commercial sterility and shelf-life extension

Retorting is essentially a microbiology-based food preservation process, wherein the main focus of the procedure is not only on stabilization but the attainment of commercial sterility in low-acid foods. The significance of this process for meat products is due to the fact that because of their high water content and near neutral pH value along with their nutrient richness, meat products provide suitable conditions for growth and survival of spoilage and pathogenic microorganisms in case the process is not conducted properly. It is through this inactivation process of vegetative cells and heat resistant spores like C. botulinum that retorted meat products get their shelf stability.

8.1 Commercial sterility and microbiological safety of retort-processed meat products

The primary objective of retort processing is the attainment of commercial sterility, defined as the absence of viable microorganisms—including heat-resistant spores—capable of proliferating under normal, non-refrigerated storage conditions (FDA, 2022). This degree of microbiological safety is required in order to provide a product with good shelf life that remains safe for consumption even after prolonged storage at ambient conditions (Girish et al., 2018; FDA, 2022). A large amount of scientific research proves that the achievement of commercial sterility is possible if thermal treatment of meat is carried out appropriately and the wide variety of meat products could be produced.

Research demonstrates that retort treatment is an efficient method for the reduction of microflora. During storage for 12 months in retort pouches, there were no microorganisms present in Rogan josh due to the effective elimination of the microflora by heating and high barrier properties of the packaging system (Shah et al., 2017). Likewise, Chettinad chicken had undetectable levels of total plate count and no pathogens (Escherichia coli, Salmonella spp., Clostridium spp., and Staphylococcus spp.) were detected after 6 months of storage at ambient conditions (Rajan et al., 2014). Research on pork curry and beef curry “Gulai-Besar” showed that there were no pathogens and no spoilage organisms detected in the food for 9 months.

In summary, these results have shown that a standardized and well-formulated retort procedure guarantees commercial sterility in different types of meat products, regardless of differences in formulations and compositions. Retorting is, therefore, a very reliable method of production of microbiologically safe meat products. Commercial sterility in retort-processed meat products can only be guaranteed through precise control of heat inactivation of both vegetative cells and bacterial spores.

This requirement is demonstrated by the evidence from Chuncheon dakgalbi as shown below. The 10-min heat treatment at 110 °C could effectively destroy vegetative microorganisms but was unable to inhibit spore germination at ambient storage condition. The microbial population of this improperly treated sample would increase up to 6.4–7.7 log CFU/g in 3 weeks at ambient condition while there were no any spoilages for the samples treated with 20 min or 30 min at 4 °C (Muhlisin et al., 2013). Therefore, it can be seen that sufficiency of thermal process cannot be considered separately. It is because sufficiency of heat treatment is intrinsically related to storage temperature.

Where process design is concerned, microbiological safety of retort processes is related to the balance among the lethality due to heating (F₀), the thermal resistance of the microorganisms (D- and z-values), and the heat transfer properties of the food matrix. Thus, it is essential that the region which heats up slowly (cold spot) of the meat gets sufficient heat processing to get rid of the spores and other microorganisms. This is particularly true for heterogeneous products such as curries, stews, gravies, and particulate RTE products since variations in viscosity, size of the particles, and pack configuration result in temperature variations which can lead to under-processing in some parts.

The microbial safety of retorted meat products is evaluated using universally accepted standards and procedures. The analysis includes total viable count tests, detection of microorganisms, and testing of commercial sterility according to the standard methods from American Public Health Association (APHA), International Commission on Microbiological Specifications for Foods (ICMSF), and the Bacteriological Analytical Manual by the United States Food and Drug Administration (FDA) (Rajkumar et al., 2010; Tayeh et al., 2019). As per these standards, the food item is deemed commercially sterile if there is no microbial growth in validated tests and the lack of pathogenic bacteria in particular sample sizes (Nalini et al., 2018; Girish et al., 2018). It must be noted that regulatory standards call for the total absence of Salmonella in 25 g samples, and the total viable count should not exceed 3.0 log₁₀ CFU/g.

Repeatedly, it has been found that meat products prepared through the process of retorting have met these highly stringent standards for several meat product formulations, including Rogan josh, Chettinad chicken, pork curry, and Gulai -Besar, wherein no pathogenic bacteria or microorganisms responsible for spoilage were found during their prolonged period of storage (Rajan et al., 2014; Shah et al., 2017; Nalini et al., 2018; Girish et al., 2018; Tayeh et al., 2019). All these results clearly indicate that retorting can be safely used to ensure the safety of meat products.

8.2 Microbial safety as the foundation for shelf-life extension in retort-processed meat

Shelf-life extension in retort products is ultimately based on microbial stabilization. In traditional chilled meats, shelf-life limitations are essentially determined by bacterial, yeast, and mold spoilage organisms, along with the potential presence of pathogens. Retort processing greatly reduces the number of microorganisms and kills spores of significance to public health, thus enabling products to have safe storage periods of several months under ambient conditions. Thus, while physicochemical factors like reduction in pH, color changes, lipid oxidation, or changes in texture may affect product acceptability, the key to extended shelf-life stability is ultimately commercial sterilization to prevent microbial growth.

Nevertheless, the shelf-life of such packages is not dependent solely on the lethality factor; it is also influenced by the combination of retort lethality and package integrity and handling. Once the product has been treated with heat, the preservation of sterility in storage can only be ensured if there is proper hermetical packaging, very little chance for post-processing contamination, and effective barrier protection from oxygen and moisture ingress. In that way, retort pouches and high barrier laminates ensure the microbiological stability of the product not just through efficient heat penetration during processing, but also through preventing any recontamination and ensuring a sterile environment in storage.

There is an abundant evidence from the reviewed literature which highlights the presence of a correlation between commercial sterility and shelf life elongation. Such food products as Rogan josh, Chettinad chicken, pork curry, and Gulai-Besar retained their microbiological stability during 6–12 months of ambient storage due to adequate retorting and packing practices (Rajan et al., 2014; Shah et al., 2017; Girish et al., 2018; Tayeh et al., 2019). In the case of improper thermal treatment (e.g., underprocessed Chuncheon dakgalbi), microbial growth took place in the course of storing the product even after the application of heat treatment (Muhlisin et al., 2013). The results obtained prove that shelf life elongation in retorted meats is impossible without process validation, when the required lethality is attained and packaging integrity is retained.

On the whole, retorting enhances the shelf life of meat products mainly through the attainment of commercial sterility, inhibition of microbial spoilage, and inhibition of survival and growth of any pathogens and spores under ambient conditions. Hence, although parameters such as color, texture, pH, lipids oxidation, and sensory parameters are key considerations in assessing product acceptability, the principle of microbial sterilization through retorting forms the core scientific principle behind the process of enhancing shelf life of meat products.

9 Sensory evaluation

Sensory evaluation is an integral component of quality assessment for retort-processed meat products, providing structured insights into consumer acceptability based on key attributes such as appearance, colour, flavour, aroma, texture, juiciness, tenderness, and overall acceptability. The standardised protocols involve the use of panels of trained or semi-trained judges, controlled conditions of service and objective presentation of samples in code-labelled containers, where responses are usually quantified by means of the 9-point hedonic scale (Rajkumar et al., 2010; Dileep et al., 2012; Muhlisin et al., 2013; Shah et al., 2017; Girish et al., 2018; Nalini et al., 2018; Tayeh et al., 2019; Melro et al., 2020; Lee and Shin, 2023).

In several studies, there is a consistent decrease in sensory values during prolonged ambient storage, with the largest drop being seen in the parameters of appearance, flavour, juiciness and tenderness (Shah et al., 2017; Nalini et al., 2018; Tayeh et al., 2019). Such pattern was found for different types of retorted foods like Rogan josh, Pepper chicken, meat blocks of buffalo meat, Chettinad chicken and fish peera (Bindu et al., 2010; Devadason et al., 2013; Rajan et al., 2014; Shah et al., 2017; Nalini et al., 2018). Such change in sensory characteristics is mainly due to biochemical processes of protein degradation and lipid oxidation resulting in decreased flavour and texture of foods (Shah et al., 2017; Tayeh et al., 2019).

It is notable that the extent to which sensory degradation occurs among retorted meat is dependent on temperature, with food stored in accelerated conditions displaying much more rapid sensory degradation compared to food held under ambient or refrigerated conditions. For instance, in the case of a traditional beef curry (Gulai-Besar), there was significant decrease in the sensory ratings of the food after 9 months when it was stored at 55 °C. On the other hand, foods kept at lower temperatures maintained relatively high levels of acceptability (Tayeh et al., 2019; Melro et al., 2020). This illustrates how high temperatures accelerate chemical processes such as lipid oxidation and protein degradation. Therefore, the temperature at which foods are stored should be considered very important in determining their sensory shelf-life (Ahn and Min, 2005; Sinha et al., 2015; Tayeh et al., 2019).

The formulation of the product, especially the use of a sauce containing moisture, is a critical factor influencing the ability to maintain sensory properties during storage. Research on pork curry and Chettinad chicken indicates no loss of important sensory properties such as taste, juiciness, and overall acceptability during storage due to the protective action of the sauce, preventing dehydration and delaying oxidation (Rajan et al., 2014; Girish et al., 2018). Conversely, products not having any sauce matrix like buffalo meat blocks displayed significant losses in sensory qualities of taste, juiciness, texture, and overall acceptability during storage up to 90 days (Devadason et al., 2013). Such contrasting results indicate the critical importance of product formulation and structure regarding sensory stability, and hence standardized formulation is an effective approach to improve the shelf life and sensory stability of retort meat products.

In multi-component retort foods, it is quite possible that the sensory characteristics may not be determined by the meat components but rather by the non-meat components. For instance, in ginseng chicken porridge, the textural changes experienced during the storage period were mainly as a result of rice starch retrogradation and not changes in the chicken components (Jang and Lee, 2012). Such results indicate that the quality and stability of multi-component retort foods are greatly influenced by the individual non-meat components, making it necessary to have knowledge and control of all individual components.

10 Modern advancements in retort technology and future directions

New technological innovations in retort operations have been mainly developed to solve the problems related to traditional static retorting such as increased processing time, inadequate heat transfer, increased thermal injury and poor physiochemical and sensory properties of ready-to-eat meat products. With regard to the current review, the phrase “modern approaches” is used to indicate technological innovations which ensure not only commercial sterility but also quality improvement by enhancing the retort design, thermal scheduling, process monitoring, predictive modeling, hybrid heating processes, and retort packaging. The modern retort approaches included in the current paper are variable retort temperature operation, agitated and rotary retort, computational fluid dynamic supported process optimization, real-time thermal monitoring and digital control, high barrier retort packaging and new sterilization processes such as microwave assisted thermal sterilization and ohmic heating.

Modern advancements in retort technology have focused on achieving more precise thermal control, improving energy efficiency, and enhancing product quality during sterilization. Among others are varied developments such as the development of variable temperature retorting, automated steam-air retorts with accurate over-pressure control, water immersion and spray methods for improvement of heat distribution, agitation/rotating retorting for faster heat distribution, and even using Computational Fluid Dynamics (CFD) to model temperature distribution, cold spots and lethalities in food products. Some of the notable developments include the automated steam–air retorts with accurate over-pressure control and water immersion and water spray methods that help in heat transfer effectively while saving on energy. F₀/Fh real time monitoring using wireless data loggers has made process validation more effective, and the development of high barrier retortable and sustainable packaging films has helped to protect products for long periods.

These technologies mentioned in the ensuing sections cannot be considered as modifications of traditional retort systems directly. On the contrary, they are considered to be supplementary or alternative container-based thermal sterilization techniques designed to overcome the drawbacks of the traditional methods of retort technology especially high levels of heat processing and uneven heat distribution. Even though there are different heating modes, the main idea behind these systems is to achieve commercially sterile shelf stable meat products. Therefore, these techniques are included in this review to give an overview of recent developments in thermal preservation of meat products.

In addition to developments in process engineering and machinery, the present day retorting systems are also equipped with new types of thermal processes such as MATS, ohmic heating, and intelligent retorts with sensor-based process control. These innovations have been created in order to ensure shorter processing time, greater heating uniformity, reduced over-processing risks, and improved retention of quality-sensitive attributes in comparison with static retorting process. In addition, innovations in packaging technologies such as high barrier multilayer retort pouches, semi rigid retort trays, oxygen scavengers, and recyclable retort polymer containers are important innovations in the field of retorting.

Taken together, all these technological innovations try to minimize the quality degradation caused by thermal sterilization without compromising on the commercial sterility, which becomes the core of successful meat retorting. Thus, the modern innovations discussed above may be grouped into six major categories: (i) the innovative design of retorts and over-pressure control; (ii) retort temperature regulation and thermal processing scheduling; (iii) agitation and rotating retorts; (iv) CFD modeling and computer-controlled retort technology optimization; (v) combined thermal processing technologies such as MATS and ohmic heating; and (vi) advanced packaging systems for retorted products. Based on the results of the current literature review, it is evident that the quality of retorted meat products depends on the interplay between processing temperature, packaging efficacy, product formulation and storage conditions.

11 Key recommendations

The key measures for achieving quality enhancement and improved stability in retorted meat products include optimization of F₀ value, which is generally 8–12 min for thick viscous meat curries to ensure optimum levels of safety along with retaining the sensory attributes, and employing the technique of variable retort temperature technology (VRTT) for reducing heat destruction and sterility maintenance. The employment of high oxygen-barrier retort pouches ensures better color stability and inhibition of oxidation processes, whereas addition of gravies in the formulation helps maintain juiciness through moisture retention. Further quality enhancement measures may include reduction of headspace oxygen content along with adding natural antioxidants to limit lipid oxidation. Finally, maintaining appropriate storage temperatures is critical, as products stored under ambient or moderate conditions retain superior quality attributes compared with those exposed to accelerated or elevated temperatures.

Taking into account the technical aspect of the issue discussed in this review, it can be stated that contemporary retort optimization has to include the selection of the retort system and thermal schedule depending on the rheology of the product, the shape of the package, and quality goals. The use of variable retort temperatures, agitation retorts, and CFD validation can become a solution for conduction-heated and/or viscous meat products in order to decrease process severity and provide adequate lethality in the cold spot. Moreover, the implementation of new retortable packages with improved oxygen and water barrier has to be taken into account as one of the necessary process elements. Generally speaking, the most effective contemporary approach includes not one intervention, but optimization of multiple aspects of the process.

12 Industrial feasibilities, future perspectives and research needs

Although some of the new technologies have shown a lot of potential in terms of quality preservation, their industrial application is different from one another. The traditional static retorts and agitated retorts continue to be the most prevalent commercial technologies because of their dependability, regulation approval, and cost-effectiveness. However, the commercialization of the MATS technology, ohmic heating, and artificial intelligence-based smart retorts is still in its infancy because of their high capital costs, packaging limitations, and validation difficulties. Consequently, future work should focus on industrial validation, sustainable packaging for the retorting process, digital twin, and artificial intelligence-based optimization of the process.

Future research should focus on optimizing F₀ values, advancing VRT technology, improving oxygen-barrier packaging, and retaining gravy-based formulations to enhance stability. The combination of retort technology with either the MATS system, ohmic heating, or the sous-vide method shows promise in achieving faster and more efficient heating processes but needs some answers regarding engineering issues, compatibility with packaging systems, and sustainability aspects. More knowledge about heat-matrix-packaging interactions is necessary using metabolomics and lipidomics. Finally, improved lethality validation, real-time monitoring, and smart packaging are essential to ensure safety while advancing quality and sustainability.

In addition, future work needs to provide more definitive guidelines on the choice of modern retort technologies based on the nature of food products, type of packaging, and desired shelf life. Comparative studies of static, agitated, and combined retort operations under equal lethalities will be especially useful in order to understand which methods can be used more efficiently in case of meat products of particular categories. Finally, further research efforts need to focus on integrating the modeling of microbiological safety, shelf-life, packaging performance, and quality into a coherent decision support system for retort process optimization.

13 Conclusion

Retorting technology still continues to serve as the backbone for thermal processing of low acid meat foods on an industrial scale. The information provided in this article shows that apart from advancements made in traditional retort technology, new methods have also been developed in parallel with the traditional one and provide for an alternative form of thermal sterilization of food products within the container in order to eliminate the drawbacks associated with traditional retorting processes. But the facts that have been presented in this article prove that traditional retorting is often responsible for deterioration of quality due to such factors as lipid and protein oxidation, color change, pH alteration, moisture loss, hardening of the texture, and sensory deterioration. The product composition also affects this deterioration, and the meat products based on gravy tend to be more resistant to sensory deterioration due to higher moisture retention and less thermal damage. Modern retort developments discussed in this review have increasingly been concerned about limiting the negative impacts on food quality while ensuring food safety from the microbiological point of view. Variable retort temperature processing, agitation in retorts, computational fluid dynamics-supported process optimization, digital lethality measurements, advanced retortable packaging, and hybrid technologies such as MATS, ohmic heating, and sous-vide–retort combination are some of the latest trends in retort development. They result in higher uniformity in heat transfer, lower overheating of foods, and better retention of colour, texture, taste, and nutritional values. While on the one hand, the literature review shows that there is not one single technology capable of solving all the problems associated with retort food processing, the most optimal solution for preserving meat through retort processing in the future would be an integration approach, which would include efficient F₀ value delivery, retort system choice, appropriate formulation of the product, oxygen and moisture barrier packaging, and proper storage. The future of research into such issues as mechanisms of thermal oxidation and protein denaturation, real-time process validation and smart monitoring of lethality of thermal processing, optimization of packaging to advanced thermal technology, and better correlation between instrumental indices of product quality and consumer perception is therefore highly desirable. In conclusion, future developments in retort technology, package development, and process control are necessary to produce quality shelf-stable meat products.

Correction note

This article has been corrected with minor changes. These changes do not impact the scientific content of the article.

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Footnotes

Edited by: Miguel Elias, University of Evora, Portugal

Reviewed by: Evans Frimpong Boateng, University of Energy and Natural Resources, Ghana

Ajay Singh, Mata Gujri College, India

Maria Inês Rouxinol, University of Evora, Portugal