Prepared meals, consisting of various ingredients impacting the shelf life, represent a very complex topic. Amongst the essential issues regarding ready meals, the possibility of microbiological contamination during the technological preparation phase, storage, and insufficient thermal regeneration are included. For example, prepared foods (i.e., meals), have undergone all the necessary technological processes during the manufacturing process, including preservation. They have a specific shelf life and are ready for immediate consumption or regeneration before eating [28].

According to Leistner et al. [29], the shelf life of most foods is a result of the combined effects of many demanding parameters. Each of them is an inhibiting factor for the development of the microbial association and native enzymatic backgrounds but insufficient for visible effects. Therefore, as many barriers as possible are needed if the product is to be microbiologically stable for the so-called "barrier technology".

It is crucial to determine the conditions and requirements for the shelf life and to meet them precisely. There are essential differences between the stability under ideal conditions (laboratory) and typical storage in restaurants. Concerning the latter, it is challenging to define shelf life limits because freezer temperatures during a restaurant’s operational business processes can change frequently. According to Stražiščar et al. [30], the Hazzard Analysis Critical Control Point system allows such deviations in storage temperature for a short time. Therefore, a link between specific microbiological requirements and the simultaneous monitoring of sensory properties (e.g., appearance, taste, smell, and texture) is necessary to determine the shelf life [31]. It is most effective to monitor both parameters (temperature and shelf life) for ready-to-eat and frozen food produced solely in the restaurant at specific time intervals to better determine the tenacity over time. Protocols must be established for these processes to ensure safety and quality. However, the practice in the restaurant business is complicated by several factors, such as peak hours and the difficulty of maintaining a constant temperature in the freezer. Therefore, this study is important for gaining accurate insight into possible consumption improvements in restaurant business processes.

The production of ready meals includes the following phases: (1) pre-preparation of raw materials, (2) preparation (heat treatment process), (3) packaging, (4) preservation, (5) storage, and (6) regeneration or heating. Restaurant pre- and preparation processes are, in principle, the same for all categories of ready meals. The following technological processes, such as packaging, preservation, and storage, are specific to each dish. Depending on the distribution, preservation, and storage type, ready meals are divided into hot, chilled, pasteurised, sterilised, frozen, and dehydrated [28].

Frozen ready meals can be defined in technical terms as prepared culinary products stabilised in a new state by the freezing process (T is lower than −40 °C) and stored at a temperature lower than −18 °C after maximum crystallisation is reached. In this temperature range, the growth of microorganisms is stopped, and quality changes are due to the actions of enzymes. Today, the restaurant business offers a diverse range of frozen ready meals, as with few exceptions, practically everything is prepared in regular culinary practice. A wide range of multi-component dishes is made possible by the process of preservation by freezing (below −18 °C), which practically does not change the sensory or nutritional quality of the food. Freezing is an effective means for the preservation of the colour and flavour of the dish. However, great care must be taken when preparing combined dishes (e.g., sauce, meat, vegetables, and starchy foods) if the raw materials are prepared differently (e.g., fresh or heat-treated).

Frozen ready meals must also be hygienic. The effects of freezing on microorganisms depend on the following: the type and number of microorganisms, the degree and speed of freezing, the nutrient composition, the pH level, the presence of cryophilic agents, the temperature and storage time, and the speed and method of thawing.

The most significant reduction in the microbial population occurs during freezing itself or immediately after freezing. Gram-positive bacteria are more resistant to freezing and thawing than Gram-negative bacteria, and bacterial spores are the most resistant. The cells show the most significant resistance at the stationary phase, while young cells are most susceptible to freezing and thawing. Microorganisms grow more intensively at temperatures above −8 °C. Many pathogenic microorganisms are significantly reduced during storage, but this should never substitute good hygiene during production [30]. Some enzymes of microorganisms are active even at lower temperatures. Thawing with microwaves has the most destructive effect on microorganisms. Microwaves induce rapid and robust vibrations of nuclear matter and thus decompose it [32].

Freezing may prolong the shelf life but can lead to undesirable deterioration of the sensory properties. The changes are apparently due to the formation of ice crystals during the freezing process. The change in flavour is mainly due to hydrolytic and oxidative changes in the fats and the action of peroxidases in vegetables, which are not entirely inactivated by freezing. In addition to the stability of the individual ingredients, several additional factors must be considered in the compositions of prepared foods. These factors could result in an association between the flavour and the ingredients.

In particular, the problem with compound dishes is the change in taste and smell due to the volatility and transition of the aromatic components. These findings are illustrated in Table 1 [32]. Onions and garlic are good examples of enzymatic reactions that cause a loss of characteristic flavour and odour. A frozen meat dish lacks harmony between its spices. Most spices lose intensity, while the aroma of pepper, for example, does not change.

Packaging is a process that plays a crucial role in extending shelf life, especially as a parameter of barrier technology. Vacuum packaging, active packaging, or packaging in modified atmosphere packaging (MAP) are optimal today [33,34]. When packaging cold foods and meals, it is essential to know that Clostridium botulinum’s bacterium grows even without oxygen.

Foods are packaged before or after freezing, but it is essential to remember that packaged foods freeze more slowly because of the insulating effect of the packaging. The packaging must be as close as possible to the product’s surface to prevent freezing and prevent access to oxygen and light. Packaging materials must be mechanically resistant to low temperatures and the thermal regeneration temperatures of the ingredients (up to 100 °C when heated in boiling water, from 150 °C to 160 °C when heated in convection and microwave ovens, and 225 °C when heated on a plate).

Various packaging materials are used for the packaging of frozen ready meals. Due to its resistance to high temperatures (up to 124 °C), polyethene is very suitable for ready meals that need to be heat treated and regenerated directly in the packaging before use. Polyethene is impermeable to liquids and only slightly permeable to water vapour. It is suitable for packaging frozen foods due to its exceptional flexibility at low temperatures [35]. Trays coated with aluminium or polypropylene are also used. Polypropylene is impermeable to grease and has low permeability to steam and gasses. It can withstand temperatures from −10 °C to 150 °C. A biaxial orientation improves resistance at lower temperatures, withstanding temperatures as low as −60 °C, and it is also used in freezing [36,37].

The combination of polyamide and polyethene (PA/PE) results in an oxygen- and water vapour-impermeable film. Polyester/polyethene film has excellent mechanical properties. If polyvinylidene chloride is added, the properties of the film are further improved, and an excellent film for packaging frozen meals is obtained, whereas the newest product (PA/PE re-granulated product) is foreseen for the circular economy [38].

During storage and distribution, ready meals require packaging to maintain their quality and protect against damage. It also provides a barrier against microorganisms, insects, moisture, gasses, and foreign flavours. Cardboard packaging is commonly used to protect the primary packaging from the physical and mechanical effects of the environment [39].

The storage temperature mainly influences the shelf life of frozen meals [40]. Temperature fluctuations are not allowed; otherwise, ice crystals will form and affect the product’s texture. The duration of freezing at −18 °C is from a few months to a year, but frozen ready meals can be stored at −30 °C for about 6 months without the risk of noticeable sensory changes. In determining the shelf life of a frozen meal, the practical storage time (PST), the barely perceptible quality difference (QD), and the high-quality period (HQP) are determined. The PST determines the time when the product still has the expected quality. The QD is the storage time when at least 70% of trained inspectors can distinguish stored products from fresh samples. The PST is mainly determined by sensory evaluation of the product stored from −30 °C to −40 °C 2–5 times. For foods that are very sensitive to colour loss (e.g., peaches, cauliflower, and redfish meat), the PST is very close to the QD. It is also necessary to determine the PST at the actual commercial temperatures (usually at −18 °C) and not only at the desired storage temperatures (from −30 °C to −40 °C) [41].

Prepared foods represent a very complex area in terms of shelf life, as they are made up of various components that individually affect the shelf life. It is necessary to identify the ingredient most susceptible to loss of flavour, discolouration, and rancidity. When determining stability, it is necessary to indicate the storage time at specific temperatures marked in the trade (* −6 °C, ** −12 °C, and *** −18 °C). The quality of frozen food depends on the storage temperature, time, composition, and quality of raw materials and technologies (i.e., production and freezing of the packaging) [42].

The best final quality is obtained in compound meals when the individual ingredients are separately prepared. In a meal consisting of noodles, meat, and vegetables, the noodles are packed raw, and the vegetables are only blanched, while the meat can be fully processed. During storage, the main changes that affect the quality are loss of weight (water), loss of volatile components, oxidative changes in lipids, denaturation of proteins, change in colour, and the transfer of aromatic constituents from product to product. Volatile aromatic compounds originate from the surface of the product. They can pass through the air to other products, where they cause strange odours and flavours by being deposited in the aqueous or fatty phase. For the original product, however, this means a loss of characteristic taste and odour [43].

Physical changes in food depend on changes in the physical state of its water. Complimentary water, which is a free liquid in food, freezes depending on the solute content [44]. The more the free water freezes, the better the quality of the frozen food. Despite the high free water content in most foods, the freezing point decreases due to the dissolved substances from natural and colloidal solutions. When the food is cooled to the freezing point, the water turns to ice while heat continues to be released. When we freeze food, we lower the temperature below the cryoscopic point of its juices. Most water crystallises in a temperature range that is 2–3 °C below the freezing point. This process is the so-called ice crystal formation range or critical zone (usually between 0 °C and −5 °C, depending on the composition of the food). The point at which the temperature transitions into this range is critical to the quality of the frozen food. First, when the surface is frozen, the food crystallises in depth. Consequently, the liquid in the centre is concentrated and frozen at a temperature between −60 °C and −65 °C. Freezing to this point is expensive, so temperatures around −20 °C are usually used [45].

When freezing food, the effect on the quality as determined by the characteristics of the food is to be checked. Most experts advocate for fast freezing because the ice crystals are much smaller and spread intracellularly. The water is inside the cells and does not have time to enter the intracellular spaces. During slow freezing, large crystals form intercellularly because the water diffuses the cells into the intercellular spaces and settles on the crystals already there, making them more prominent. Smaller crystals do minor damage to the cell structure [46].

The higher the water content of the food, the better its ability to freeze produce a higher percentage of ice. To keep the loss of frozen food as low as possible, a low and constant temperature in the storage room and high relative humidity are required. Recrystallisation processes (damage to the colloidal composition of the cell) play an essential role in the structural changes of food. The consequence of these irreversible processes during food thawing is a greater or lesser loss of juice, which alters the quality of the frozen product [39]. At low temperatures, enzyme activity is reduced but not completely stopped. It affects changes in the odour, taste, and colour (lipases: hydrolysis of fats; pectinase: the destruction of the cell wall of plants; and polyphenol oxidase: catalyses rust reactions). Ice crystals also play an important role in enzymatic changes by disrupting membranes and allowing enzymes to access cells.

The growth of microorganisms in frozen foods is inhibited by temperature and water activity. Since the freezing point of solid food is lower than the freezing point of water, they turn into ice. This change causes an increase in the solute concentration and a decrease in water activity, interrupting the metabolic processes of microorganisms. Gram-positive bacteria are more resistant to freezing than Gram-negative bacteria, and bacterial spores are most resistant to freezing damage [47].

Freezing slows the chemical reactions in foods, but protein denaturation, starch breakdown, and fat oxidation can still occur. Slow freezing alters the colloidal structure of proteins and reduces their ability to bind water. The extent of denaturation depends on the length of time frozen foods are stored. The extent of denaturation depends on how long the frozen food is stored. Oxidative spoilage occurs in frozen foods that contain high levels of fat. Boltman [48] stated that the development of radiation in frozen foods is significantly reduced at −34 °C. Low temperatures, especially freezing temperatures, also affect the quality of starches and starch products. The consequences are the loss of a smooth, tender texture and loss of water during thawing. The starch granules, which consist of amylose and amylopectin, swell at a temperature of 50–60 °C and then change into a soluble form (starch gluten). When the starch is cooled, a thick starch paste forms a gel that releases water (this is avoided if the products are heated to 140–150 °C). Common household starches (e.g., wheat and corn) contain only 16–25% amylopectin and lead to a high retrogradation rate, which means that these starches are not suitable for freezing. If recrystallisation processes occur during freezing at low temperatures and shorter storage times, the quality of the frozen starch product is not significantly affected [49]. On the other hand, household starches are commonly used in restaurants. Therefore, this assessment is an opportunity to exchange information between practitioners and scientists.

Sustainable consumption is a process consisting of striving to achieve long-term, sustainable socioeconomic goals, taking into account extremely important environmental aspects [50]. The findings of Wang et al. [50] strongly suggest that European countries are international leaders in sustainable consumption and production practices. These studies motivated the identification of opportunities in the sensory analysis of stuffed pepper.

The current use of human resources is not in line with the goal of sustainable development. Agriculture and food production have a particularly high share of the impact, and this is also true for food consumption. Transforming food production plays a critical role in addressing the challenges, and sensory consumer science can contribute.

Restaurant businesses seeking sustainable transformation are turning to consumer sensory science in droves to achieve sustainability. Based on a comparison of previous research [51], a categorisation of six contributions that consumer sensory science can make to sustainable development is proposed, which includes (1) promoting a dietary shift towards more sustainable foods and diets, (2) increasing food diversity, (3) reducing food waste, (4) improving the circular economy of the food system, (5) increasing and prioritizing food-related well-being, and (6) addressing the impact of climate change on CO2 emissions and energy consumption during the freezing period. On the other hand, some other authors propose a differentiated performance of six factors [52,53]. According to their modelling analysis, the six factors can promote the following sustainable consumption practices: (1) the cultivation of sustainable consumption awareness, (2) the government’s role in modelling, (3) a set of laws, regulations, and policies, (4) a thorough action plan for sustainable consumption, (5) the insistence of businesses and consumers on sustainable production and consumption, and (6) the concerted efforts of various stakeholders. Moreover, standardisation leads to a differentiation of lifestyles not only in the restaurant business but also among individuals, such as well-being, consistent minimalism in consumption [54], and online shopping [55]. Nevertheless, government interventions [56] could help to achieve "green" outcomes [57].

Sensory studies in consumer science have focused on foods that are considered sustainable (e.g., organic, sustainably grown, and meat alternatives) and aspects that contribute to sustainability (e.g., shelf life, consumption of fruits and vegetables, and unfamiliar foods).