Towards Reducing Food Wastage: Analysis of Degradation Products Formed during Meat Spoilage under Different Conditions

Abstract

Foodstuffs, particularly perishable ones such as meat, are frequently discarded once the best-before date has been reached, despite the possibility of their continued suitability for human consumption. The implementation of intelligent packaging has the potential to contribute to a reduction in food wastage by enabling the monitoring of meat freshness during storage time independently of the best-before date. The process of meat spoilage is associated with the formation of specific degradation products, some of which can be potentially utilized as spoilage indicators in intelligent packaging. The aim of the review is to identify degradation products whose concentration correlates with meat shelf life and to evaluate their potential use as spoilage indicators in intelligent packaging. To this end, a comprehensive literature research was conducted to identify the factors influencing meat spoilage and the eight key degradation products (carboxylic acids, biogenic amines, total volatile basic nitrogen, aldehydes, alcohols, ketones, sulfur compounds, and esters) associated with this process. These degradation products were analyzed for their correlation with meat shelf life at different temperatures, atmospheres, and meat types and for their applicability in intelligent packaging. The review provides an overview of these degradation products, comparing their potential to indicate spoilage across different meat types and storage conditions. The findings suggest that while no single degradation product universally indicates spoilage across all meat types and conditions, compounds like carboxylic acids, biogenic amines, and volatile basic nitrogen warrant further investigation. The review elucidates the intricacies inherent in identifying a singular spoilage indicator but underscores the potential of combining specific degradation products to expand the scope of applications in intelligent packaging. Further research (e.g., storage tests in which the concentrations of these substances are specifically examined or research on which indicator substance responds to these degradation products) is recommended to explore these combinations with a view to broadening their applicability.

1. Introduction

Meat is a highly perishable foodstuff. This is due to its high nutrient content (e.g., lipids and proteins), which favors microbial growth, the high amount of available water (a_w > 0.85), a pH value of >4.6 and the presence of autolytic enzymes [1]. Additionally, meat is a heterogeneous product, exhibiting a diverse range of structural characteristics even among individuals of the same species. The variation observed within a single animal is attributable to the differing compositions of connective tissue, proteins, and adipose deposits within specific muscle groups [2]. Furthermore, there are differences in the composition of meat between different animal species [3]. In the case that the referenced literature contains the requisite information, this review differentiates between various animal species (e.g., beef and poultry), which can also be classified as red (e.g., beef) or white (e.g., poultry) meat based on their distinct compositional characteristics. The color of meat results from the different concentrations of myoglobin present in different types of meat [4]. Meat analogs, which are processed foods that imitate meat products [5], and meat products are excluded from this review.

The process of meat spoilage is a complex process that renders meat unfit for human consumption. Meat spoilage is caused by physical changes during processing, chemical and biological agents (e.g., lipid oxidation, enzymatic reactions by lipases and proteases) and/or microbial activity [6,7]. The process of meat spoilage results in the formation of degradation products. It is essential to provide a clear definition of the term ‘degradation products’ in this review, as the term is used inconsistently in the literature. Degradation products are defined as chemical substances formed during the chemical degradation or partial degradation of substances [8]. The pH value, the substantial quantity of free water present (a_w value) and the high nutrient content of meat facilitate the growth of a diverse range of microorganisms. The growth of microorganisms is influenced by a number of factors, including the storage conditions. The most significant of these is the atmospheric conditions, especially the amount of free oxygen and carbon dioxide, and the temperature [9,10]. Moreover, the initial microbial load, particularly of bacteria, and the slaughtering process are of significant consequence with regard to the microbiological quality of meat [11]. The definition of the point at which meat is considered to have spoiled is dependent on two key aspects: the sensory and microbiological conditions of the meat. If the colony forming unit per square centimeter (CFU/cm²) value rises above 5 × 10⁶ to 10⁸, the meat is officially considered spoiled [12]. In numerous countries, including those within the European Union, there are legally defined limits for various microorganisms in carcasses of different meat types set forth in Regulation (EC) 2073/2005 on microbiological criteria for foodstuffs. For instance, the regulation specifies a limit of 2 CFU/cm² of Enterobacteriaceae for pork carcasses [13]. It is essential to consider the specific limits of microbial species, as some microorganisms, such as lactic acid bacteria (LAB), can also act as protective agents. Once the shelf life has been exceeded, the product may exhibit sensory impressions such as ‘buttery’, ‘sour’ and ‘greasy’ characteristics. The initial sensory indications of spoilage in beef (at 10 °C) are primarily discernible through the perception of the aforementioned characteristics, whereas in chicken (at 4 °C), the orthonasal impression of freshness serves as the primary indicator [14]. In general, changes in color, texture, the formation of liquid or slime and off-flavors are described as signs of meat spoilage [15]. The ‘best before’ date and the ‘use by’ date can be determined according to ISO 16779:2015-08 Sensory analysis—Assessment (determination and verification) of the shelf life of foodstuffs [16]. An alternative approach to determining the expiry date is the ‘quality index method’, which also assesses non-microbial spoilage by evaluating a range of quality parameters that change during storage [17].

In an empirical study, consumers consistently indicate that one of the primary reasons for discarding food items is that they have exceeded their best before date. It is not uncommon for consumers to lack confidence in the safety of foodstuffs [18]. Food packaging affects the shelf life and therefore the quality and safety of food. Inappropriate packaging is a significant contributor to food wastage, which has a range of adverse environmental impacts and social implications, particularly as the global population continues to grow and the need for safe and secure food supplies increases. Conversely, optimized packaging contributes to environmental, economic and social sustainability and is therefore an essential element in any sustainable development strategy [19].

The concept of smart packaging, especially in the context of food, has gained considerable traction since 2017. The advent of the COVID-19 pandemic served to further accelerate the pace of research and development in this field [20]. The term ‘smart packaging’ is used to describe packaging that is equipped with the ability to monitor and assess the food conditions it contains as well as the environment within the packaging [21], thereby ensuring higher product safety and quality standards. Moreover, the implementation of smart packaging has the potential to enhance sustainability throughout the entirety of product’s value chain [22], as it may contribute to a reduction in food wastage. Two types of smart packaging exist: active and intelligent packaging [20]. Active packaging is packaging that absorbs or releases substances to extend the product’s shelf-life [23]. For example, the packaging is designed to gradually release substances to the food, thereby increasing its resistance to oxygen [24]. In contrast, intelligent packaging is used to monitor food or its environment [23] during storage and/or transport of the product [25]. The implementation of intelligent packaging concepts has the potential to enhance food safety and reduce avoidable food wastage [26,27]. As previously stated in the introduction, in addition to the term ‘degradation products’, there is also a definitional gap for the terms ‘indicator’ and ‘indicator substance’. The aforementioned gap is subsequently addressed in this paper. In the context of smart packaging, an indicator is defined as a device that provides information about food quality, microbial activity and/or other characteristics. Indicators are designed to respond to the presence, absence or changing concentrations of chemical and/or biological substances. The response of the indicator is often discernible through a change in color [8]. Consequently, in this review, the term ‘indicator substance’ is defined as a substance that reacts to the presence and the change in concentration of degradation products formed during the change (spoilage processes) of packaged products. In order for an indicator to be used for the purpose of indicating meat spoilage, it must meet a number of specific requirements. To guarantee quality and safety, the utilization of methods for testing the freshness of meat offers a viable option, provided that they are cost-effective, non-destructive, efficient [28] and precise [29]. It is further required that indicator substances and degradation products do not react too slowly, but rather indicate meat spoilage at the earliest possible juncture. Moreover, the indication spectrum must be as extensive as possible [30], with a sufficiently large number of degradation products to ensure the ability to make an accurate diagnosis [31]. It is possible for indicators to be in direct contact with the product, as well as in the gas phase of the headspace of the packaging [32]. In either case, the indicators must be capable of being integrated into the packaging material and must not present any harmful effects [29]. When employing indicators for consumer applications, it is of paramount importance that the resulting indicator reactions are readily comprehensible by the consumer [29,33]. Therefore, it is essential to identify suitable degradation products that elicit these reactions in order to monitor meat spoilage.

It is possible to predict which microorganisms will grow or dominate meat spoilage during storage if certain chemical and physical parameters are known. Moreover, the range of degradation products generated by these microorganisms, encompassing esters, ketones, aldehydes, sulfur compounds, amines, and volatile fatty acids, can be anticipated [11]. This knowledge is essential for the identification of appropriate indicators for intelligent packaging concepts. The aim of this review is to present an overview of the search for suitable degradation products whose concentration correlates with the shelf life of meat, and to evaluate their potential use as indicators of meat spoilage. To this end, a comprehensive literature review is carried out and the aforementioned parameters (temperature, atmosphere, meat type) and degradation products (carboxylic acids, biogenic amines, total volatile basic nitrogen, aldehydes, alcohols, ketones, sulfur compounds, and esters) are determined in order to ascertain suitable degradation products for indicating meat spoilage (hereinafter referred to as ‘degradation products’)packaging concept.

In light of the review, the following steps may then be undertaken: A systematic analysis should be conducted to determine the extent to which intelligent packaging can enhance the shelf life and safety of meat. This will entail identifying specific degradation products that are indicative of meat spoilage. It is recommended that the potential integration of these degradation products into intelligent packaging concepts be evaluated by investigating the influence of different factors, such as temperature, atmosphere and meat type. In addition, a clear understanding of the mechanisms underlying meat spoilage and the effectiveness of different degradation products as indicators for intelligent packaging applications should be gained. The development of more accurate and consumer-friendly spoilage detection methods will contribute to reducing food wastage and improving food safety and packaging concept.

2. Materials and Methods

The initial search was conducted using Google Scholar, which yielded a comprehensive range of scientific studies on the microbiological degradation products of meat and their potential role in shelf life indication systems. In conclusion, studies published between 1987 and 2022 were cited. In the case of basic literature, older literature up to 1988 was cited in individual cases where no more recent literature could be found.

Firstly, the factors influencing meat spoilage were identified and defined. The principle search terms employed for this purpose were ‘intrinsic factors’ ‘extrinsic factors’ ‘processing factors’ ‘implicit factors’ and ‘emergent factors’. Secondly, the degradation products of meat spoilage (aldehydes, alcohols, ketones, sulfur compounds, esters, carboxylic acids, biogenic amines, total volatile basic nitrogen) were identified and their influence on the factors ‘temperature’, ‘atmosphere’ and ‘meat type’ was investigated. In this review, the most commonly accepted trivial names are used to refer to chemical compounds and the International Union of Pure and Applied Chemistry (IUPAC) name is provided in brackets when used for the first time, unless it is identical to the commonly accepted trivial name. The primary focus has been on the collation and evaluation of scientific literature on these degradation products in the context of meat spoilage and their potential integration into intelligent packaging concepts, particularly for consumer use packaging concept. In addition to the names of degradation products, the primary search terms included ‘meat’, ‘spoilage’, ‘microbial’, ’microorganism’ ‘intelligent packaging’, ‘smart packaging’, ‘shelf life’, ‘vacuum’, ‘modified atmosphere’, ‘air’, ‘biogenic amines index’, ‘Total Volatile Basic Nitrogen (TVB)-N spoilage limits’, which were searched individually and in combination with one another. Finally, the results are discussed in order to reach a conclusion and outlook for future research.

3. Basics of Microbial Spoilage in Meat—Influencing Factors

The contamination of meat with microorganisms, such as bacteria is a natural consequence of a number of factors, including animal diseases, the slaughtering process and the opening of the carcass. These microorganisms can originate from a range of sources, including cutting knives, lymph nodes and bacteria present in the intestines [1]. The types of bacteria that grow during meat processing are contingent upon the storage conditions of the meat in question [33]. The growth of microorganisms is influenced by a multitude of factors, as illustrated in Figure 1 and can be classified into the following categories: intrinsic, extrinsic, processing, implicit and emergent factors. The intrinsic factors are related to the physicochemical properties and structure of meat [34]. To illustrate, the quantity of glycogen has an impact on the rate of spoilage of meat [2]. The extrinsic factors include all factors that are influenced by the type of packaging and storage conditions, such as temperature and/or atmosphere [35]. The processing of the product exerts an influence on the primary microbial community [36]. Implicit factors are intrinsic biotic parameters, such as the synergistic or antagonistic interactions between microorganisms [37]. The last category of emergent factors comprises those that exert a greater influence when interacting with other factors than when acting in isolation [34].

Meat spoilage is dependent on a microbiota which is dominated by psychrophilic microorganisms [38]. Under aerobic conditions, the predominant microorganisms associated with spoilage of red and white meat are Pseudomonas spp. [39,40,41,42,43,44,45]. Under anaerobic conditions lactic acid bacteria (LAB), such as Lactobacillus, are the primary agents responsible for the spoilage of meat [46]. These LAB represent a dominant population in anaerobic packaging [1,47,48]. In addition to LAB, anaerobic Brochothrix spp. are present in modified atmosphere packaging (MAP) with a low oxygen content and vacuum packaging [39,49]. A key distinction between LAB and Brochothrix spp. is the threshold at which meat spoils. LAB will cause spoilage at a concentration of 10⁸ CFU/g, whereas Brochothrix spp. are capable of causing spoilage at significantly lower germ counts [6]. The presence of facultative anaerobic Enterobacteriaceae is indicative of the freshness of the meat and reflects the hygienic conditions under which the meat was produced and transported. Enterobacteriaceae is capable of growing under both aerobic and anaerobic conditions. However, two studies have reported that the growth of Enterobacteriaceae is more pronounced in modified atmosphere packaging than in vacuum-packed meat [39,50]. This is particularly evident in pork stored at 5 °C [39]. Another bacterium associated with meat spoilage is Clostridium spp. Some strains are capable of surviving under psychrophilic conditions. Clostridium spp. has been identified as a potential cause of blown vacuum packaging and it is a spore-forming bacterium [51,52].

Molds and yeasts are infrequently implicated in meat spoilage, and are thus receive minimal attention in this review. The species isolated from fresh and spoiled meat include Candida, Cryptococcus, Debaryomyces and Yarrowia. Trichosporon spp. is exclusively associated with spoiled poultry [33].

In addition to the main spoilage organisms previously described, there are several other bacterial genera that have been identified, including Acinetobacter spp., Psychrobacter spp. and Moraxella spp. The majority of these organisms demonstrate optimal growth under aerobic conditions and exhibit a tendency to increase in proportion at ambient temperatures [39].

Table 1. Overview of microorganisms mainly responsible for meat spoilage under aerobic, anaerobic and facultative anaerobic conditions.

Preferred Growth Atmospheres
Aerobic Microorganisms	Anerobic Microorganisms	Facultative Anaerobic Microorganisms
Pseudomonas spp. [42,43,44,45,46,47,48] Acinetobacter spp. [42] Psychrobacter spp. [42] Moraxella spp. [42] Molds [36]	Lactic acid bacteria (e.g., Lactobacillus) [1,49,50,51] Clostridium spp. [53,54] b Brochothrix spp. [42,52] a/b	Enterobacteriaceae [42,55] a Yeasts [55]

^a: preferred occurrence in MAP [42,52,55]. ^b: preferred occurrence in vacuum packaging [42,52,54,55].

provides an overview of a selection of the microorganisms that are responsible for meat spoilage under aerobic, anaerobic and facultative anaerobic conditions.

3.1. Extrinsic Factors

Temperature is a factor that affects the shelf life of meat [55]. The utilization of intelligent packaging represents a potential solution for providing consumers with reliable food safety information, particularly given that the temperature within domestic refrigerators frequently exceeds the optimal maximum temperature for meat storage, which is 4 °C [53,54]. This is corroborated by a study on domestic refrigerator storage temperatures, which revealed that 71% of participants had a refrigerator temperature higher than 5 °C [54]. Temperature has a direct effect on microbial growth and the species of microorganisms present [56,57]. Accordingly, temperature has a more pronounced influence on the total viable count (TVC) of microorganisms compared to the composition of the headspace within the packaging [58,59]. It is therefore crucial to ensure the maintenance of the cold chain throughout the entire fresh meat supply chain [57] in order to effectively inhibit the growth of pathogens and prevent the spoilage of the meat [60]. Consequently, the maximum temperatures permitted for transportation and storage of goods, including meat, are regulated in a number of countries. In Europe, for example, the core temperature of meat must not exceed 7 °C during the transportation and storage [61,62]. In the USA, the maximum temperature permitted during the transportation and storage of meat is 5 °C [60].

The storage atmosphere within the packaging is an important factor in extending the shelf life of fresh meat [63,64]. For example, in MAP, the atmosphere surrounding the product has been replaced by oxygen (O₂), carbon dioxide (CO₂) and/or nitrogen (N₂) [65,66]. The microbial growth of microorganisms within the package can be influenced by the presence of different gases, primarily oxygen (O₂) and carbon dioxide (CO₂), within the headspace. CO₂ has been demonstrated to possess antimicrobial properties, especially against gram-negative aerobes such as Pseudomonas spp. [65]. Consequently, the utilization of CO₂ in MAP retards the spoilage of meat [67] by curbing the growth rate [39,68]. On the one hand, reducing the level of O₂ to a minimum can enhance the shelf life of meat in MAP [69] and maintain the red color of fresh red meat [19]. On the other hand, high levels of O₂ (up to 70%) also help inhibit the growth of strict anaerobic bacteria like Clostridium spp. Nevertheless, numerous prevalent food spoilers have been observed to exhibit resistance to these atmospheres [70]. In addition, a high O₂ content in the packaging atmosphere is deleterious to sensory properties, as it may induce lipid oxidation, resulting in the development of rancid off-flavors [65]. In addition to the effects of O₂ and CO₂ on microorganisms described above, effects can be indicated even in the absence of O₂ and CO₂. These effects mainly occur in vacuum-packed meat and in MAP with 100% N₂ as shown by [71], who conducted a comparative analysis of various studies on meat packaging atmospheres and concluded that vacuum-packed and meat packed with 100% N₂ (a rarely employed experimental approach) exhibit a delayed spoilage compared to meat packed with O₂ and CO₂. Nevertheless, N₂ has a negligible impact on microbial growth. largely due to its extremely low solubility in both fat and water [72]. The absence of O₂ in such packaging concepts mainly affects aerobic bacterial species such as Pseudomonas spp., Acinetobacter spp. and Psychrobacter spp. [67,70]. However, the growth of facultative anaerobic food spoilage organisms, including LAB [71] and anaerobic ones such as Clostridium spp. [51] remains unaffected.

3.2. Processing Factors

The processes during the meat processing, from the breeding of animals to the final packaging of meat, are of great importance with regard to the composition of the meat as well as the TVC of microorganisms present. The contamination of meat is related to the environmental conditions under which the meat is handled and processed [36]. One contributing factor is the ability of bacteria to adhere to surfaces and form biofilms, which can serve as a source of contamination during processing [73]. Surface characteristics exert a decisive influence on the adhesion of bacteria and the subsequent formation of biofilms [74,75]. Other process-related factors that exert an influence on the rate of meat spoilage include the temperature during deboning and carcass cutting [76]. The influence of process factors is also reflected in the different microbial compositions across different slaughterhouses [3].

3.3. Intrinsic Factors

The intrinsic factors that contribute to meat spoilage are those that pertain to the physicochemical properties of the meat. The growth of microorganisms on meat is influenced by parameters such as pH value, water activity [77], available nutrients, and antibacterial elements [78], e.g., antibiotics. The growth of microorganisms has been observed to vary in response to different pieces of meat from different parts of the animal [39] as well as between different animal species [3]. These differences in meat spoilage are mainly due to differences in glycogen reserves and the accessibility of micronutrients and minerals, including iron [2].

3.4. Implicit Factors

The implicit factors are those that are themselves related to microorganisms [79]. The most intensively studied group of bacteria are LAB, which are capable of producing a range of inhibitory substances, including bacteriocins [80,81]. In addition, metabiotic, commensalism, predation and competition of microbes are among implicit factors. The term ‘metabiotic association’ is used to describe the phenomenon whereby presence of one microorganism creates an environment conducive to the growth of another microorganism. The term ‘commensalism’ is used to describe the interaction of two groups: one that benefits and one that is not affected. In contrast, the term ‘predation’ is used to describe an organism that feeds on other organisms [79].

In addition to the aforementioned factors, there are also emergent factors that warrant consideration. To date, there has been a paucity of research conducted on these factors. One study offers a definition what is meant by ‘emergent factors’: When factors have a greater effect when interacting than when acting alone, they are designated as emergent factors [34].

4. Degradation Products That Can Be Indicated during Meat Spoilage

The spoilage of meat is regularly accompanied by the formation of a range of degradation products, which are the result of the underlying changes that occur during the spoilage process [30]. The aforementioned degradation products can be utilized to indicate the process of spoilage, for example, in the context of food analysis, shelf-life modeling or intelligent packaging concepts. A variety of spoilage indicators can be indicated, during the analysis of spoiled meat [14]. The following categories have been identified for the classification of typical spoilage products: Biogenic Amines (BAs), Total Volatile Basic Nitrogen (TVB-N) and Volatile Organic Compounds (VOCs) encompass a range of compounds, include aldehydes, alcohols, ketones, esters, sulfur compounds and carboxylic acids.

Several studies show a correlation between the release of VOCs associated with spoilage and the growth of specific microbial species during the storage of meat [82,83]. Furthermore, these studies demonstrate that the generation of VOC is dependent on the on strain type involved [83,84]. The external factors influencing meat spoilage, such as the packaging atmosphere, have a significant impact on the profile of VOCs formed [85,86]. This phenomenon can be attributed to the impact of the atmosphere on the microorganisms that grow in it. However, it is also influenced by to other processes such as the accelerated oxidation of lipids at higher O₂ concentrations [87]. In addition, a correlation is identified between the VOC profile and the fatty acid composition of the meat [88].

The spoilage characteristics become observable when the microorganisms have metabolized the glucose ((2R,3S,4R,5R)-2,3,4,5,6-Pentahydroxyhexanaldehyde) and lactate (2-hydroxypropanoate) present in the meat and the microorganisms begin to utilize the proteins [56]. The initial indicators of meat spoilage are the presence of decomposition products, such as sulfides, methyl esters or ammonia (azane). These compounds result from the conversion of amino acids [6] and lead to sensory signs of spoilage like off-flavors [89].

4.1. Aldehydes

Aldehydes are formed through a number of different processes, including triglyceride hydrolysis, oxidation of unsaturated fatty acids, and lipid autooxidation in meat. These processes can occur under a variety of conditions, including air, vacuum, or modified MAP. Hexanaldehyde (hexanal), nonanaldehyde (nonanal), heptanaldehyde (heptanal), benzaldehyde, and Isovaleraldehyde (3-methylbutanal) are the most commonly occurring aldehydes in naturally spoiled meat and in inoculated model meat systems [11]. The formation of aldehydes in meat during storage and spoilage is the result of lipid oxidation and/or processes catalyzed by growing bacteria [90,91], which is influenced by the amount of O₂ in the packaging atmosphere [92,93,94]. The highest amounts of aldehydes are produced by Pseudomonas spp., Carnobacterium spp. and Enterobacteriaceae [11,90]. However, not all commonly found aldehydes are linked to bacterial growth. Otherwise, the low absolute concentration of aldehydes and their rapid oxidation to acids at an early stage of storage would be inexplicable [11]. Consequently, the concentration of aldehydes such as hexanaldehyde is not a reliable indicator of meat spoilage since this phenomenon cannot be solely explained by microbiological influences. Moreover, aldehydes are found in both sterile and spoiled pork [95]. A comparable estimation is conducted for chicken fillets in different packaging atmospheres. Although a correlation cannot be established between aldehyde concentration and meat storage time due to the high degree of variability in the results [90].

4.1.1. Temperature

The existing literature indicates that the concentration of aldehydes during the storage of meat increases with rising temperature, regardless of the storage atmosphere [15,82]. Nevertheless, the impact of temperature on aldehyde formation is seldom addressed in the existing scientific literature.

4.1.2. Atmosphere

The two aldehydes acetaldehyde and 2-butenal are not yet widely used as indicator substances, due to the limitations imposed by the packaging atmosphere and the composition of the growing microorganisms, which are influenced by it. The results indicate, that 2-butenal ((2E)-but-2-enal) is a more suitable degradation product for meat packed aerobically, while acetaldehyde is more appropriate for vacuum-packed meat [95]. This limits the applicability of aldehydes as a universal degradation product for diverse atmospheric conditions and for a range of products. A comparable dependence of aldehydes on packaging atmospheres and their O₂ contents is observed. Malonaldehyde (propandial) in beef, for example, demonstrates a continuous increase exclusively under aerobic conditions [36]. Aldehydes are also indicated when beef is stored at 4 °C under vacuum. This correlates with the increase in Lactobacillus [46]. Another study of beef storage at 4 °C under MAP (70% O₂, 20% CO₂, 10% N₂) found a significant increase in aldehydes and recommended octanal, nonanal, 2 and 3-methylbutanal as degradation products of beef spoilage [86]. When studying the evolution of 3-methylbutanal during the storage of poultry in MAP (40% CO₂, 30% N₂, 30% O₂) at 4 °C, a significant increase can be indicated during the first days. Therefore, 3-methylbutanal can be used for the early indication of spoilage in chickens packed in MAP [96]. The selection of aldehydes as indicators is restricted to the extent of fat oxidation, as exemplified by malondialdehydes [97]. However, this approach cannot be extrapolated to microbial conditions without further research. Various aldehydes display a dependence on specific packaging atmospheres and their oxygen levels. Consequently, it is challenging to identify a universally applicable aldehyde for indicating meat spoilage [92].

4.1.3. Meat Type

A trend analysis of acetaldehyde and 2-butenal in air- and vacuum-packed meat (beef, pork, and poultry) at 4 °C indicates that these two aldehydes indicates an increase over the storage time in all meat types. The formation of these compounds is more prominent in beef during the first days of storage, whereas the subsequent increase is less prominent in pork and poultry [98]. Hexanaldehyde is the dominant aldehyde in chicken meat [33], showing a continuous increase in concentration during the seven days storage period of chicken meat [99]. The differences in the profile of aldehydes formed can be attributed to the varying composition of fatty acids available for oxidation, which depends on the type of meat. Hexanal, for instance, is the result of linoleic acid oxidation [88]. A review of the literature reveals a paucity of information regarding the formation of aldehydes and the spoilage of different types of meat.

4.2. Alcohols

Microbial metabolism plays a crucial role in facilitating the breakdown of proteins and amino acids, as well as the reduction of ketones and aldehydes derived from lipid peroxidation. This process ultimately leads to the formation of alcohols [1]. An increase in microbial counts has been observed to result in a greater increase in alcohols in stored meat [90]. As an example, a significant increase in ethanol and 2-propanol (propan-2-ol) is indicated in chicken meat after 12 days at 4 °C [96]. The evolution of ethanol and methanol in beef and poultry under a modified atmosphere (70% O₂ and 30% CO₂) is also observed, and a slight increase is indicated during the first seven days. The formation of alcohols increases marginally with extended storage periods, yet the overall level remains notably low. In beef, a notable increase in ethanol levels is only observed at elevated storage temperatures of 10 °C and longer storage periods [14]. A concentration increase over time would be advantageous as it would allow for a correlation between alcohol content and microbial spoilage. Given that ethanol formation is contingent upon the presence of specific microorganisms [100], it is plausible that this phenomenon may also be associated with the microbial flora. The main microorganisms responsible for alcohol production are Pseudomonas spp. and Carnobacterium spp. [1]. In chicken, 3-methyl-1-butanol (3-Methylbutan-1-ol) correlates more strongly with the growth of several spoilage bacteria, e.g., Pseudomonas spp., than ethanol [101]. The Pearson correlation coefficients, which quantify the strength of the linear correlation between two variables [102], show relatively weak correlations between ethanol and the total viable count (TVC) (r = 0.66) and between ethanol and psychrotrophic microorganisms or Pseudomonas spp (0.6) [101]. Different bacterial strains can even produce different profiles of alcohols [83]. For example, not all strains of Carnobacterium maltaromaticum are capable of forming alcohols under aerobic (air) and/or anaerobic (vacuum) conditions. The alcohols 1-octen-3-ol and 2-ethyl-1-hexanol (2-ethylhexan-1-ol) are produced by the greatest number of strains, irrespective of the applied atmosphere. The number of observed strains for 3-methyl-1-butanol is consistent, but only under anaerobic conditions [103].

However, other alcohols such as 1-octen-3-ol (oct-1-en-3-ol) and 1-pentanol (pentan-1-ol) are also formed in the absence of microorganisms in sterilized pork [87]. As a result, these alcohols are not suitable as degradation products for the spoilage process. It is therefore not always reliable to draw conclusions about microbial spoilage levels based on these alcohols.

4.2.1. Temperature

The formation of various VOCs, including alcohols, in poultry under MAP (70% O₂ + 30% CO₂) is influenced by temperature. When it is stored at 4 °C, a correlation between spoilage and ethanol formation is evident. However, at 10 °C the presence of ethanol cannot be confirmed. The concentrations measured at 4 °C are already very close to the limit of quantification in the range around the critical limit, and the confidence intervals chosen for the individual measuring points are high, making it impossible to make an exact statement about the actual concentration [14]. Another study shows that the concentration of alcohols present during the storage of meat increases in line with rising temperatures, irrespective of the storage atmosphere [82]. Refs. [14,94] show a dependency between alcohol formation and temperature, whereby an increase in temperature is accompanied by a proportional increase in alcohol formation. Nevertheless, additional research is required to substantiate this assertion.

4.2.2. Atmosphere

Alcohols are produced by spoilage microorganisms in both aerobic and anaerobic conditions [1] Nevertheless, a study of the evolution of ethanol during storage at 4 °C in beef, poultry, and pork demonstrates an absence of a discernible trend in ethanol concentration across all three types of meat under aerobic conditions [98] and in beef also under vacuum [46,104]. Under vacuum conditions, the presence of heterofermentative lactic acid bacteria in beef and poultry may be indicated by an increase in ethanol concentration. (LAB) [98]. These observations correspond to the findings of a study of beef and pork, which indicates that ethanol formation is marginally higher under aerobic atmospheres than under CO₂ or N₂ packaging [97]. Additionally, elevated oxygen levels in MAP packaging is linked to an increased concentration of 1-octen-3-ol, 1-pentanol, hexanaldehyde, and 2-pentanone (pentane-2-one), which can be attributed to heightened lipid oxidation [87]. Consequently, a limiting factor for the utilization of alcohols as degradation products is their dependence on the O₂ content of the atmosphere during storage. To illustrate, a study on beef storage at 4 °C under MAP (70% O₂, 20% CO₂, 10% N₂) demonstrates the impact of O₂ on the formation of various alcohols. The study identifies a notable increase in the concentration of alcohols, with 2-methylbutanol and 3-methylbutanol, 1-pentanol, and 1-hexanol emerging as potential degradation products in beef [86]. Given the contradictory statements, it is not possible to provide a definitive account of the formation and change in concentration of alcohols in relation to the packaging atmosphere.

4.2.3. Meat Type

The formation of alcohols during spoilage is demonstrated in a variety of meat types in different studies. While 3-methylbutan-1-ol is identified as a potential degradation product in air-stored beef at 4 °C [104], ethanol shows a significant, though not robust, correlation with the microorganism concentration in the early stages of spoilage of chicken breast meat stored aerobically at varying temperatures (4, 10, 21 °C) [101]. A further study has revealed disparities in the evolution of alcohols during storage at 4 °C, both aerobically and under vacuum, of pork, beef, and poultry. While the alcohol concentration remains unaltered in pork and poultry, it increases in beef [98]. Given that alcohols are implicated in the spoilage of diverse types of meat, they possess the potential to be utilized as degradation products for various types of meat. Nevertheless, no single alcohol is identified in the literature as being particularly suitable.

4.3. Ketones

The formation of ketones is a consequence of microbial or chemical deterioration, occurring as a result of the breakdown of fats [1,90]. The presence of ketones is indicative of the activity of Gram-negative (e.g., Pseudomonas, Shewanella and Moraxella) or Gram-positive bacteria (Carnobacterium spp.). These bacteria are capable of producing ketones as a byproduct of alkane degradation or alcohol dehydrogenation [11,42,105,106]. Although ketones like 2-pentanone, 2,3-pentanedione (pentane-2,3-dione), 2,3-octanedione (octane-2,3-dione), and 3,5-octanedione (octane-3,5-dione) are associated with off-flavors [86]. Despite this, no specific ketone has been identified as a reliable indicator of meat spoilage [13]. The primary ketones responsible for the off-flavor of meat are acetoin (3-hydroxybutan-2-one) and diacetyl (butan-2,3-dione). With respect to bacterial production, Brochothrix thermosphacta has the capacity to produce acetoin [1]. Other ketones are also found in different types of meat, e.g., 2-butanone (butan-2-one), acetoin and diacetyl are commonly found in fresh pork [102] and acetone (propan-2-one) whose concentration remains constant during poultry storage at 1.5 ± 0.2 mg/m³. The acetoin concentration does not follow a clear trend, initially demonstrating a sharp increase from 5–8 mg/mg³ (between days 3 and 9 of storage), followed by a sharp decrease to 0–2 mg/m³. The formation of acetoin is associated with the metabolic activity of Brochothrix thermosphacta, Carnobacterium spp. and Lactobacillus spp. Additionally, a decrease in acetoin may occur concurrently with the potential formation of diacetyl [96]. A study of acetoin and diacetyl formation in pork at 4 °C reveals that acetoin increases rapidly, reaching 50 mg based on one litter of packaging volume on day 3. Thereafter, it decreases to 10 mg on day 4 and then increases again to almost 400 mg on day 6 [107]. This phenomenon is similar to that previously reported by [106]. In contrast to the aforementioned studies on chicken meat, the presence of acetoin is not indicated by [84]. In conclusion, the typical ketones, including acetone, acetoin, 2-butanone, and diacetyl, do not demonstrate a statistically significant positive correlation with meat spoilage.

4.3.1. Temperature

The formation of ketones depends on the storage temperature. The concentration of diacetyl and acetoin is significantly higher at 10 °C than at 4 °C [14]. In the context of air, an increase in ketone concentration is indicated by a rise in temperature, which cannot be confirmed under MAP conditions [82]. Therefore, it appears that the formation of ketones is dependent on temperature. However, it can be reasonably deduced that the packaging atmosphere plays a more decisive role.

4.3.2. Atmosphere

No general correlation has been identified between the influence of different atmospheres and the formation of ketones during spoilage. In contrast, there are ketones that are produced irrespective of the prevailing atmosphere, while others are produced by microorganisms only in specific atmospheric conditions [103]. Additionally, a study reports that packaging atmosphere shows an effect on ketone formation. In this study, acetoin and diacetyl are identified as possible degradation products in air-stored beef, while 2-butanone is additionally identified in vacuum-packed beef [108]. The majority of Carnobacterium maltaromaticum strains have been observed to produce acetoin in air and/or vacuum packaging. In contrast, almost none of the strains are able to form 2-heptanone (heptan-2-one) and 3-octanone (octan-3-one) under vacuum atmospheres. However, the majority of the strains are capable of doing so in air [103]. In addition to the microorganisms Pseudomonas putida, Lactobacillus sakei, Pseudomonas fragi, and Leuconostoc mesenteroides [95], and Carnobacterium maltaromaticum [109], the presence of ketones is also associated with Lactobacillus. This is shown by the storage of beef at 4 °C under vacuum [46]. Furthermore, a study examining the storage of beef at 4 °C under different atmospheres identifies correlations between the presence of acetoin and the growth of Brochothrix thermosphacta and Pseudomonas spp. under MAP and in bacteriocin-activated antimicrobial packaging [42]. Another study, also on beef storage at 4 °C under MAP (70% O₂, 20% CO₂, 10% N₂), shows a significant increase in ketones during the storage and suggests the utilization of 2,3-octanedione and 3,5-octanedione, among other potential degradation products, for beef [86]. The existing literature does not present a unified perspective on the extent to which the atmosphere exerts an influence on ketones. Some studies indicate that certain ketones are formed with greater frequency in a vacuum, while others suggest that the presence of oxygen (O₂) is a significant factor. Additionally, there are observations indicating that certain ketones are not influenced by the atmosphere.

4.3.3. Meat Type

The formation of ketones is subject to different dynamics, depending on the type of meat in question [14]. With regard to pork, research indicates that storage at 4 °C under a high oxygen atmosphere results in an increase in the concentration of acetoin, diacetyl, and 3-methyl-1-butanol during the spoilage process [87]. In pork inoculated with Pseudomonas putida, Lactobacillus sakei, Pseudomonas fragi, and Leuconostoc mesenteroides and stored aerobically at 4 and 10 °C, the presence of ketones can also be indicated in the headspace of the packaging of pork that has undergone spoilage. Nevertheless, the presence of acetoin is also indicated in the headspace of packaging used for fresh pork [83]. The relationship between ketones and pork spoilage at 4 °C under MAP (70% O₂, 30% CO₂), is also corroborated by another study, wherein multiple ketones were identified over the course of a 12-day storage period. The data indicate an increasing rise for both acetone and acetoin, with acetone exhibiting a relatively high odor threshold and acetoin a low one. An increase in concentration up to day 6 and subsequent decrease up to day 12 can be determined for 2-butanone. In contrast, 2,3-butanedione (butane-2,3-dione) is not indicated until the end of the storage period [110]. A comparable phenomenon is observed with 2,3-Butanedione in the context of beef storage at 4 °C, under aerobic and vacuum conditions. Following an initial increase, a subsequent decrease in the ketone concentration is evident [108]. In a contrasting observation, the storage of poultry meat at 4 °C and MAP (40% CO₂, 30% N₂, 30% O₂) reveals an increase in the concentration of 2-butanone until the conclusion of the 15-day storage period, with no subsequent decline in concentration [96]. A review of the scientific literature reveals a lack of consensus regarding the formation of ketones in diverse types of meat. A number of ketones have been found to exhibit elevated levels in pork samples (acetone, acetoin, diacetyl, and 3-methyl-1-butanol), whereas in poultry meat samples, 2,3-butanone is the predominant ketone present at elevated concentrations. No discernible trend is identified with respect to beef, and no pertinent literature is uncovered for other types of meat.

4.4. Sulfur Compounds

The presence of sulfides is among the earliest indications of meat spoilage [6]. This observation is a consequence of the primary source of sulfur compounds being the degradation sulfur-containing amino acids [111], including methionine ((2S)-2-amino-4-methylsulfanylbutanoic acid) and cysteine ((2R)-2-amino-3-sulfanylpropanoic acid) [10]. The concentration of sulfur compounds increases with storage time [98]. The most prevalent sulfur compound in spoiled meat is dimethyl sulfide, followed by dimethyl disulfide (methylsulphonylmethane), dimethyl trisulfide, and methyl thioacetate [11,112]. In contrast with the findings of [11,113], a study in which pork is inoculated with various spoilage microorganisms (Pseudomonas fragi, Lactobacillus sakei, Pseudomonas putida and Leuconostoc mesenteroides) and stored in air at 4 and 10 °C, the results show that dimethyl sulfide is mainly indicated in the fresh pork samples. In contrast, the presence of other substances, including alcohols, ketones, and esters, are identified in spoiled pork samples [83], which calls into question the use of dimethyl sulfide as a potential spoilage indicator. The hypothesis that dimethyl sulfide is a degradation product is also refuted by the finding that the concentration of dimethyl sulfide decreases with increasing storage time [83,109], which is in contrast to the findings of [98]. Another sulfide mentioned in the context of meat spoilage is hydrogen sulfide [105], which is formed by the enzymatic desulphidation of cysteine [114]. The presence of sulfur compounds such as dimethyl sulfide, dimethyl disulfide or dimethyl trisulfide, can cause unpleasant odors like petrol, rotten, fishy or sulfurous odors. Furthermore, sulfur compounds contribute to malodor and play a crucial role in the off-flavor of stored meat [115]. A comparison of Pearson correlation coefficients also shows that the linear correlations of sulfides with total viable count (TVC) are comparatively weaker (0.23–0.58) than those of alcohols (0.55–0.66) or free fatty acids (0.44–0.73), and also in comparison with TVC [100]. Nevertheless, sulfur compounds are indicated during meat spoilage [115,116], which suggests that they may still be regarded as potential degradation products.

4.4.1. Temperature

Dimethyl sulfide concentration increases with prolonged storage time, temperature and microbial counts [117]. In various studies examining the storage of meat, sulfur compounds are identified at disparate temperatures, including 4 °C [116] and room temperature [111]. In the development of a dye to indicate the spoilage of poultry meat by color change due to the presence of volatile organic compounds in the headspace of the package. The study has examined the effect of room temperature and refrigerator temperature on consumer. In both cases, the color change occurs under the presence of sulfur compounds [115], which lends to support to the use of dimethyl sulfide as a degradation product for poultry meat. A correlation between the increase in dimethyl disulfide concentration and temperature can be observed [96]. The concentration of dimethyl disulfide increases with increasing temperature [117]. The available evidence indicates that sulfur compounds are present at varying temperatures, suggesting their potential as degradation products for monitoring meat spoilage. However, there is a paucity of literature examining the temperature dependence of sulfur compounds.

4.4.2. Atmosphere

Sulfur compounds are mainly produced by Enterobacteriaceae, but also by Pseudomonas spp. Enterobacteriaceae grow in both aerobic and anaerobic conditions and are involved in meat spoilage [118]. For example, hydrogen sulfide can be used to indicate a variety of Enterobacteriaceae and is considered a possible indicator of meat spoilage under aerobic conditions [91]. Sulfur compounds are also indicated under aerobic conditions in association with Pseudomonas spp., while no indication occurs under MAP (70% O₂, 30% CO₂ and 30% CO₂, 70% N₂) [116]. Contrary to the statement of [119], the studies of [45,106] report that dimethyl sulfides can be indicated under MAP atmospheres (40% CO₂, 30% N₂, 30% O₂ and 60% O₂, 40% CO₂) [42,96]. In contrast to the constant increase in the concentration of dimethyl sulfide under these conditions, carbon disulfide initially shows a decrease and a non-significant increase only on the last two days of measurement and is therefore not suitable as an indicator of meat spoilage [96]. This conclusion cannot be universally applied to the entire category of sulfur compounds. In the case of a developed indicator for monitoring the freshness of chicken meat, sulfur compounds have been observed to indicate at 6 °C in MAP, three-to-four days after the expiration of the best-before date. Consequently, this indicator is deemed suitable for indicating chicken meat spoilage under MAP packaging [120]. Furthermore, studies indicate the presence of sulfur compounds during meat storage under aerobic atmospheres, including 2-ethyl-1-hexanol [42], dimethyl trisulfide, and dimethyl sulfide [111]. In comparison to other degradation products such as acetoin with a concentration of 215 ppm in air, the sulfur-containing compounds are only indicated at extremely low levels, ranging from 0.5 to 2.96 ppm in air [42]. Despite the formation of sulfur compounds under disparate atmospheric conditions, it is challenging to identify a singular substance as the optimal degradation product due to discrepancies in the literature.

4.4.3. Meat Type

Three studies report that dimethyl sulfide can be indicated in both poultry and beef [96,111,112]. In addition, other studies report the observation of sulfur compounds in pork [116] and in poultry [115]. It is therefore evident that sulfur compounds may be present in a range of meat types, also mentioned by [14], which elucidates that the formation of specific sulfur compounds is contingent upon the type of meat in question.

For instance, the concentration of dimethyl sulfide is known to increase when beef and poultry meat are stored under MAP (30% CO₂, 70% O₂) conditions and subsequently exhibits a decline in beef samples only. Nevertheless, the measured concentration of dimethyl sulfide in the packaging headspace is below 25 pp, which is a very low level [14].

It can therefore be surmised that sulfur compounds represent a less suitable option as degradation products within the headspace of intelligent packaging.

4.5. Esters

Esters are formed by the esterification of organic acids and alcohols and are associated with fruity, sweet or ethereal odors. The main ester producer is Pseudomonas fragi [11,14]. Nevertheless, the presence of Pseudomonas fragi is not generally associated with high ester formation during spoilage [106]. In this regard, the utilization of esters to indicate the presence of meat spoilage is not consistently reliable. Methyl ester is reported as the first degradation product [6]. Other esters, including ethyl acetate, ethyl butanoate, ethyl 3-methyl butanoate, ethyl octanoate, ethyl hexanoate, and ethyl decanoate, are formed later in the meat spoilage process [1,11].

4.5.1. Temperature

Below a temperature of −1.5 °C, various bacteria such as LAB and Enterobacteriaceae show reduced growth, as they cannot use their metabolism properly at this temperature [15]. The same applies to psychrotolerant and mesophilic bacteria above 7 °C. Above this temperature, their metabolism does not function sufficiently to produce degradation products, such as esters [118]. However, several studies show that at least small amounts of esters can be indicated even above 7 °C [83,87,101]. Despite this, it is reasonable to assume that ester formation is strongest between −1.5 °C and 7 °C, which is exactly the recommended core temperature for meat of <5 °C in USA [60] or <7 °C in Europe [61,62].

4.5.2. Atmosphere

The packaging atmosphere of stored meat exerts a significant influence on the formation of esters, with Pseudomonas spp. exhibiting a marked prevalence under aerobic conditions [41]. This is also confirmed by [11] who states that esters are mainly formed in fresh meat stored under aerobic conditions [11]. Ethyl esters can also occur under MAP (60% O₂, 40% CO₂) atmospheres, but are more commonly found under aerobic conditions. In the aforementioned atmospheres, it is not only Pseudomonas fragi, but other species of Pseudomonas spp. as well [85]. Two studies show that the indication of esters such as ethyl acetate during meat storage is possible under MAP (60–80% O₂, 20–40% CO₂) [87] as well as under air atmospheres. Given the low concentration (<peak area × 10⁴), the study further considers other degradation products, including alcohols, but not esters [101]. Nevertheless, none of the aforementioned studies propose esters as a degradation product for meat, in contrast to [121], which suggests esters as a potential degradation product for the indication of food spoilage. However, [122] does not differentiate between the various substances, such as biogenic amines (BA), alcohols or esters, that may be most suitable for meat [122]. The aforementioned studies indicate that esters are preferentially formed under aerobic conditions, although they can sometimes also be indicated under MAP atmospheres with O₂. It may thus be postulated that the utilization of esters as degradation products is confined to packaging configurations with oxygen.

4.5.3. Meat Type

Some esters, including ethyl acetate, ethyl propionate, ethyl butyrate and ethyl hexanoate can potentially be considered as characteristic compounds of fresh pork. A negative correlation, such as decrease over storage time, may be indicated for these esters [90]. Two other studies also report the presence of esters during pork storage. One study reports the presence of ethyl acetate [87], while the other describes esters in general and initially notes an increase in concentration and subsequently a decrease with storage time. [83]. In addition to pork, esters can also be indicated in chicken [101]. While the literature does make mention of the presence of esters in pork and chicken meat, no statements have been made regarding the presence of these compounds in other meat types, such as beef. Therefore, it is not possible to state that esters can be used universally to indicate the spoilage of different types of meat.

4.6. Carboxylic Acids

The hydrolysis of triglycerides and phospholipids results in the formation of volatile fatty acids [1], including carboxylic acids. Other possible pathways for the production of carboxylic acids are the degradation of amino acids or the oxidation of ketones, esters and aldehydes [123]. Brochothrix thermosphacta and Carnobacterium spp. are associated with the production of volatile fatty acids [123]. Other microorganisms that are known to produce acids are lactic acid bacteria (LAB), which are mainly responsible for the formation of lactic acid, but can also form acetic acid and other acids during storage [39,124]. Acetic acid is described as one of the most promising volatile degradation product for chicken meat. Nevertheless, propanoic acid, 2-methylbutanoic acid, 4-methylpentanoic acid and butanoic acid are also indicated in chicken meat, which is stored at 21 °C for three days [111]. Thiobarbituric acid (2-sulfanylidene-1,3-diazinane-4,6-dione) is also employed in the assessment of the sensory shelf life of foodstuffs, and is frequently utilized to quantify the extent of lipid oxidation [125].

4.6.1. Temperature

The concentration of carboxylic acids formed is influenced by the duration and temperature of meat storage. This results in varying concentrations of carboxylic acids (specifically, octanoic acid and butanoic acid) during the storage of beef at temperatures of −2 °C and 4 °C. A comparison of different studies supports this assertion. Carboxylic acids is indicated during meat storage at a range of different temperatures including −3.5 °C [88], 4 °C [42,104,119], 6 °C [47] up to 21 °C [111]. At lower temperatures up to 6 °C, the formation of hexanoic acid [11], 3-methylpentanoic, pentanoic acid, octanoic, nonanoic, decanoic and oleic acids [119] is indicated. At higher temperatures (21 °C), propanoic acid, 2-methylbutanoic acid, 4-methylpentanoic acid is indicated [111]. The formation of acetic acid and butanoic acid occurs at temperatures that are both cooler and warmer [104,111], than those affecting the carboxylic acids mentioned previously, and thus are less affected by temperature. It can be reasonably concluded that acetic acid and butanoic acid are the most suitable degradation products for universal use.

4.6.2. Atmosphere

The following fatty acids can be identified under different storage atmospheres: acetic acid [11], butanoic acid [42], and hexanoic acids [47] The formation of branched fatty acids, including 2- and 3-methylbutanoic acid, is exclusively observed under aerobic conditions [11]. In both vacuum-packed meat and under aerobic conditions, acetic, butanoic, and pentanoic acids are identified. As storage time increases, the concentration of carboxylic acids rises, resulting in perceptible changes in sensory properties. These changes are observed after seven days under aerobic conditions and after nine days under vacuum [104]. Such sensory alterations encompass, for instance, the occurrence of off-odors, which are attributable to the release of acetic acid and butanoic acid during the storage of beef [108]. In vacuum-packed meat, butanoic acid is produced by LAB via the degradation of amino acids by the Stickland reaction or by Clostridia via fermentative butanoic acid metabolism [42,91,113]. In aerobic conditions, Pseudomonads spp. are capable of forming gluconic acids specifically ((2R,3S,4R,5R)-2,3,4,5,6-pentahydroxyhexanoic acid) and 2-oxogluconic acids through the oxidation of glucose [126]. Additionally, carboxylic acids like hexanoic [47] and butanoic acid [11] are formed under MAP (80% O₂, 20% CO₂). Furthermore, pentanoic acid, n-heptanoic acid, n-octanoic acid, n-nonanoic acid, n-dodecanoic acid, propionic acid (propanoic acid) and isovaleric acid (3-methylbutanoic acid) are also indicated under MAP (high O₂). In particular, n-octanoic acid, n-nonanoic acid, and pentanoic acid are prevalent, particularly in meat that has been packaged aerobically [88]. Furthermore, the study of the effects of different atmospheres on ground meat revealed an oxygen dependence of carboxylic acids. The study reveals that, in the majority of instances, the acetic acid concentration is higher in oxygen-rich atmospheres than in other atmospheres [97]. The existing literature indicates that certain carboxylic acids, such as acetic acid, are formed under different atmospheric conditions. This evidence supports the argument that these acids can be utilized as degradation products. However, the oxidation of acids that occurs when glucose is removed, as well as the varying glucose content of meat [91], are consequences that render these acids unsuitable as indicators of spoilage.

4.6.3. Meat Type

Carboxylic acids are present in a range of meat types. Some studies demonstrate the presence of carboxylic acids in stored chicken [88,111,119], while others show that carboxylic acids also occur when storing beef [42,47,104]. A 45-day storage of beef at 4 °C under different atmospheres (air, MAP, vacuum, antimicrobial bacteriocin) shows a correlation between butanoic acid and the growth of lactic acid bacteria (LAB, with butanoic acid concentration increasing with increasing LAB concentration [42]. LAB are among the microorganisms that are most responsible for the spoilage of meat of various types [11,127]. In chicken, propanoic acid, 2-methylbutanoic acid, 4-methylpentanoic acid [111] 3-methylpentanoic, octanoic, nonanoic, decanoic and oleic acids [119] are indicated. Meanwhile, hexanoic acid is indicated in beef [47]. In both meat types, chicken and beef, acetic, butanoic [104,111], and pentanoic acids [104,119] are indicated. It is evidenced that carboxylic acids are formed during the spoilage of beef and chicken. Currently, however, there is a paucity of scientific literature pertaining to the formation of these acids in other types of meat.

4.7. Biogenic Amines

Polyamines, spermine (N,N′-bis(3-aminopropyl)butane-1,4-diamine) and spermidine (N′-(3-aminopropyl)butane-1,4-diamine) are naturally present in fresh meat. In contrast, biogenic amines (BAs) such as histamine (2-(1H-imidazol-5-yl)ethanamine), putrescine (butane-1,4-diamine), tyramine (4-(2-aminoethyl)phenol), tryptamine (2-(1H-indol-3-yl)ethanamine), and β-phenylethylamine (azane) are formed during storage [128]. BAs are the result of microbial activity where amino acid decarboxylases generate them from free amino acids (FAA) [129], therefore they could be used as bacteriological quality indicators [121]. BAs have already been proposed as an indicator of spoilage [43] or as a useful tool for monitoring meat quality and safety [130]. In particular, putrescine, histamine, cadaverine and tyramine have a high potential for indicating meat spoilage [29].

The formation of BAs is dependent on a number of factors that influence the FAA content and the activity amino acid decarboxylases in meat [130]. These factors include the composition of the meat and the characteristics of the microbial growth, in addition to the processing, preservation and storage conditions [130,131,132,133]. The spectrum of BAs produced is contingent upon the availability of precursors in the form of FAA, as well as the composition of the microbial flora. The discrepancies in FAA levels may be attributed, at least in part, to the differing proteolytic and amino-genic capabilities of the specific microbiota [134]. The formation of BAs increases at higher bacterial loads beginning at a concentration of log 6 CFU/cm² [135]. Therefore, the formation of BAs is not linearly correlated with the number of microorganisms present, but there is a slight time lag between content of BAs and microbial growth [17]. A study of 50 poultry-associated bacterial strains (25 Pseudomonas, 13 Salmonella, and 12 Listeria) shows that different strains do not necessarily produce an identical profile of degradation products. Cadaverine (pentane-1,5-diamine), putrescine, phenylethylamine, histamine and tyramine are formed by all Salmonella strains tested. The formation of cadaverine, putrescine, and phenylethylamine is observed in over 90%. Additionally, the studied Pseudomonas strains demonstrate the production of histamine (60%), tryptamine (36%) and tyramine (16%). The Listeria strains do not form BAs [136]. Cadaverine is also primarily associated with Enterobacteriaceae and tyramine, putrescine, histamine and cadaverine to a lower extent with Pseudomoas spp., Brochothrix thermosphacta and Pychrotrophs [137]. Ref. [138] also found correlations between the amount of Enterobacteriaceae present with cadaverine, and between Pseudomonas spp. and Brochothrix thermosphacta with tyramine, and to a smaller extent with histamine and cadaverine [139]. In addition to a correlation between Enterobacteriaceae and aerobic mesophilic bacteria with cadaverine, a high positive correlation between putrescine and these microorganisms can also be observed. This implies that an increase in the concentration of BAs is associated with an increase in the number of microorganisms [121]. Furthermore, amino acids are metabolized by lactic acid bacteria (LAB), resulting in the production of biogenic amines, including tyramine, spermine, histamine, and putrescine [118]. It can thus be concluded that amines are dependent on the microorganisms involved in the process of meat spoilage. The presence of Enterobacteriaceae and Pseudomonas spp. in particular leads to an increase in the concentration of cadaverine, putrescine, tyramine and histamine.

BAs are also affected by different storage conditions [133]. For instance, the concentration of tyramine and cadaverine demonstrates a pronounced correlation with storage duration, exhibiting an increase in concentration with prolonged storage. In contrast, the effects on spermidine and putrescine are less pronounced [137]. Likewise, the pH value shows a pronounced effect on the formation of BAs [130].

4.7.1. Temperature

The formation of BAs is influenced by both storage temperature and time. An increase in temperature, whether gradual or abrupt, has the effect of stimulating microbial growth. Similarly, the concentration of BAs is known to rise in response to elevated temperatures and extended storage time. Therefore, the concentrations of putrescine and cadaverine in poultry are also influenced by storage temperature [140]. Furthermore, histamine demonstrates a temperature-dependent behavior, exhibiting a concentration increase at temperatures above 4.9 °C, as opposed to a concentration decrease at temperatures below 18 °C [141]. Refs. [142,143] also conclude that the total amount of BAs is lower when meat is stored at lower temperatures (below 6 °C) [142,143]. Higher BAs concentrations are found at higher temperatures. In Addition, the influence of temperature on the formation of BAs in relation to the type of meat confirms that the temperature influence is most pronounced in poultry, in comparison to beef and pork. [144] Consequently, the concentration of BAs demonstrates a pronounced correlation with temperature, with an increase in concentration observed as temperature rises.

4.7.2. Atmosphere

The atmosphere utilized for packaging can impact the qualitative and quantitative formation of Bas [145]. The study [46] on the effects of aerobic and anaerobic atmospheres on BAs at 4 °C confirms the observations of [146], who also show a significant influence of the atmosphere on the formation of BAs, especially putrescine and cadaverine [46,146]. In this regard, the concentration of these two BAs is higher under aerobic conditions than under MAP (30% CO₂, 70% N₂), which may be attributed to the enhanced growth characteristics of Pseudomonas spp. in this atmosphere [43,147]. The influence of Pseudomonas spp. on the formation of BAs is related to the decreasing availability of glucose. As the number of bacteria increases, the available glucose becomes insufficient, resulting in the subsequent degradation of amino acids and proteins into BAs and sulfides [39]. This correlates with the finding that the levels of putrescine and cadaverine increase linearly with storage time [43]. Facultative Enterobacteriaceae can also grow under all atmospheric atmospheres, with or without the presence of O₂. Enterobacteriaceae growth leads to increased formation of cadaverine and putrescine, tyramine and histamine [148,149]. For example, the formation of putrescine and cadaverine in pork meat increases in all MAP atmospheres, whereas tyramine shows only minor increases [139]. However, less putrescine and cadaverine is formed in chicken tile under high O₂ MAP (75% O₂, 25% CO₂) than under air or vacuum [150]. For vacuum-packed beef, the formation of BAs as a function of microbial load is confirmed [135]. The levels of other BAs, such as spermidine and spermine in pork, exhibit minimal variation in response to the different atmospheres [139]. Additionally, a reduction in these two BAs may occur in chicken meat [43]. The results demonstrate that, in particular, cadaverine and putrescine, but also tyramine and histamine, are promising degradation products under different conditions. In addition to the aforementioned increasing BAs, there are spermidine and spermine that do not demonstrate significant concentration changes during processing and storage [107,146,148].

4.7.3. Meat Type

The formation of BAs is influenced by the conditions under which raw meat is handled. The different compositions of FAA that are dependent on the type of meat may result in either differentiating or unifying effects on red or white meat, respectively [129,131]. In a study examining the storage of meat, higher levels of cadaverine are observed in both red and white meat. This phenomenon may be attributed to the elevated levels of the precursor lysine present in these meats. In this experiment, all BAs exhibited an increase in concentration earlier in white meat than in red meat. This can be attributed to the shorter muscle fibers in white meat, which are more susceptible to proteolytic enzyme attack [151]. Another study yields comparable outcomes, with accelerated and elevated formation observed in chicken, turkey, and pork. The aforementioned observations pertain to the total free amino acid precursors of BAs [17]. For chicken meat stored at 4 °C ± 1 °C for 17 days, cadaverine, putrescin, and tyramine are identified as the major BAs for indirect bacterial indicators [121]. Other bases, including spermidine and spermine, as well as trace amounts of putrescine, have been identified in unripened chicken meat that has been subsequently stored at temperatures ranging from 1 to 1–5.7 °C for 8 h and at −18 °C ± 1 °C for 89 days. During the 8-h storage, an increase in tyramine and histamine a there is a notable increase in the levels of tyramine and histamine [141]. However, histamine is more commonly associated with the degradation of fish freshness [139] than with meat. With regard to the various muscle groups within a given species, there is no significant divergence in the profile of BA formation. Only minor differences in the levels of cadaverine and putrescine are observed, which can be attributed to different microbial counts resulting from different contamination during cutting [152]. Another experiment shows higher levels of amines in the breast compared to the leg of chicken meat stored for 15 days [147]. Besides the indication in chicken, BAs can also be indicated in beef. As storage time increases, BAs concentrations also increase, especially of tyramine and cadaverine. At the outset of the storage period, the highest concentrations of tryptamine and spermine is observed. However, with prolonged storage, no notable alterations are discerned in these BAs and histamines. For putrescine, there is a significant increase in concentration after eight days, but not as much as for cadaverine and tyramine [137]. The more specific protein composition and softer structure of chicken meat render it more susceptible to the formation of BAs than pork or beef [153]. Therefore, chicken meat exhibits a more rapid deterioration due to BAs than beef and pork, and to a similar extent as lamb [17]. It can be reasonably concluded that, although BAs, particularly cadaverine and tyramine, are indicated in the spoilage of various meat types, they appear to be superior degradation products for chicken in comparison to other meat types. It is challenging to utilize these compounds as broad-spectrum degradation products.

4.7.4. Biogenic Amine Index

As different BAs occur in different meats, the use of a biogenic amine index (BAI) may be helpful in assessing meat freshness [154]. An advantage is that the BAI is not influenced by microbial activity and tends to remain stable during storage [130]. Over time, a variety of calculation models for BAI in meat is reported, some of which are shown in

Table 2. Examples of calculation models for the biogenic amines index (BAI) used in different studies to combine different BAs for assessing meat freshness more easily.

Calculation Models for the BAI	BAI Value above Which Meat Is Considered Spoiled	Meat Type	Atmosphere	References
Putrescine + cadaverine + tyramine + histamine	>50 mg/kg	Pork	No specification	[138,154]
		Chicken	Air, MAP (75% O2, 25% CO2), vacuum	[150]
		Different meat types	No specification	[142]
(histamine + putrescin + cadaverine): (1 + spermine + spermidine)	>10 mg/kg (considered as low quality)	Meat in general	No specification	[155]
Putrescine + cadaverine + tyramine	No specification	Turkey	MAP (different compositions)	[143]
		Chicken	Air, MAP (70% N2, 30% CO2)	[43]

.

The profile of the formed BAs is not identical for all types of meats and the different packaging concepts [17,151]. Accordingly, disparate calculation bases are necessary for disparate scenarios, as well as disparate limit values for the classification of freshness based on the BAI value achieved. In various studies, the BAI limits range from 50 mg/kg [147,154] to 96–101 mg/kg [43] with the latter representing the threshold for spoilage in the studied meat types. Although it is possible to use BAIs as quality indices and indicators of undesirable microbial activity in meat [131], BAIs also have their limitations. A low concentration of BAIs does not necessarily indicate microbiological quality, as not all spoilage or starter microorganisms are capable of decarboxylating free amino acids [156]. Furthermore, common food infections such as Listeria do not lead to the formation of amines [136]. The BAI is therefore restricted to the indication of spoilage and is not a suitable indicator of the general risk of infection.

4.8. Total Volatile Basic Nitrogen

TVB-N can be used to indicate the freshness of meat [146,157]. Nevertheless, the widespread application of TVB-N as a spoilage indicating degradation product is constrained by the inconsistency and scarcity of findings pertaining to the correlation between TVB-N content and the freshness of diverse types of meat, e.g., beef, pork, chicken, and fish [146]. The degradation of nitrogenous protein and non-protein compounds results in the formation of a range of microbial degradation products, including nucleotide catabolites and amino acids, which contribute to an increase in TVB-N content. This is indicative of meat spoilage [157]. It can be reasonably inferred that microorganisms are also responsible for elevated TVB-N levels, as evidenced by the proliferation of Pseudomonas spp., including Pseudomonas taetrolens and Pseudomonas fragi during the storage of beef under air atmospheres and at 4 °C [108]. Furthermore, TVB-N is linked to the decarboxylase activity of microorganisms with regard to amino acid during the storage period [158]. Additionally, a correlation between TVB-N in chicken meat and microbiological and sensory values can be demonstrated, sensory values are observed to decrease with increasing TVB-N content [159]. Moreover, [160] demonstrates that the concentrations of putrescine, cadaverine and tyramine in beef, pork and chicken have a strong positive correlation with the TVB-N content during storage. The concentration of putrescine, cadaverine and tyramine increases with increasing TVB-N content [161]. Consequently, the increase in TVB-N is also delayed, with an increase occurring after a few days of storage [147]. This phenomenon is attributed to the initial consumption of glycogen by putrefactive bacteria, which subsequently initiates a shift towards protein degradation, leading to an increase in TVB-N. The existing literature shows consistency regarding the impact of TVB-N on the microbiological and sensory characteristic of meat. However, there is a lack of consensus regarding the optimal TVB-N threshold, which is supposed to indicate the freshness of meat, but no uniform limit has been established [146].

4.8.1. Temperature

A slight correlation between the TVB-N value and the temperature is discerned. TVB-N increases at a gradual rate at low temperatures, which is likely due to a delay in bacterial growth at lower temperatures [162]. A similar phenomenon is observed in a study examining the storage of beef at temperatures of 4 °C and 28 °C. At 4 °C (start value: 0.001% N) the initial TVB-N limit of 0.017% N is exceeded after seven days, whereas at 28 °C (start value: 0.010% N) this limit is reached after 8 h. Consequently, the rate of TVB-N increase is slower at lower temperature. [163] A number of studies have examined the impact of temperature on the concentration of TVB-N, with experiments conducted at a range of temperatures, including −18 °C [107], 3 °C [164], 4 °C [107,108], 5 °C [162] and 28 °C. The findings indicate that the content increases at a faster rate with increasing temperature, for example, the TVB-N limit of 15 mg/100 g, other factors being equal, is exceeded at −18 °C between 466 and 494 days, while at 4 °C it is already exceeded after five days [107]. This evidence supports the conclusion that the TVB-N content is temperature dependent and increases with rising temperatures.

4.8.2. Atmosphere

The development of TVB-N compounds on chicken breast is highly dependent on the storage atmosphere. Observation of the evolution of TVB-N on chicken breast during different storage phases and in different atmospheres at 5 °C shows that in all atmospheres (A—0.03% CO₂, 21% O₂, 78% N₂; B—50% CO₂, 50% O₂; C—100% CO₂) there is an increase in TVB-N concentration during storage. On day 0, TVB-N levels are below 15 mg per 100 g and increase to 28.689 ± 0.15 (C), 30.831 ± 0.70 (B) and 40.475 ± 0.80 (A) mg per 100 g by day 9, depending on the atmosphere. With increasing CO₂ content (50 and 100%), the magnitude of the TVB-N increase decreases [158]. Similar results can be indicated in turkey meat, where this effect is associated with inhibition of Pseudomonas spp. and Enterobacteriaceae under MAP atmospheres [2]. In general, studies are available on different atmospheres, air [108], vacuum [108,164] and MAP [158,164]. In a comparison between beef storage in air and vacuum-packed, it is indicated that the established TVB-N limit of 20 mg/100 g is reached faster in air (after nine days) than in vacuum (after 21 days) [108]. In contrast, the atmosphere does not appear to have a significant effect on TVB-N levels during turkey meat storage [164]. The TVB-N content thus increases with increasing O₂ content and decreasing CO₂ content and is thus dependent on the atmosphere. However, the dependence is not too strong, since in all the studies mentioned a comparable increase in TVB-N content occurred in all atmospheres.

4.8.3. Meat Type

There is a diversity of recommendations pertaining to TVB-N spoilage limits, which are contingent upon the specific categories of fish, seafood, and meat in question. Consequently, it is not feasible to consider these recommendations as universally applicable [146]. The disparate recommendations are attributable to the variations in physiology, meat characteristics, and nutrient content among the diverse types of meat [157]. In a study on chicken and beef, a limit of 20 ± 2 mg/100 g is set, which is reached by chicken when beef is still at a value of 5 mg/100 g [165]. A further TVB-N limit is utilized to ascertain the spoilage point of pork, with an assumed value of 15 mg/100 g of meat, derived according to Chinese standard GB 2707-2016. The study reveals that the specified limit is reached after eight days storage of at 4 °C [166]. A further analysis of the spoilage of pork at the TVB-N limit of 15 mg/100 g indicates that this limit is exceeded after five days at 4 °C. The initial TVB-N level is 3 mg/100 g, and a rapid increase is indicated from day 3 onwards [107]. In particular, the more pronounced increase observed after a few days lends support to the suggestion that TVB-N may be employed as a degradation product. Other TVB-N limits are reported in the scientific literature, including 17 mg/100 g for beef [167], and 20–30 mg/100 g for beef and pork [158]. Moreover, there are studies on duck meat [157], seafood [159] and, turkey meat, although the TVB-N value for turkey meat appears to be questionable [143]. A further possibility, as demonstrated by the analysis of TVB-N content in chicken breast and thigh meat stored at 4 °C, is that different cuts of meat from the same species may exhibit differing TVB-N values. The samples exhibited disparate TVB-N levels at both day zero and day 15, with higher levels observed in the thigh than in the breast meat [147]. The TVB-N content can be utilized as a degradation product for diverse types of meat. While the permissible limits per type of meat and, in select instances, per individual piece of meat, do vary, they typically range from 15 mg/100 g [107,166] to 30 mg/100 g [158].

5. Discussion

The potential for intelligent packaging to reduce meat spoilage represents a promising avenue for future research. However, they are still in the developmental phase, which is why it is crucial to identify degradation products of meat spoilage in order to develop indicators for indicating meat spoilage in intelligent packaging. This review identifies potential degradation products and discusses them in detail.

Table 3. The following is a summary of the most significant findings of the review, with the objective of presenting a preliminary classification of the degradation products under consideration. Nevertheless, the possibility of the existence of additional degradation product categories or specific substances within these categories under the specified conditions cannot be ruled out. Moreover, no definitive correlations can be identified, as the formation of degradation products is contingent upon specific conditions and the presence of various substances.

Degradation Product	Related Microorganisms	Conditions in Which the Degradation Products Are (Frequently) Measured
		Temperature	Atmosphere	Meat Type
Aldehydes	Pseudomonas spp. Carnobacterium spp. Enterobacteriaceae [11,90]	Increases with increasing temperature [15,94,101]	MAP (30–70% O2, 20–40% CO2, 10–30% N2) [98,106] Vacuum [168] Aerobic [103,168]	Beef [102] Pork [102] Poultry [102,107,169]
Alcohols	Pseudomonas spp. Carnobacterium spp. [1]	Various statements: increases with rising temperature [94] or indication at 4 °C, but not at higher temperatures (10 °C) [14]	Mainly aerobic [99,114] MAP with high amount of O2 (70% O2, 20% CO2, 10% N2) [98] Vacuum [168]	Beef [102] Chicken [111,170]
Ketones	Brochothrix thermosphacta Carnobacterium spp. Lactobacillus spp. [106] Photobacterium spp. [99]	Increases with increasing temperature [14]	MAP (70% O2, 20% CO2, 10% N2) [98] Vacuum [109] Aerobic [94]	Beef [49] Pork [91] Poultry [36,106],
Sulfur Compounds	Enterobacteriaceae Pseudomonas [127]	Increases with increasing temperature [126] Indication at 4 °C [119] Indication at room temperature [123]	MAP (30–60% O2, 40% CO2, 30% N2) [45,106] Aerobic [45,123]	Beef [106,113,123] Pork [119] Poultry [106,113,123]
Esters	Pseudomonas fragi [11,96] Pseudomonas spp. [96]	Mostly formed between—1.5 °C and 7 °C	MAP (60–80% O2, 20–40% CO2) [96,99] Aerobic [11,43,111]	Pork [95,99] Chicken [111]
Carboxylic acids	Brochothrix thermosphacta Carnobacterium spp. [131]	Indicated between—3.5 °C [98] and 21 °C [123]	MAP (80% O2, 20% CO2) [90] Aerobic [115,136] Vacuum [115] Increases with CO2 increase [136]	Beef [45,50,115,123,135] Chicken [90,115,123,135]
Biogenic amines	Enterobacteriaceae [138,145,154] Pseudomonas spp. [138,153] Lactic acid bacteria [127]	Increases with increasing temperature [142] Indicated below 6 °C [142,143] to—18 °C [155]	MAP (0–75% O2, 25–30% CO2, 0–70% N2) [46,145,146] Aerobic [46,146,161] Vacuum [161]	Beef [154,163] Pork [17] Poultry [17,145,146,155,163,164] Fish [138] Lamb [17]
Total volatile basic nitrogen	Pseudomonas spp. [120]	Indicated between—18 °C [91] and 28 °C [171] Increases faster with increasing temperatures [91]	MAP (0–50% O2, 0.03–100% CO2, 0–78% N2) [2,136,172] Aerobic [120] Vacuum [120,172]	Beef [136,173] Pork [91,136] Poultry [146,174,175] Seafood [176,177]

provides an overview of the results of the review. Nevertheless, when looking at the individual conditions in relation to the respective degradation products, no general correlation can be established, as there are more promising (e.g., acetic acid in relation to the wide temperature range which it can be identified) and less promising (e.g., ethyl acetate, which is only mentioned in the literature in connection with pork, so it remains unclear whether it can also be identified in other types of meat) results for specific substances within a degradation product.

The implementation of a method for identifying instances of meat spoilage through the utilization of freshness indicators across a spectrum of food packaging requires the formation of degradation products across a range of temperatures, atmospheres, meat types and microbial species associated with meat spoilage. As detailed in chapter 3, these factors exert a considerable influence on the concentration of these degradation products.

The degradation products considered are formed via disparate mechanisms, yet they can all be generated during the process of meat spoilage, as evidenced by the various studies presented in chapter 4. These degradation products are mainly due to the presence of Pseudomonas (aldehydes, alcohols, ketones, sulfur compounds, esters, BAs, TVB-N), Carnobacterium spp. (aldehydes, alcohols, ketones, carboxylic acids), and/or Enterobacteriaceae (aldehydes, sulfur compounds, BAs, TVB-N), which are involved in the spoilage of meat. In order to assess the feasibility of indicating meat spoilage using freshness indicators, a range of factors influencing the formation and concentration of degradation products are taken into consideration. These influences include temperature, atmosphere, meat type, and the presence of meat spoilage microorganisms. A discussion of the results pertaining to the individual categories of degradation products is presented in the following sections.

5.1. Aldehydes

Not all aldehydes are formed by microorganisms (mainly Pseudomonas spp., Carnobacterium spp. and Enterobacteriaceae [11,88], e.g., hexanaldehyde can also be formed by non-microbial factors such as the hydrolysis of triglycerides [106,178], although it is among the most commonly found aldehydes in spoiled meat [11]. The aldehydes also formed by non-microbial factors are therefore unsuitable for indicating meat spoilage. However, some aldehydes such as 3-methylbutanal are directly linked to spoilage different meat types in various studies. Certainly, the atmosphere plays a limiting role in this respect. For example, 2-butenal and malonaldehydes are associated with aerobically packaged [95,169], acetaldehyde with vacuum packaged [95], and octanal, nonanal, 2 and 3-methylbutanal with MAP packaged [86] spoiled meat. Not only the atmosphere, but also the type of meat leads to the formation of different aldehydes, due to the different composition of fatty acids available for oxidation depending on the type of meat [88]. Temperature also has an effect on aldehyde concentration, which increases as the temperature rises [15]. Overall, the presence and type of aldehydes vary too much with meat type, atmosphere and (non-)presence of microorganisms in meat spoilage to be applicable for a general indicator of spoilage in different packaging concepts.

5.2. Alcohols

There are rather fewer recent studies [14,101] investigating alcohol as a degradation product in meat. Instead, the majority of studies (e.g., [174,179]) investigate alcohol more as an indicator producer of food spoilage. Since alcohols are formed by the breakdown of carbohydrates, oxidation and hydrolysis of fats, or reduction of ketones and aldehydes, their concentration increases only with prolonged storage [1,90] of seven [14] to 12 days [96]. Similarly, temperature also has an effect on alcohol formation, but studies disagree on whether formation increases with rising temperature [82] or whether alcohols can no longer be indicated at a higher temperature [14]. Microorganisms for alcohol production are mainly Pseudomonas spp. and Carnobacterium spp. [1], although alcohols are described as unsuitable as degradation products, as some of them (e.g., 1-octen-3-ol and 1-pentanol) occur in the presence and absence of microorganisms [14,87]. As with aldehydes, there is alcohol formed under different atmospheres for these degradation products. In general, alcohols appear to be formed mainly in the presence of O₂ [87,97], although another study has found an increase or decrease in ethanol under aerobic and vacuum conditions, depending on the type of meat [95]. For chicken meat, ethanol seems to be a possible degradation product [101,180], but this correlation between ethanol and Pseudomonas spp. formation does not always seem to exist, which argues against the use of ethanol as a degradation product [14]. Thus, the studies on the development of alcohols during meat storage do not show a clear general trend, but depend on environmental influences or are contradictory depending on the study.

5.3. Ketones

Acetoin and diacetyl are considered the major ketones for off-flavors in packed meat [1], but are also present in fresh meat [102]. In addition, acetoin shows inconsistent increases and decreases in concentration during storage of poultry [96] and pork [107], with increases associated with Brochothrix thermosphacta, Carnobacterium spp. and Lactobacillus spp. and decreases associated with possible concomitant diacetyl formation [96]. Furthermore, ketone formation is associated with Photobacterium spp. [87]. Diacetyl shows an increase in concentration and subsequent decrease during chicken storage [84]. In addition to pork and poultry meat, ketones are also indicated in the spoilage of beef [46]. Thus, these typical ketones do not show a significant increasing correlation with meat spoilage. Furthermore, ketone formation increases with increasing temperature [14], but there are conflicting statements as to whether this effect is shown only under air [82] or also under MAP atmospheres [14]. In general, no clear correlations can be made with the behavior of ketone concentration and different storage atmospheres. For example, acetoin can be formed under air and under vacuum conditions [103], while another study indicates acetoin and diacetyl under air but not under vacuum conditions [108]. Overall, there are conflicting conclusions as to whether or not ketones can be used as an indicator of meat spoilage, like [14] denies ketones as an indicator of meat spoilage [14] while [98] indicates 2,3-octanedione and 3,5-octanedione, among others, as possible degradation products at least for beef [14,86].

5.4. Sulfur Compounds

Dimethyl sulfide is the most abundant sulfur compound in spoiled meat [11,112] and has already been identified as a possible degradation product in several studies [14,42,96,111,112]. Dimethyl sulfide increases with increasing temperature and storage time [117] and can be associated with spoilage of various meats, such as beef and poultry [96,111,112], and sulfur compounds are also indicated in spoiled pork [116]. Sulfur compounds are mainly produces in the presence of Enterobacteriaceae and Pseudomonas, which grow under aerobic and anaerobic conditions [118]. Thus, sulfur compounds are indicated under air conditions [42,111]. Inconsistencies exist in indicability under MAP, with some studies [42,96] indicating sulfur compounds under MAP and other studies not [42,116]. There have been initial developments of indicators to show the spoilage of chicken meat by sulfur compounds [115,120] and also that sulfur compounds are considered to be one of the first indicatable degradation products [6]. Sulfur compounds are not necessarily suitable as indicators of meat spoilage because they occur in somewhat lower concentrations than other degradation products such as alcohols, ketones [101] or esters [83,111]. Moreover, sulfur compounds can also be indicated in fresh meat [83] and show a decrease with progressive meat spoilage [109]. All these inconsistencies suggest that the use of sulfur compounds as an indicator of meat spoilage is rather inappropriate.

5.5. Esters

Although esters are also considered to be early indicators of meat spoilage [6], only one [122] of the studies identifies them as degradation products. The other studies show ester indication in pork under MAP [87] and chicken under air conditions, but only at low concentrations of <<peak area × 10⁴ [101]. Ambiguous results, such as a concentration increase and subsequent decrease [83] or negative correlations [90], are also present in the literature. Since esters are mostly formed by Pseudomonas fragi, ester formation usually occurs under aerobic conditions [11,85], which argues against their wide application in different packaging concepts. However, as esters are formed in the temperature range (−1.5 to 7 °C [127]) in which fresh meat should be stored, they are suitable as decomposition products to indicate meat spoilage at least in relation to the temperature.

5.6. Carboxylic Acids

Different carboxylic acids are formed depending on the storage temperature [181]. Hexanoic acid [11], 3-methylpentanoic acid, pentanoic acid, octanoic acid, nonanoic acid, decanoic acid and oleic acid [119] are formed at cooler temperatures (up to 6 °C) and propanoic acid, 2-methylbutanoic acid, 4-methylpentanoic acid are formed at warmer temperatures (21 °C) [111], although in reality meat is not stored at such high temperatures. Both apply to acetic acid and butanoic acid, which are formed at cooler and warmer temperatures [104,111]. Fatty acids that can be indicated under different atmospheres include acetic acid [11], butanoic acid [42], and hexanoic acids [47], while branched-chain fatty acids are formed only in the presence of air [11]. Different carboxylic acids can be determined in different meat types (chicken and beef), with acetic acid, butanoic acid [104,111] and pentanoic acid [104,119] indicated in both meat types. Of the carboxylic acids, acetic acid, butanoic acid, and pentanoic acid appear to be the main degradation products, as they are formed in at least two types of meat and in different atmospheres. Advantages of carboxylic acids as suitable degradation products to indicate meat spoilage are that they can be indicated in the headspace of a package [88] and in one study dominate other volatiles [119]. On the downside, acids self-oxidize when glucose is removed, and as meat has a variable glucose content [91], this argues against the use of acids as degradation products.

5.7. Biogenic Amines

Biogenic amines (BAs), especially putrescine, histamine, cadaverine and tyramine [29] have already been identified as possible degradation products [130], as their concentration increases with storage time [139,150], whereas for other BAs, such as spermine or spermidine, it remains constant [139] or even decreases [43]. However, contrary studies find no significant histamine increase [137,139]. The formation of BAs depends on several factors. These include the composition of the meat, its processing and preservation, the characteristics of the growing microorganisms and the storage conditions [130,131,132,133]. The BAs concentration increases with increasing temperature, especially above 6 °C [140,182] and shows a greater influence on poultry meat than on other types of meat, such as pork or beef [144]. Also depending on different bacterial strains, different BAs are formed [134]. Cadaverine is mainly associated with Enterobacteriaceae and tyramine, putrescine, histamine with Pseudomoas spp., Brochothrix thermosphacta, pychrotrophs [137] and lactic acid bacteria (LAB) [118]. Due to more favorable aerobically growth characteristics of Pseudomonas spp., higher concentrations of putrescine and cadaverine are indicated under aerobic conditions than under MAP [43,147]. However, an increase in the concentration of BAs is also evident under MAP [139]. Another study shows that an increase in concentration can be indicated not only in air, but also in a vacuum [150]. That BAs can be indicated in the spoilage of various meat types, such as chicken, turkey, pork, lamb, and beef [17], argues for their application as degradation products. Moreover, for broader application, the presence of different BAs can be combined to form a BAI. However, there is currently no single BAI, but a variety of different ones, as shown in Table 2. Despite some of the advantages mentioned, BAs also have the disadvantage that not all microorganisms can decarboxylate free amino acids, so a low BA concentration does not automatically indicate high microbiological quality [156]. The suitability of BAs as indicators of meat spoilage is therefore limited. If used, it should be noted that cadaverine, putrescine, and tyramine have the greatest potential and can therefore be used individually or combined in a BAI.

5.8. Total Volatile Basic Nitrogen

TVB-N content is mainly associated with Pseudomonas spp. and Enterobacteriaceae [90]. The researched literature [90,108,158] shows consistency regarding the effect of TVB-N on microbiological and sensory characteristics of meat. Furthermore, a slower TVB-N increase is evident at lower temperatures [162], and TVB-N concentration raises with increasing temperature [107,163]. TVB-N compounds are formed under different atmospheres, with less formation in aerobic packages [108,158]. Due to the different physiology, meat characteristics, and nutrients in different meat types, different TVB-N perishability limits, per meat type [146] or even piece of meat within a species [147], must be established. This makes a general application challenging, but an application is still conceivable if an individual TVB-N perishability limit is determined and set for each type of meat and perhaps also for each piece of meat within a type of meat. In the researched literature, the following limits are given for different meat types 20 mg/100 g for chicken [175], 17 mg/100 g [167], 20 mg/100 g [108] or 20–30 mg/100 g [158] for beef and 15 mg/100 g [107,166] or 20–30 mg/100 g [158] for pork. TVB-N compounds can thus find application as an indication of meat spoilage in different packaging concepts and for different types of meat. A possible application to indicate meat freshness has also been reported in the researched literature [146,157], with conflicting and limited results reported on the relationship between TVB-N content and meat freshness [146].

6. Conclusions

The objective of this review is to establish a basis for the search for optimal degradation products whose concentration correlates with the shelf life of meat, and to evaluate their potential for indicating meat spoilage. In order to achieve this objective, this review considers eight different degradation products.

In light of the results presented in this review, it can be concluded that an indication pathway via a single degradation product is a challenging proposition. While some degradation products (aldehydes, alcohols, ketones, sulfur compounds, esters) seem to be less suitable as spoilage indicators, others (carboxylic acids, BAs, TVB-N) demonstrate greater potential. Although the more suitable degradation products also have disadvantages, as previously discussed, individual substances in these groups can be identified as potential degradation products. These include acetic acid, butanoic acid, and pentanoic acid, which belong to the carboxylic acids, and cadaverine, putrescine, and tyramine, which are classified as BAs. Additionally, these three BAs can be utilized in combination as BAI.

The use of TVB-N has the greatest potential, but the spoilage limits of TVB-N need to be determined and standardized for widespread use. Additional research is also required to ascertain the potential of combinations of different degradation products. Prioritization should be given to combinations of carboxylic acids, BAs, and TVB-N, as these have the greatest potential for indicating meat spoilage using freshness indicators in intelligent packaging concepts.

Further research may be warranted regarding the following steps. A systematic analysis should be conducted to determine the role of intelligent packaging in improving the shelf life and safety of meat. This analysis should identify specific degradation products that correlate with meat spoilage, taking into account the various influencing factors. It would be beneficial to assess the potential integration of these degradation products into intelligent packaging concepts, with a view to ensuring their suitability for real-world applications. This could be achieved by investigating the influence of different factors, such as temperature, atmosphere, and meat type. Furthermore, it would be advantageous to gain a clear understanding of the mechanisms underlying meat spoilage and the effectiveness of different degradation products as indicators for intelligent packaging applications. Ultimately, this research could contribute to reducing food wastage and improving food safety by providing insights into the development of more accurate and consumer-friendly spoilage detection methods.