Flavour development in meat

J.S. Elmore , D.S. Mottram , in Improving the Sensory and Nutritional Quality of Fresh Meat, 2009

Abstract

Meat flavour forms during cooking, as a result of the Maillard reaction and lipid oxidation. Compositions of both Maillard precursors, i.e. sugars and amino acids, and lipids can be influenced by several factors. This chapter examines how meat flavour can be affected pre-slaughter (by the animal's diet, breed, and slaughter age, for example) as well as post-slaughter (as a result of conditioning and preservation). The characteristic aroma compounds of cooked meat from different species are discussed, as well as those compounds that cause undesirable flavours in meat, due to spoilage, contamination or diet. Methods for analysing cooked meat aroma are described and future developments in meat flavour research are discussed.

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CHEMICAL AND PHYSICAL CHARACTERISTICS OF MEAT | Palatability

R.K. Miller , in Encyclopedia of Meat Sciences (Second Edition), 2014

Chemical Development and Reactions of Meat Flavor

Cooked meat flavor is the result of chemical reactions that occur within and between the lipid and lean portions of meat during cooking. Raw meat contains very little aroma. Raw meat aroma can be described as bloodlike or one that has a serumy taste, but precursors to cooked meat flavor are contained within raw meat even though in the raw state these precursors are nonvolatile or nondetectable. In general, cooked meat flavor develops as a result of interactions between amino acids, peptides, reducing sugars, vitamins, and nucleotides or their breakdown products during cooking from the lean component. Lipids also play a role in meat flavor and much of the species-specific flavor of meat is derived from adipose tissue. Lipid degradation and oxidation both contribute to meat flavor, usually negatively by contributing off-flavors.

Sulfurous- and carbonyl-containing volatile compounds are thought to be mainly responsible for flavor aromatics in meat. These chemical reactions are complex and intermediate reaction products can interreact with multiple products.

From a sensory standpoint, meat flavor is segmented into aroma or smell before consumption by the olfactory senses, the flavor aromatic perceived by the olfactory senses during chewing, basic tastes of sweet, sour, bitter, and salty sensed from taste receptors on the tongue; mouthfeels identified from the trigeminal receptors in the mouth that provide astringent and metallic sensory attributes; and aftertastes that are perceived after swallowing that are almost always flavor attributes perceived by the olfactory senses. The underlying chemical components that contribute to these sensory attributes have been extensively studied.

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MUSCLE FIBER TYPES AND MEAT QUALITY

T. Astruc , in Encyclopedia of Meat Sciences (Second Edition), 2014

Flavor

Meat flavor stems largely from its lipid content. Oxidative muscle fibers packed with cellular organelles such as mitochondria are widely recognized as having higher phospholipid content than that by type-IIX and -IIB muscle fibers. Type-I fibers also contain intracellular lipid reserves in the form of lipid droplets that are only rarely observed in type-IIX and -IIB fibers. However, there are muscle lipid reserves stored in adipocytes that are usually found in the perimysium of all muscles, regardless of fiber type composition. Finally, the range of different results obtained to date is too wide to converge on a clear and specific relationship linking fiber type to total lipid content.

Given the range of compositional differences in phospholipids, glycogens, soluble proteins and so on, it is more likely that flavor is partly dependent on fiber type composition. Further targeted studies are needed in order to investigate the effect of fiber type on meat flavor.

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SPECIES OF MEAT ANIMALS | Sheep and Goats

E.L. Walker , M.D. Hudson , in Encyclopedia of Meat Sciences (Second Edition), 2014

Factors affecting meat flavor

Components of meat flavor are both fat and water soluble, but the water-soluble components are relatively similar across species and are the main reason why fat-free products taste similar across species. It is the fat component of meat that primarily contributes to species differences in flavor. The total and ratios of fatty acids present (often referred to as the fatty acid profile) in the adipose tissue of lamb and chevon influence flavor perception by consumers. Forages contain a variety of odoriferous and reactive fat-soluble components that ultimately are deposited as flavor precursors in the muscle. These compounds can accumulate within adipose tissue over time and are perceived as either positive or negative meat flavors (depending on culture or geographical origin of consumers). These flavor-influencing compounds are more prevalent in chronologically older animals and can contribute to the distinct flavor differences between young and old animals. Generally, as animal's age, flavor intensity increases, often to the point of undesirability for many consumers. In some areas of the world it is common to harvest and consume intact males, the meat from which may be perceived as less tender with stronger flavor.

In the USA, more than 80% of lambs marketed are finished on high-concentrate (predominantly corn) diets, yet, in many countries, lambs are fed 100% forage diets. Lamb consumers accustomed to corn-finished lamb perceive forage-finished lambs to have 'lamby' or 'grassy' flavors. Those accustomed to the more common worldwide production practice of pasture- or forage-finished lambs find the corn-finished lamb to be mild, lacking in traditional lamb flavor, and too fat. Little research has been conducted regarding the effects of diet on sensory characteristics of goat meat.

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COOKING OF MEAT | Flavor Development

R.B. Pegg , F. Shahidi , in Encyclopedia of Meat Sciences (Second Edition), 2014

Abstract

Processes shaping the development and deterioration of meat flavor are extremely complex. It is a continual process involving both the generation and loss of desirable flavor compounds and the formation of warmed-over flavor notes. Many factors, such as the animal's status preharvest to postmortem handling of the meat will impact the flavor of the cooked product. Although the majority of meat flavor is lipid in origin, the contribution of proteinaceous compounds and their reaction with reducing sugars to form Maillard reaction products and Strecker aldehydes are important. Meat scientists continue to strive to better understand the complex issues pertaining to meat flavor and to seek strategies at mitigating the loss of desirable aroma constituents.

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Lipid-derived flavors in meat products

F. Shahidi , in Meat Processing, 2002

5.2 The role of lipids in generation of meaty flavors

The role of lipids in meat flavor generation has been the subject of extensive studies. It has been suggested that the basic meaty aroma of beef, pork and mutton is the same and is derived from the water-soluble fraction of the muscle which is a reservoir of low-molecular-weight compounds ( Hornstein and Crowe, 1960; 1963). Meanwhile, species-specific flavors in meats originate from the involvement of their lipid components in Maillard reaction during heat processing. Lipids may contribute both to desirable and undesirable flavors of meat from different species. Their effect on generation of desirable aromas in cooked meats may arise from mild thermal oxidative changes which produce important flavor compounds; they may also react with components of lean tissues to afford other flavor compounds and may act as a carrier for aroma compounds, thus affecting their sensible threshold values.

The effect of both triacylglycerols (TAG) and phospholipids (PL) on the development of meaty aroma in meat and model systems has been studied (Mottram, 1983, 1991; Mottram and Edwards, 1983). In these studies, a meat sample was heat processed as such or was extracted with hexane or methanolchloroform prior to cooking. While there was little difference in the development of meaty aroma in the untreated and hexane-extracted samples, meat extracted with methanol-chloroform had little meaty aroma, but possessed a sharp roast and biscuit-like odor. In particular, the concentration of dimethylpyrazine in the headspace volatiles was significantly increased with a concurrent decrease in the content of lipid oxidation products. Thus, it was concluded that phospholipids present in the intramuscular lipids of meat were primarily responsible for the development of meaty aromas.

Based on these experiments, other model systems were devised in order to unravel the role of phospholipids in the formation of Maillard reaction products (MRP) (Farmer and Mottram, 1990). Cysteine and ribose, with or without phospholipids, were used to assess the involvement of lipids in the formation of aroma compounds via Maillard reaction following heating in buffered solutions.

There was a marked reduction in the amount of thiols when phospholipids, and to a lesser extent triacylglycerols, were present (Table 5.1). The compounds that were formed only in the presence of lipids were 2-pentylpyridine, 2- alkylthiophenes, alkenylthiophenes, pentylthiapyran and alkanethiols (Table 5.1). Furthermore, the impact of PL was much greater than that of TAG in affecting the flavor of systems under investigation. Formation of 2-alkylheterocyles was generally due to the reaction of 2,4-decadienal with ammonia or hydrogen sulfide formed from cysteine or other precursors (Fig. 5.1). Direct reaction of hexanal with amino acids would also lead to the formation of 2- hexylpyridine. Formation of other alkyl substituted heterocyclic compounds from participation of lipid- derived aldehydes is also contemplated. Reaction of 2,4-decadienal directly with amino acids has also been reported. The compound 2,4-dacadienal is a major breakdown product of the omega-6 fatty acids. Furthermore, 1-heptanethiol and 1-octanethiol were present only in the systems containing phospholipids and their formation is presumed to be due to the interaction of alcohols with hydrogen sulfide. Furthermore, MRPs may also interact with meat lipids by acting as antioxidants in order to stabilize them. Bailey (1988), and Bailey et al. (1997) have demonstrated that MRPs act as important antioxidants in meat model systems. It has also been shown that furanthiols and thiophenethiols exert antioxidative activity in lipids (Eiserich and Shibamoto, 1994), as well as in aqueous solutions as evidenced by their tyrosyl radical scavenging effect (Eiserich et al., 1995). The antioxidant activity of these compounds was similar to that of ascorbic acid. Figures 5.1 and 5.2 show examples of involvement of lipids in Maillard reaction and formation of volatile flavor compounds.

Table 5.1. Relative concentration of selected acyclic and heterocyclic volatiles from the reaction of cysteine and ribose in the absence or presence of beef triacylglycerols (BTAG) and beef phospholipids (BPL)

Compound No lipid BTAG BPL
3-Pentanone, 2-mercapto 1 0.72 0.49
2-Pentanone, 3-mercapto 1 0.77 0.47
2-Furylmethanethiol 1 0.67 0.63
3-Furanethiol, 2-methyl 1 0.40 0.15
2-Thiophenethiol 1 0.32 0.03
3-Thiophenethiol, 2-methyl 1 0.08 0.01
Pyridine, 2-pentyl 0 0.09 1
Thiophene, 2-pentyl 0 0.00 1
Thiophene, 2-hexyl 0 0.15 1
2-H-Thiapyran, 2-pentyl 0 0.10 1

Fig. 5.1. Production of long-chain alkyl heterocycles from the reaction of 2,4-decadienal with ammonia and hydrogen sulfide.

Fig. 5.2. Involvement of hexanel, an oxidation product of linoleic acid, in production of a trisulfide heterocyclic compound.

Other potentially desirable flavor components that might be formed in processed meats are free fatty acids and related compounds which are prevalent in dry-cured-ham. These compounds are formed in such products via fermentation reactions. The flavor characteristics of dry-cured ham has recently been reported (Toldra et al., 1997).

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Sensory Properties Affecting Meat and Poultry Quality

Maurice G. O'Sullivan , in A Handbook for Sensory and Consumer-Driven New Product Development, 2017

Factors Affecting Meat Flavour

Over 1000 volatile compounds have been identified in cooked meat (Elmore and Mottram, 2009). Meat flavour is a combination of taste and odour; however, mouthfeel and juiciness of meat also affect the individual flavour perception (Farmer, 1992; Robbins et al., 2003). Meat flavour is characteristic of volatiles produced as a result of reactions of nonvolatile components that are induced thermally (Khan et al., 2015). Uncooked meat has little or no aroma and only a blood-like taste. Only after cooking and a series of thermally induced complex reactions that occur between many different nonvolatile compounds of the lean and fatty tissues does meat become flavoursome (Mottram, 1998; Calkins and Hodgen, 2007). Fats and additionally low-molecular-weight water-soluble compounds constitute the most important precursors of cooked meat flavour (Resconi et al., 2013). The broad array of flavour compounds found in meat include hydrocarbons, aldehydes, ketones, alcohols, furans, thriphenes, pyrrols, pyridines, pyrazines, oxazols, thiazols, sulphurous compounds and numerous others (MacLeod, 1994). Many of these sulphur compounds contribute sulphurous, onion-like and, sometimes, meaty aromas (Fors, 1983). Roast flavours in foods are usually associated with the presence of heterocyclic compounds such as pyrazines, thiazoles and oxazoles (Mottram, 1998).

Additionally, lipid oxidation, Maillard's reaction, interaction of lipid oxidation products with Maillard's reaction products and vitamin degradation are thermally induced reactions producing volatile flavour components responsible for the characteristic cooked meat aroma (MacLeod, 1994).

Typically, fresh red meats are stored in MAP containing 80% O2:20% CO2 (Georgala and Davidson, 1970) and cooked meats are stored in 70% N2:30% CO2 (Smiddy et al., 2002). Zakrys et al. (2008) reported that sensory panellists expressed a preference for cooked beef steaks stored in packs containing O250% and O280%, despite detecting oxidised flavours under these conditions. In general, muscle foods are susceptible to oxidative activity of their lipid, protein, pigment, vitamin and carbohydrate composition (Kanner, 1994). Additionally, there is evidence that off-flavours or taints may develop in MAP meat due to CO2 dissipation into tissue and the formation of carbonic acid (Nattress and Jeremiah, 2000; O'Sullivan et al., 2010).

Polyunsaturated fat levels vary with species and are higher in poultry meat followed by pork, lamb and beef. Thus meats like chicken are particularly susceptible to lipid oxidation. Duck meat has higher lipid content than chicken and turkey meat and is more susceptible to oxidation as it contains high levels of unsaturated fatty acids (around 60% of total fatty acids) and also high levels of haemoglobin and myoglobin (Baéza et al., 2002). The oxidation of polyunsaturated fatty acids in meat causes the rapid development of meat rancidity and also affects colour, nutritional quality and meat texture (Kanner, 1994; Zakrys et al., 2009). The products of fatty acid oxidation produce off-flavours and odours usually described as rancid (Gray and Pearson, 1994). As rancid flavours develop, there is also a loss of desirable flavour notes; however, it is difficult to determine the limiting point at which beef can be rejected due to lipid oxidation, based on sensory perceptions (Campo et al., 2006).

The proportion of polyunsaturated fat in muscle can be also altered by feed intervention. Feeding a more unsaturated diet will result in greater concentrations of polyunsaturated fat within the muscle tissue which in turn affects meat flavour and reduces oxidative stability. Antioxidants are compounds that inhibit or retard the free radical generating chain mechanism of lipid oxidation (O'Sullivan et al., 1998). Many studies have been conducted on the basis that incorporation of α-tocopherol into the cell membrane will stabilise the membrane lipids and consequently enhance the quality of meat during storage. Oxidation studies with chicken (Jensen et al., 1995), turkeys (Marusich et al., 1975), pigs (O'Sullivan et al., 1997, 1998), cattle (Faustman et al., 1989), veal (Shorland et al., 1981) and fish (Frigg et al., 1990) have all demonstrated reduced lipid oxidation in muscles and adipose tissue from animals supplemented with dietary α-tocopherol compared to the same muscles from nonsupplemented animals.

The oxidation of fatty acids occurs due to the exposure to O2 and is accelerated in the presence of light and catalysts, such as free iron, and similar to the mechanism described previously for pigment oxidation (O'Sullivan and Kerry, 2011). The oxidation of unsaturated fatty acids is a free-radical chain reaction with three stages. (1) initiation, the formation of free-radicals; (2) propagation, the free-radical chain reactions; (3) termination, the formation of nonradical products (Tappel, 1962; O'Sullivan and Kerry, 2008). Preformed fatty acid hydroperoxides react with haeme compounds and undergo homolytic decomposition (Fig. 11.2). The alkoxy (LOradical dot) radical formed in turn propagates the peroxidation reaction. Lipid hydroperoxides may also be decomposed by ferrous iron (Fig. 11.3), which form very reactive alkoxy radicals. However, ferric iron produces the less reactive peroxy (LOOradical dot) radicals from fatty acid hydroperoxides (Ingold, 1962). Transition metals, notably iron, are believed to be pivotal in the generation of species capable of abstracting a proton from an unsaturated fatty acid (Gutteridge and Halliwell, 1990; Kanner, 1994). Seman et al. (1991) suggested that ferritin may be responsible for catalysing lipid peroxidation in muscle foods. Model systems with water-extracted muscle residues implied that myoglobin was not the principal prooxidant in meat and that nonhaeme iron was the main catalyst (Sato and Hegarty, 1971; Tichivangana and Morrissey, 1985).

Figure 11.2. Diagram displaying the mechanism and stages of lipid oxidation (O'Sullivan and Kerry, 2008).

Figure 11.3. Diagram displaying the decomposition of lipid hydroperoxides by ferrous iron and the resultant formation of the very reactive alkoxy radicals (O'Sullivan and Kerry, 2008).

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The use of natural antioxidants in food products of animal origin

Professor Susan L. Cuppett , in Antioxidants in Food, 2001

12.2.1 Background

Since its naming by Timms and Watts in 1958, warmed over flavor (WOF), now also called meat flavour deterioration (MFD), has been a primary research issue for food scientists and the meat industry. There are many endogenous and exogenous factors that have been shown to affect WOF. Early research showed that this was a phenomenon related to the meat's phospholipids as opposed to its triacylglycerides. 1 In addition it also found that a primary catalyst to WOF was iron 2 , 3 and that during cooking there was a release of (free) iron from the heme compounds. 4

Since these early findings, researchers have begun to establish the role of endogenous factors in the development of WOF. Some endogenous factors help to control oxidation and include the presence of compounds that are active antioxidants, i.e. dipeptides, tocopherols, etc. while others are enzymes capable of deactivating active oxygen species. 5 In contrast, prooxidants are also present in the cell; these can include free iron, ascorbic acid and active oxygen species. The final stability of the system is therefore dependent on the balance between these factors. 5 The balance or lack of balance translates into the fact that there are differences in oxidative stability between animal species and between muscle types within a species. For example, generally between species differences are that beef is the most stable, followed by pork, chicken, turkey and finally fish, and within a species such as poultry, the dark (thigh) meat is more susceptible to oxidation than the white (breast) meat.

Exogenous factors affecting the oxidative stability of meats include the level of processing, cooking technique/time, pre- and post-cooking storage time and temperature, packaging system, and/or use of antioxidants. When meat is processed the balance of the system becomes altered. Grinding disrupts the muscle tissue and allows mixing of the cell contents with oxygen; it also allows pro-oxidants access to the more unsaturated fatty acids in the membranal phospholipids. Increases in free iron have been found to result from cooking and during storage. During storage reducing agents (ascorbate and superoxide anion) can act to release the iron from its chelated state. Myoglobin is a major source of this released iron while ferritin has been shown to be less susceptible to releasing iron when denatured. The released iron is very active and in close proximity to the lipid substrate. Finally cooking also acts to destabilise the unsaturated fatty acids in the membranal phospholipids.

Control of oxidation in meat systems can occur in the raw or cooked meat system. In the raw meat system, factors affecting the levels of endogenous antioxidants have been receiving a great deal of attention in the literature in the 1990s. Prior to this research, most of the activity was focused on the application of exogenous antioxidants during processing. The remainder of this section will be divided to discuss most recent findings on the role of endogenous and exogenous antioxidants and combinations of both in controlling lipid oxidation in meat systems. Antioxidants are either lipid soluble (tocopherols and carotenoids) or water-soluble (ascorbic acid, dipeptides, and plant phenolics or polyphenolics; raw meat will also contain antioxidant enzymes).

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The Eating Quality of Meat

Mónica Flores , in Lawrie´s Meat Science (Eighth Edition), 2017

13.2.1 Lipid Degradation (Oxidative Reactions)

Fats or fat-soluble precursors were also shown to be implicated in accounting for species differences and in contributing generally to meat flavor. There are, of course, considerable differences between species in intramuscular fat ( Chapter 4). The oxidative reactions during cooking of unsaturated fatty acids produce the generation of a wide number of volatile compounds, aldehydes, ketones, alcohols, aliphatic hydrocarbons, acids, and esters (Mottram, 1998). The induced oxidation of unsaturated fatty acids is responsible for the cooked meat aroma, development of rancid notes during storage periods, and cured aroma of meat products (Toldrá and Flores, 2007). Those unsaturated fatty acids with more than two double bonds are rapidly oxidized acting as regulators of the meat shelf life (Wood et al., 2004). Thus, phospholipids with a higher polyunsaturated content than triacylglycerides are more susceptible to oxidation and therefore important for meat flavor development.

The importance of phospholipids rather than triglycerides in explaining the contribution of lipids to cooked meat flavor has been shown by Elmore et al. (1999). Removal of the intramuscular triglycerides had relatively little effect on the pattern of volatiles, but subsequent removal of the phospholipids caused a loss of aliphatic aldehydes. There was also a marked decrease in pyrazines, which suggests that, in cooked meat, lipids are normally involved in Maillard reactions and inhibit the production of pyrazines. Using model systems, Campo et al. (2003) assessed the importance of oleic, linoleic, and alpha-linolenic acids, with or without the presence of cysteine and ribose, in relation to the development of odor in cooked meat. "Fishy" notes were experienced only with mixtures including linolenic acid an effect exacerbated by the presence of ferrous iron.

Many carbonyl compounds have been identified as meaty odorants in oven roast beef (Rochat and Chaintreau, 2005), cooked pork (Elmore et al., 2001), goat meat (Madruga et al., 2009), and sheep meat (Bueno et al., 2011; Watkins et al., 2013). They have not been considered as essential contributors to meat aroma due to their high odor threshold values in comparison to heterocyclic compounds although they contribute to specific aroma notes: rancid, grass, green, citrus, fried, fatty, depending on chemical structure (Table 13.1). Traditionally it has been believed that the full flavor of meat cannot be developed without its associated fat, but the possible health hazards of the ingestion of fat have led producers to rear leaner animals.

Table 13.1. Volatile Components of Cooked Meats From Different Species (Beef, Pork, Lamb) Characterized as Main Meaty Odorants

Compound Aroma Meat
Sulfur
2-Furfuryl 2-methyl-3-furyl disulphide Meaty, roasted, burnt Cooked bovine muscles (Farmer and Patterson, 1991)
Bis(2-methyl-3-furyl)disulphide Meaty, roasted, burnt Cooked bovine muscles (Farmer and Patterson, 1991)
Methional Toasty, burnt, cooked vegetables, potato Roasted beef (Cerny and Grosch,1992), oven roast beef (Rochat et al., 2007), goat meat (Madruga et al., 2009), sheep meat (Watkins et al., 2013)
2-Acetyl-2-thiazoline Toasty, burnt, earthy, caramel-like, meaty, pop-corn Roasted beef (Cerny and Grosch, 1992), oven roast beef (Rochat et al., 2007), grilled lamb meat (Bueno et al., 2011)
2-Furfurylthiol Boiled meat Cooked beef, pork, lamb (Kerscher and Grosch, 1998)
2-Methyl-3-furanthiol Boiled meat Cooked beef, pork, lamb (Kerscher and Grosch, 1998), oven roast beef (Rochat et al., 2007)
2-Mercapto-3-pentanone Garlic, fried onion Cooked beef, pork, lamb (Kerscher and Grosch, 1998)
3-(Methyl)thiophene Alliaceous, rubbery Oven roast beef (Rochat et al., 2007)
Dimethyl trisulfide Sulfur Oven roast beef (Rochat et al., 2007), goat meat (Madruga et al., 2009), sheep meat (Watkins et al., 2013)
2-Methyl-3-mercapto-1-propanol Beef broth, meaty Oven roast beef (Rochat et al., 2007)
2-Methyl-3-[(2methylbutyl)thio]furan Meaty, green Oven roast beef (Rochat et al., 2007)
2-Phenyl and 3-phenyl-thiophene Vague, meaty, rubbery Oven roast beef (Rochat et al., 2007)
4-Isopropyl-benzenethiol Mushroom, alliaceous Oven roast beef (Rochat et al., 2007)
Nitrogen Compounds
2-Ethyl-3,5-dimethylpyrazine Toasty, burnt, earthy and caramel-like Roasted beef (Cerny and Grosch, 1992)
2,3-Diethyl-5-methylpyrazine Toasty, burnt, earthy and caramel-like Roasted beef (Cerny and Grosch, 1992), sheep meat (Watkins et al., 2013)
2,5-Dimethylpyrazine Beef, fried Grilled lamb meat (Bueno et al., 2011)
Carbonyl Compounds
Linear aldehydes (C3–C11) Caramel, spicy, smoky, fish, green, fresh grass,
citrus, sea, fatty, rancid, floral		 Oven roast beef (Rochat and Chaintreau, 2005), sheep meat (Watkins et al., 2013)
2-Alkenals (C6–C10) Green floral-fatty rancid Oven roast beef (Rochat and Chaintreau, 2005), sheep meat (Watkins et al., 2013), grilled lamb meat (Bueno et al., 2011)
12-Methyltridecanal Rancid, meat Goat meat (Madruga et al., 2009)
(E,E)-2,4-Decadienal Rancid, meat Goat meat (Madruga et al., 2009), sheep meat (Watkins et al., 2013), grilled lamb meat (Bueno et al., 2011)
Heterocyclic Compounds
Furaneol Caramel Roasted beef (Cerny and Grosch, 1992), sheep meat (Watkins et al., 2013)
2-Acetyl-1-pyrroline Popcorn, roasted Sheep meat (Watkins et al., 2013)
Other Compounds
Guaiacol Toasty, burnt, Roasted beef (Cerny and Grosch, 1992)
4-Methyl-phenol (p-cresol) Stable, animal Sheep meat (Watkins et al., 2013)
4-Ethyl-octanoic acid Mutton-like Sheep meat (Watkins et al., 2013)

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The Storage and Preservation of Meat

Youling L. Xiong , in Lawrie´s Meat Science (Eighth Edition), 2017

7.2.4 Oxidative Changes in Chilled Meat

During postmortem aging, proteolytic degradation of muscle fibers into short peptides, nucleotides, free amino acids, and various other nitrogen-containing compounds contribute to meat flavor enhancement. On the other hand, chemical and biochemical reactions, particularly lipid oxidation and also protein oxidation, could cause flavor deteriorations due to the generation of secondary lipid oxidation products and sulfur compounds on the exposed surfaces of carcasses and meat cuts ( Martinaud et al., 1997; Spanier et al., 1997). Of course, for meat that is chilled in a vacuum-sealed package, oxidation would have a negligible role in meat flavor change.

Deterioration in meat lipids may be due to direct chemical action or through the intermediary activity of enzymes (either indigenous or derived from microorganisms). In general, direct chemical deterioration is not so important in fresh meat carcasses but can be a part of quality loss when deboned meat is subjected to extended storage. Two types of deterioration occur: hydrolysis and oxidation. Lipolytic enzymes split fatty acids from triacylglycerols; with phospholipids that are abundant in membrane, inorganic phosphate is produced in addition to the release of free fatty acids, which are predominantly unsaturated. The fatty acids liberated in meat are generally not so offensive as compared with those produced in milk (short chain fatty acids). On the other hand, due to the susceptibility of unsaturated fatty acids to radicals, the higher their amount, the more oxidative the muscle, hence, the more off-flavors (Min and Ahn, 2005).

The rate of oxidation of intramuscular fat tends to be higher in nonruminants than in ruminants (e.g., poultry meat and pork in comparison with beef and mutton), in the less improved breeds, in muscles with relatively low contents of intramuscular fat, in the lumbar region of longissimus dorsi in the pig compared with the thoracic region (the reverse being true in beef animals), in animals on a low plane of nutrition, and in animals receiving large proportions of unsaturated fat in their diet, particularly in nonruminants (Lawrie, 1992; Love and Pearson, 1971; Rhee et al., 1996).

A considerable number of such differences may operate simultaneously. The relative tendency of pork muscles to become rancid and discolored exemplifies this complexity. Porcine psoas muscle has a higher proportion of polyunsaturated fatty acids (PUFAs), especially in the phospholipid fraction, than longissimus muscle (Owen et al., 1975), and more prooxidative heme protein. Yet, during prolonged frozen storage (e.g., at −10°C), minced porcine longissimus muscle undergoes oxidative rancidity, and concomitant metmyoglobin formation, to a markedly greater extent than psoas. This anomalous behavior appears to be related to the higher ultimate pH of the latter (Owen et al., 1975). At high pH the activity of the cytochrome system of enzymes is much enhanced and this increases their metmyoglobin-reducing activity (Faustman and Cassens, 1990). Moreover, such enzymes are found at higher concentration in psoas. In porcine psoas muscle, therefore, the relatively high ultimate pH, by minimizing prooxidant conditions, more than offsets the inherently greater tendency of its lipids to oxidize. On the other hand, for beef, the ultimate pH of both psoas and longissimus dorsi muscles is generally normal, and this potentiates the effect of the higher proportion of PUFAs in the former (Rhee et al., 1988). Clearly, multiple factors must be known before accurate prediction of the behavior of a given muscle can be made.

The prooxidant effect of heme compounds in fat oxidation is reciprocal since unsaturated fatty acids accelerate the oxidation of myoglobin. As myoglobin and fats are brought into intimate contact with one another in meats, their coupled reaction will contribute to rancidity and discoloration simultaneously (Faustman et al., 2010). Secondary lipid oxidation products, such as malonaldehyde and 4-hydroxynonenal, can bind to nucleophilic histidine residues in myoglobin causing meat discoloration during chill storage. Suman et al. (2014) reported that the number of histidine residues adducted by reactive aldehydes was greater in beef myoglobin than in pork myoglobin. This seems to contribute to the notion that metmyoglobin formation induced by oxidized lipids is more relevantly important for color instability in beef than in pork. During cooking, both heme-bound and nonheme iron accelerate lipid oxidation. Nevertheless, it should be pointed out that the behavior of heme pigments and unsaturated fats, when in juxtaposition, is not fully understood. Kendrick and Watts (1969) had long postulated that at low lipid:heme ratios, heme compounds can stabilize peroxides or free radicals and exert an antioxidant effect.

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