Investigation of The Key Flavor Precursors in Chicken Meat |
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1. What is Flavor?1.1 Key Flavor Compounds Identified in Chicken Meat1.1.1 Differences between studies on key odor compound1.1.2 Similarities between studies on key odor compound1.1.3 Main compounds important for chicken flavor1.2 Overview of Flavor Forming Reactions1.2.1 Maillard Reaction & Strecker Degradation1.2.2 Degradation of Thiamin1.2.3 Oxidation of Lipids1.3 Precursors of Meat Flavor1.3.1 Ribonucleotides1.3.1.1 Concentrations of ribonucleotides in chicken meat1.3.2 Reducing and phosphorylated sugars1.3.2.1 Concentrations of sugars in chicken meat and other meats1.3.3 Amino acids1.3.3.1 Sulfur-containing amino acids- Role of cysteine- Role of methionine1.3.3.2 Concentrations of amino acids in meat1.3.4 Thiamin1.3.4.1 Concentration of thaiamin in chicken meat1.3.5 Fatty acids and Lipids1.4 Formation of Flavor Precursors in Meat Post-Slaughter1.4.1 Post-mortem changes during conversion of muscle to meat1.4.2 Glycogen pathway1.4.3 ATP degradation pathway
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1. What is Flavor?In 1969, the U.S. Society of Flavor Chemists proposed the following definition: ‘Flavor is the sensation caused by those properties of any substance taken into the mouth which stimulates one or both senses of taste and smell, and also the general pain, tactile and temperature receptors in the mouth’ (Heath 1978). Flavor is a complex mixture of sensory input composed of taste (gustation), smell (olfaction) and tactile sensation of food as it is being chewed, a characteristic that the food scientists often term “mouthfeel’’. Scientists generally describe human taste perception in terms of four qualities: saltiness, sourness, sweetness and bitterness. Some have suggested, however, that other categories exist as well, most notably ‘umami’, the sensation elicited by glutamate, one of the 20 amino acids that make up the proteins in meat, fish and legumes (Smith and Margolskee 2001). Flavor is a very important component of the eating quality of meat and there has been much research aimed at understanding the chemistry of meat flavor which has resulted in the identification of over 1000 volatile compounds from cooked meats (Mottram 1991). A considerable amount of work, especially by the flavor companies, has been conducted and reviewed on the production of artificial flavorings to imitate cooked meat flavor (MacLeod and Seyyedain-Ardebili 1981). In recent thirty years, reviews have been written to explain how desirable flavor is generated (Thomas et al. 1971; Gordon 1972; Patterson 1974; Farmer 1992; Shi and Ho 1994; Mottram 1998; Farmer 1999). A review of the flavor of poultry could cover a variety of aspects. In this introduction, the volatile compounds and precursors important for chicken flavor formation will be reviewed briefly. The quantities of the precursors present in chicken and their role for flavor formation are the major focus of this thesis and will be reviewed in more detail.
1.1 Key Flavor Compounds Identified in Chicken MeatAroma compounds in chicken are largely formed during the cooking process. Indeed, raw meat has none of the aroma of cooked meat, and the characteristic aroma is largely generated during cooking (Crocker 1948). In fact, the flavor compounds are generated via chemical reactions occurring between natural precursors present in raw meat during heating.
Figure 1a: Chicken Flavor FormationIdentification of the volatile compounds which give cooked poultry its desirable aroma may help to determine which chemical reactions are responsible for forming these compounds during cooking (Farmer 1999). Once these chemical pathways have been elucidated, it may be possible to investigate potential precursors involved in these reactions and deduce which of them in raw meat, are needed to produce the desirable poultry aroma (Farmer 1999). About 500 volatile aroma compounds have been reported in chicken (Schroll et al. 1988; Schliemann et al. 1988; Maarse and Visscher 1989; Mottram 1991; Ramarathnam et al. 1991; Ramarathnam et al. 1993; Werkhoff et al. 1993) Many of these have relatively high odor thresholds and make little contribution to the overall flavor Others may be present at very low concentration but, due to their very low odor thresholds, have a very large effect on overall flavor (Farmer 1999).
1.1.1 Differences between studies on key odor compoundFirst of all, the source and background of the chicken used for the studies cited in Table 1A and Figures 1 b-g is unknown. The type of sample used by these authors also varies considerably. Some included whole chicken (Figures 1c, d, e, f, g), breast or thigh (Figure 1b), with skin (Figures 1c, d, e, f, g) or without skin (Figures 1b, d). Comparison of breast versus thigh (Farmer et al. 1999) does not explain the differences between these data and those for whole chicken (Kerler and Grosch 1997). Presence of skin in the samples analysed in (Figures 1c-g) may explain the greater importance of lipid oxidation products (e.g., compound 35). |
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Table 1A: Odor descriptors and identities of some important volatile compounds |
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*(a: Gasser and Grosch 1990; b: Kerler and Grosch 1997; c: Farkas et al. 1997; d: Kerscher and Grosch 1999; e: Farmer et al. 1999) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Sulphur-containing compounds are more important when the chicken skin is removed, but may also be affected by extraction method (e.g., products 21, 29, 33 and 38). The cooking method used by these authors also varied. Some chicken samples were cooked in a flask using a boiling water bath (Farmer et al. 1999), whereas, the use of pressure cooker at 116°C or 119°C, and roasting the whole chicken with coconut oil at 180°C, were preferred by Kerler and Grosch (1997) and Kerscher and Grosch (1999), respectively. The aroma of chicken will differ depending on the cooking method, but the four samples cooked by pressure cooker (Figures 1c, e, f and g) are not markedly similar. The other divergence between studies carried out by these authors is their choice of extraction method. Static headspace (Kerler and Grosch 1997; Farmer et al. 1999) seems to be more sensitive to highly volatile compounds (compounds 1-10) than simultaneous distillation extraction (Gasser and Grosch 1990; Farkas et al. 1997) or high vacuum distillation (Farkas et al. 1997). High vacuum distillation and simultaneous distillation extraction give many similarities (compounds 11, 19, 21, 35, Fig 1e), but also differences for volatile compounds such as, 3-mercapto-2-pentanone, trimethylthiazole, nonanal, 2-undecenal (compounds 13, 14, 16 and 32, respectively) being detected only with simultaneous distillation extraction method (Fig 1d). In two studies (Kerler and Grosch 1997; Kerscher and Grosch 1999), quantities and odor thresholds were determined and odor activity values were calculated. Many methods of analysis failed to detect some volatile S-containing compounds (e.g. dimethylsulfide, 2-methyl-3-(methyldithio)furan, 2-acetylthiazoline). In addition, since birds of different origin might have different concentrations of precursors and if different amounts of precursors can lead to different levels of each volatile compound, then, the lack of information concerning the origin of the birds and the concentration of natural precursors for each bird might explain a part of the diversity of their results (Figures 1b-g). |
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Figures 1b-g: Comparison of the aromagrams derived from studies on chicken aroma (Compounds are listed in Table 1A) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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Extraction method: | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
WB: Water bath; PC: Pressure cooker; B: Boiled in water; R: Roasted in coconut oil; SHS: Static headspace; HVD: High-vacuum distillation; SDE: Simultaneous distillation extraction; OAV: Odor activity values. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1.1.2 Similarities between studies on key odor compoundGeneral agreements between authors (Gasser and Grosch 1990; Kerler and Grosch 1997; Farkas et al. 1997; Kerscher and Grosch 1999; Farmer et al. 1999) were observed regarding the importance of compounds such as 1-octen-3-one, 2-methyl-3-furanthiol, 2-furanmethanethiol, methional (compounds 10, 11, 19 and 21, respectively) which were detected in all studies. Other compounds such as hexanal, (E,E)-2,4-decadienal (compounds 8 and 35, respectively) were present in four of five studies. Thus, it might be assumed that these compounds are important for chicken flavor, by whatever method these were determined.
1.1.3 Main compounds important for chicken flavorThe most important study on chicken flavor in recent years is probably the one published by Gasser and Grosch (1990). Using aroma extract dilution analysis, they identified 16 primary odor compounds in chicken broth. Fourteen of these compounds were structurally identified as 2-methyl-3-furanthiol, 2-furanmethanethiol, methional, 2,4,5-trimethylthiazole, 2-trans-nonenal, nonanal, 2-formyl-5-methylthiophene, p-cresol, (E,E)-2,4-nonadienal, (E,E)-2,4-decadienal, 2-undecenal, ß-ionone, γ-decalactone and γ-dodecalactone. 2-Methyl-3-furanthiol, identified in Gasser and Grosch (1990) as the most important flavor compound contributing to the meaty flavor of chicken broth, has also been recognized as a character impact compound in the aroma of cooked beef (Gasser and Grosch 1988) and canned tuna fish (Withycombe and Mussinan 1988). 2-Methyl-3-furanthiol and its oxidative dimer, bis-(2-methyl-3-furyl)disulphide, possessing characteristic meat flavor notes, have been found by Evers et al. (1976) among the volatile products from heating thiamine hydrochloride with cysteine hydrochloride and hydrolysed vegetable protein. These two compounds were also found in volatile products of thiamine degradation (van der Linde et al. 1979; Hartman et al. 1984). Thiamine has, therefore, been recognized as one possible precursor of the formation of the meaty aroma compounds, 2-methyl-3-furanthiol and bis-(2-methyl-3-furyl)disulphide. However, thiamine is not the sole source of 2-methyl-3-furanthiol. When ribose or 5’-inosine monophosphate (IMP) reacted with cysteine, a significant amount of 2-methyl-3-furanthiol and the bis-(2-methyl-3-furyl)disulphide was formed (Farmer and Mottram 1990; Grosch et al. 1990; Zhang and Ho 1991). However, the formation of 2-methyl-3-furanthiol from either ribose or IMP requires the participation of sulphur-containing amino acids, cysteine or cystine, or pepetide, glutathione (Shi and Ho 1994), whereas, thiamine does not require these components.
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1.2 Overview of Flavor Forming ReactionsThe chemical reactions by which the volatile compounds responsible for chicken aroma and flavor are formed include the Maillard reaction and Strecker degradation, lipid oxidation and the degradation of thiamine. In this section a brief overview of each chemical reaction will be presented.
1.2.1 Maillard Reaction & Strecker DegradationThe Maillard reaction between reducing sugars and amino acids is known to generate flavors similar to those of cooked foods. The Maillard reaction is the reaction between an amino compound (amine, amino acid, peptide, or a protein) and a carbonyl group in a sugar. The Maillard reaction contributes to the formation of flavor in most cooked foods. Even in its simplest form, between one amino acid and one reducing sugar, the Maillard reaction yields more than one hundred volatile products (Salter et al. 1988). Although the reaction has been extremely studied and is the subject of many reviews (Hurrell 1982; Nursten 1986; Tressl et al. 1993; Mottram 1994), it is interesting to note that the mechanism proposed by Hodge (1953) still provides the basis for our understanding of the early stages of this reaction (Mottram 1998). The first step of the reaction is the formation of a N-aldosylamine, which involves an addition reaction between the carbonyl group of the open chain form of an aldosugar and the amino group of an amino acid, peptide or other compound with the primary amino group (Mottram 1994). The subsequent elimination of water and molecular rearrangement gives a 1-amino-1-deoxy-2-ketose (Amadori product). Figure 1h shows a simplified version of the formation of rearrangement products from Amadori compounds and their possible degradation by dehydration and retro-aldolization (fission). |
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Figure 1h: Deamination and dehydration of Amadori compounds to form important meat flavor intermediates (Bailey 1994) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Figure 1i shows the primary products resulting from Strecker degradation of cysteine. |
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Figure 1i: Primary products resulting from Strecker degradation of cysteine (Kobayashi and Fujimaki 1965; MacLeod and Seyyedain-Ardebili 1981; Roos 1992) |
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Table 1B shows a few important volatile compounds in chicken meat (reported in Table 1A) with the Maillard reaction as the probable origin. The role of ribose as the reducing sugar in the Maillard reaction is significant (Table 1B). | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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*References for the analysis of chicken aroma; a: (Gasser and Grosch 1990); b: (Werkhoff et al. 1990); c: (Farmer et al. 1999); d: (Siegl et al. 1995); **References for probable origin; 1: (Vernin and Parkanyi 1982); 2: (Güntert et al. 1992); 3: (van den Ouweland and Peer 1975); 4:(Farmer et al. 1989); 5:(Tressl et al. 1985)
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Thiamine plays an essential role in many foods as a water-soluble vitamin. Additionally, its function as a flavor precursor in heated foods, e.g. meat, should not be neglected. However, certainly, this aspect depends very much on its amount and the specific conditions in the food system (Güntert et al. 1992). Another important field in which thiamine plays a remarkable role is the application of flavorings. Along with carbohydrates, amino acids, ribonucleotides, and other constituents, thiamine is widely used as a flavor precursor. This fact is clearly demonstrated by many patented reaction or processed flavors (Güntert et al. 1992). The thermal degradation of thiamine produces a number of compounds with particularly potent aromas, including furans, furanthiols, thiophenes, thiazoles, and aliphatic sulfur compounds, some of which have been reported in meat volatiles (Mottram 1991).
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Grosch and co-workers (1992; 1993) studied the influence of thiamine as a possible precursor for the formation of the important meat flavor compound, 2-methyl-3-furanthiol, and its oxidised form bis-(2-methyl-3-furfuryl)disulfide. Van der Linde et al. (1979) found 5-(2-hydroxyethyl)-4-methylthiazole among the primary products of thiamine heated in buffer at 130°C, and suggested that it is formed as a result of the attack by a hydroxyl ion on the carbon atom connecting the thiazole and prymidine ring systems (Figure 1j). |
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Table 1C: Important volatile compounds formed by thermal degradation of thiamine |
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*References for the analysis of chicken aroma; a: (Gasser and Grosch 1990); b: (Werkhoff et al. 1990); c: (Farmer et al. 1999); **References for probable origins; 1: (Güntert et al. 1992); 2: (van den Ouweland and Peer 1975); 3:(Farmer et al. 1989). |
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Lipid oxidation is well known as the cause of rancidity development but it can also contribute to desirable food flavors. This complex series of reactions is ubiquitious in the natural world and has been extensively studied and reviewed (Grosch 1982; Chan 1987). Table 1D indicates those which are derived from the thermal oxidation of lipids and which are believed to contribute to the flavor of chicken.
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Table 1D: Some important volatile compounds with lipid oxidation as origin |
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*References for the analysis of chicken aroma; a: (Noleau and Toulemonde 1986); b: (Gasser and Grosch 1990); c: (Siegl et al. 1995); d: (Farmer et al. 1999); e: (Kerler and Grosch 1997); f: (Kerscher and Grosch 1999); g: (Farkas et al. 1997). **References for probable origins; 1: (Grosch 1982) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
The major difference between the flavor of chicken broth and that of beef broth is the abundance of (E,E)-2,4-decadienal and γ-dodecalactone in the chicken broth. Both are well-known lipid oxidation products (Gasser and Grosch 1990). Carbonyl compounds formed by oxidative degradation of unsaturated lipids have been discussed by Minor et al. (1965) as a cause of the ‘chicken odor’ and an intensification of the ‘meaty odor’. In particular, the (E,E)-2,4-decadienal was found by Pippen and Nonaka (1957) to contribute to the aroma of chicken. Gasser and Grosch (1990) confirmed the importance of (E,E)-2,4-decadienal, with the highest FD factor of all the aroma compounds extracted from the chicken broth. They also reported that γ-dodecalactone and undecenal were potent odorants arising from a breakdown of lipids. The role of lipid-derived carbonyl compounds in poultry flavor has been extensively reviewed by Ramaswamy and Richards (1982). |
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1.3 Precursors of Meat FlavorMeat tissue consists primarily of water, protein, fat, and carbohydrate, plus lesser amounts of nonprotein nitrogen-containing compounds and minerals and trace amounts of vitamins and other organic compounds (Mottram 1991). On heating, these components react to produce the complex volatile mixtures that are characteristic of meat aroma. A wide range of temperature conditions exist during normal cooking of meat; the centre of a rare steak may reach only 50°C, the centre of roast meat may attain 70-80°C, while the outside of grilled or roast meat will be subjected to much higher temperatures and localised dehydration of the surface will occur. It is not surprising, therefore, that a wide range of different flavor sensations are perceived in cooked meats. The nature and quantity of volatiles depend on the time and temperatures of heating. A number of these precursors may play a major role in formation of the volatile sulphur compounds important for roasted and meaty flavors (Mottram 1991). These include ribonucleotides, reducing and phosphorylated sugars, amino acids, thiamine and lipids which are briefly reviewed in this section. |
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1.3.1 RibonucleotidesThe naturally occurring ribonucleotides play an important role in the flavor of flesh foods (Kuninaka 1967; Jones 1969). Inosine-5’-monophosphate (IMP) and guanosine-5’-monophosphate (GMP) have been reported to enhance the meaty flavor and suppress sulphury, fatty, burnt, starchy, bitter and hydrolysed vegetable type flavors (Kuninaka 1967; Wagner et al. 1963). flavor enhancers, such as monosodium glutamate, inosine monophosphate and guanosine monophosphate are natural components of meat and are believed to make a major contribution to meat flavor (Farmer 1999). They have been shown to improve flavor and have been used by the Japanese for many years to give ‘umami’ (Reineccius 1994; Maga 1994). When using added IMP, Kurtzman and Sjostrom (1964) concluded that canned chicken-containing noodle soup was not flavor-enhanced. However, other products evaluated, including canned beef noodle soup, did show improvement with IMP addition. |
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1.3.1.1 Concentrations of ribonucleotides in chicken meatLiterature provides a large number of methods of analysis for determination of ATP and its degradation products in muscle (Khan and Frey 1971; Shaw et al. 1979; Jeungling and Kammermeir 1980; Attrey et al. 1981; Kitada et al. 1983; Ryder 1985; Fujimura et al. 1995). A summary of different methods of analysis of ribonucleotides is discussed further in Section 2.3.1. Fujimura et al. (1995) has reported the concentrations of ATP breakdown products in chicken breast muscle (Table 1E). |
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Table 1E: ATP metabolites in chicken meat (Fujimura et al. 1995) |
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1.3.2 Reducing and phosphorylated sugarsThe important role played by ribose and cysteine in a model system leading to meat flavor formation, by heating a mixture of these compounds, was first investigated by Morton et al. (1960). The volatile compounds formed in the reaction between cysteine and ribose were dominated by sulphur-containing heterocyclic compound, particularly certain thiols and thiophenes (Farmer et al. 1989). They also reported 2-methyl-3-furanthiol and 2-furanmethanethiol as major volatile compounds from cysteine + ribose. Using a model system containing ribose and cysteine at pH 5.6, Mottram and Nobrega (1997) reported a large quantity of 2-furanmethanethiol with the dominant volatile in the ribose + cysteine system being 2-furfural. Reductions in the quantities of carbohydrates and amino acids in model systems, were observed during heating, the most significant losses occurring for cysteine and ribose (Mottram 1998). In meat, it has been proposed that ribose phosphate, from the ribonucleotides, is the principal precursor of furan and thiophenethiols (Mottram 1998). Dephosphorylation and dehydration of ribose phosphate form the important intermediate 4-hydroxy-5-methyl-3(2H)-furanone, which readily reacts with hydrogen sulfide (van den Ouweland and Peer 1975). Ribose-5-phosphate is formed from ribonucleotides such as inosine monophosphate, which is present in significant concentration in post mortem muscle (Patterson 1974; Macy et al. 1970a). Mottram and Nobrega (1997), reported a higher reactivity of ribose-5-phosphate compared to ribose when reacted with cysteine in a model system. |
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1.3.2.1 Concentrations of sugars in chicken meat and other meatsIn order to determine the relative importance of these reducing sugars for flavor formation in meat, it is necessary, first, to know their natural concentrations. Very few studies on determination of sugars in chicken have been found in the literature. An investigation of sugars (but not their phosphates) was conducted by Lilyblade et al. (1962), using paper chromatography. The results of their studies are summarized in Table 1F. |
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Table 1F: Inositol and free sugars (mg 100 g-1 dry muscle) of chicken muscle (Lilyblade and Peterson 1962) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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a: 13 week New Hampshire pullets. b: 18 month old New Hampshire hens. c: Leg and thigh, 15 min post-mortem; breast, 30 min post-mortem. | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Glucose and glucose-6-phosphate have also been quantified in beef by enzymic methods (Lawrie 1985). However, the reported concentrations can show considerable variation. For example, the concentration of ribose in beef is reported as 1 mg 100g-1 wet weight (Jarboe and Mabrouk 1974), 126 mg 100 g-1 wet weight (Cuzzoni and Gazzani 1984) and 524 mg 100 g-1 wet weight (Gazzani and Cuzzoni 1985). Table 1G summarises some results obtained for sugars in beef. It is unclear whether this variation is due to differences between meat samples or analytical methods. The natural quantity of ribose-5-phosphate in meat has not been reported. |
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a: (Macy et al. 1964); b: (Jarboe and Mabrouk 1974); c: (Cuzzoni and Gazzani 1984); d: (Gazzani and Cuzzoni 1985); e: None reported | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1.3.3 Amino acidsAmino acids are important for the formation of meat flavor as they react with reducing carbohydrates via the Maillard reaction. The release of certain amino acids can also have an influence on meat quality attributes such as drip loss, water holding capacity (Lawrie 1992) and the development of flavor (Spanier and Miller 1993). Proteolytic degradation of myofibrillar proteins gives rise to increased substrate for enzymatic degradative systems with the concomitant release of free amino acids. It is possible therefore, that amino acid concentrations may also yield some information regarding the tenderness or flavor of meat (Lawrie 1992). On heating, protein and amino acids serve as a source of free ammonia. In addition, sulfur-containing amino acids, and proteins containing these amino acids, are precursors of hydrogen sulfide. |
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1.3.3.1 Sulfur-containing amino acidsSulfur-containing volatile compounds are a major class of food aroma compound found in vegetables, cooked meat, and other processed foods (Gasser and Grosch 1988; Farmer and Mottram 1990; Block 1992). |
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- Role of cysteineCysteine is an important precursor for many sulfur-containing aroma compounds identified in meat and foods. Sulfur-containing flavors found in meat products are normally formed through thermal processing. It has been accepted that the sulfur-containing amino acids, cysteine and cystine, are indispensable components for generation of a meat-like aroma through thermal processing. (Shahidi et al. 1986; Tressl et al. 1989; Zhang and Ho 1991; Whitfield 1992). They participate in the Maillard reaction and Strecker degradation, it is also believed that upon heating, cysteine and cystine evolve hydrogen sulfide, one of the first compounds identified in early studies attempting to characterize the volatile compounds of cooked red meats (Crocker 1948) and poultry (Bouthilet 1951a; Pippen and Erying 1957). The small amount of hydrogen sulfide formed and volatilized when chicken is heated may be sufficient to influence aroma (Bouthilet 1951a; Pippen and Erying 1957). These sulfur-containing compounds are well recognized as major contributors to meat flavors (MacLeod 1986). |
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- Role of methionineL-Methionine is one of the essential amino acids for man (Rose et al. 1955; Irwin and Hegested 1971). Methionine also has been widely used in the flavor and food industries to produce reaction flavors such as baked potato, fried potato, coffee, and meat flavors (Hertz and Shallenberger 1960; Ballance 1961; Hodge 1967; Chen 1968; Lee et al. 1973; Fan and Yeuh 1980; Silwar and Tressl 1989). The contribution of methionine to flavor formation was found to be mainly through thermal degradation or through thermal interactions with other food ingredients, especially reducing sugars. Herz and Shallenberger (1960) reported that heating methionine and glucose in an aqueous solution at 100°C or in mineral oil at 180°C would generate a potato aroma. El-Ode et al. (1966) indicated that heating methionine and sugars in water at 100°C yielded a cabbage-like aroma. Casey et al. (1965) reported that heating methionine was degraded by compounds such as glucose to produce methional, dimethyl sulfide, and dimethyl disulfide. Shigmatsu et al. (1977) reacted six sugars with methionine and cysteine. A mixture of equimolar amounts of sugar and amino acids was heated at 190°C for 15 min under reduced pressure. They reported that in most cases the main product was the Strecker aldehyde, methional, or products derived from this aldehyde, such as the corresponding alcohol. Besides dimethyl sulfides and methional, several volatile compounds were identified from the thermal degradation of methionine with or without glucose or dicarbonyl compounds (Fujimaki et al. 1969; Rijke et al. 1981; Ho et al. 1982; Hartman and Ho 1982; Tressl et al. 1989). |
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1.3.3.2 Concentrations of amino acids in meatFree amino acids (nonprotein) and some peptides are very important substances, playing important roles eliciting characteristic tastes of foods (Nishimura and Kato 1988). Their direct participation in chemical reactions leading to the formation of volatile compounds responsible for the meat flavour has been well established. Better knowledge of the concentration of these amino acids is crucial to allow their relative importance for flavour formation to be determined. |
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a:(Fujimura et al. 1995); b: (Perez-Llamas et al. 1997) |
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As expected, higher concentrations for all amino acids were reported by Perez et al. (1997) compared to those obtained by Fujmura et al. (1995). The presence of cysteine reported by Perez et al. (1997) and not by Fujmura et al. (1995) was also expected, as for the total amino acids all proteins were first hydrolysed. However, it is not clear whether the origin of cysteine detected by Perez et al. (1997) was proteins, glutathione or it was present as free (nonprotein) amino acid. The concentration of glutathione was not reported. |
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1.3.4 ThiaminThiamine (vitamin B1) is a bicyclic compound containing sulphur and nitrogen and its thermal degradation can yield a wide range of S- and N-containing volatile compounds many of which possess potent aromas (Buttery et al. 1984; Güntert et al. 1992). The role of thiamine in meat flavour formation was discussed previously in Section 1.3.2. |
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1.3.4.1 Concentration of thaiamin in chicken meatThe concentration of thiamine in chicken meat is reported in the literature. Ang and Moseley (1980) have reported the amount of thiamine in chicken leg meat as 0.85 ± 0.03 μg g-1, wet weight. Abdulrahman and Abdelbary (1993), have reported a similar results in light and dark muscles, 1.51 ± 0.09 and 1.92 ± 0.03 μg g-1 wet weight, respectively, from raw broiler chicken meat. Leonhardt and Wenk (1997) have published similar results in chicken breast and thigh: 1.4 μg g-1 wet weight for both, using the method described by Rettenmaier et al. (1979). |
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1.3.5 Fatty acids and LipidsMeat lipids may be divided into two main groups; the storage lipids and the structural and metabolic lipids (Coxon 1987). The storage lipids are the triacylglycerols present in the fat cells of adipose tissue and intramuscular marbling fat. It is the major component of the fat cells, usually constituting in excess of 70% of the weight of adipose tissue. Structural lipids are present in all tissues, but in amounts generally less than 1% of the tissue, and metabolic lipids occur in equally low concentrations. The main structural lipids are the phospholipids, sphingolipids and cholesterol, whereas the commonest of the metabolic lipids are mono- and di-acylglycerols, non-esterified fatty acids and cholesterol esters (Coxon 1987). |
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Table 1I: Fatty acid composition of the phospholipid and triglyceride fractions of chicken meat (Pikul et al. 1984) |
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Souza et al. (1999) reported the lipids and fatty acids composition and polyunsaturated fatty acids/ saturated fatty acids (PUFA/SFA) ratio of roasted chickens (breast, thigh and skin). Table 1J summarizes some of their results. |
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Table 1J: Total lipids (%), fatty acids (expressed as percent of total fatty acid methyl esters) composition and PUFA/SFA (Souza et al. 1999) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
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*Portion with skin; **Portion without skin; PUFA: Polyunsaturated fatty acids; SFA: Saturated fatty acids | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
1.4 Formation of Flavor Precursors in Meat Post-Slaughter1.4.1 Post-mortem changes during conversion of muscle to meatPost-mortem changes that occur in the conversion of muscle to meat not only alter some of its biochemical and physical properties but also play an important role in improving its keeping quality and acceptability as food (Pearson and Young 1989). The nature of these changes and their consequences for meat have been reviewed (Greaser 1986; Pearson 1987). During the post-mortem aging period meat shows significant alteration in the level of numerous chemical components such as sugars (Lilyblade and Peterson 1962), organic acids (Bodwell et al. 1965), peptides and free amino acids (Parrish et al. 1969), and metabolites of adenine nucleotides such as adenosine triphosphate, ATP (Dannert and Pearson 1967; Davidek and Khan 1967). Many of the subsequent chemical changes are brought about by enhanced hydrolytic activity. These chemical modifications in the aging meat result in a pool of flavour compounds and flavour intermediates; the latter can react/interact to form additional flavour notes during cooking, e.g., sugars and amino acids react during heating to form Maillard products (Bailey 1988).
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1.4.2 Glycogen pathwayGlycogen is the main storage carbohydrate present in animal cells. It is a polysaccharide of D-glucose linked together by alpha-1,4 linkages, with each branch point, occurring about every 8-12 glucose residues, being in the alpha-1,6 configuration. Glycogen is hydrolyzed to glucose by the action of glycogen phosphorylase (Pearson and Young 1989). The major role of glycogen in post mortem muscle is release of glucose, which can be used to replenish the high-energy phosphate compounds. Thus, glycogen is largely degraded and is mainly responsible for the formation of lactic acid in muscle. Therefore, glycogen is ultimately responsible for the changes in the properties of muscle that accompany the drop in pH as glycolysis proceeds. After death, there are only a few sources of energy, such as the glycogen stores, residual ATP and ADP, and any unused creatine phosphate. As long as residual ADP sources remain high, the reaction below can also occur to provide additional ATP (Pearson and Young 1989). The AMP formed can then be deaminated to produce inosine monophosphate (IMP) and NH3. |
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1.4.3 ATP degradation pathwayThe biophysical and biochemical changes that poultry muscle undergoes post mortem are, in general, the same as those reported for various mammalian species (De Fremery 1966). The most apparent change, of course, is the stiffening of muscle as it passes into rigor mortis. The chemical changes are (1) the disappearance of glycogen, adenosine triphosphate (ATP), and N-phosphorylcreatine, (2) the appearance of ammonia and IMP from the deamination of adenylic acid; and (3) the accumulation of lactic acid as a result of the anaerobic breakdown of glycogen. The accumulation of lactic acid lowers muscle pH from above 7.0 to ultimate values of 5.7 to 5.9. |
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These steps are fast and give rise to a rapid accumulation of IMP (Kennish and Krammer 1987). Satio and Arai (1959) found that these reactions took place rapidly during slow freezing of carp. Jones and Murray (1962) reported that trawled cod had little ATP and substantial amount of IMP at death compared with rested, well-fed fish. The dephosphorylation of IMP to form inosine is carried out by the enzyme 5’-nucleotidase. The cleavage of inosine is performed by muscle ribosidase to form hypoxanthine and ribose (Kuninaka 1957). |
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Figure 1l: Degradation of IMP by three possible pathways (Lee and Newbold 1963) | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
A comparison between Figures 1l and 1k shows that these authors (Newbold 1965; Zubay 1983) are proposing similar pathways for degradation of IMP into inosine, and also for the formation of ribose-1-phosphate from inosine. In contrast, the possible formation of ribose from inosine proposed by Lee and Newbold (1963) (Fig 1l) was not reported by Zubay
(Fig 1k). |
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Copyright 2005: 'Investigation of the key flavor precursors in chicken meat' |