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According to Shi and Ho , 16 primary odour components have been identified in chicken broth, of which 14 are structurally identified. They further demonstrated that 2-methylfuranthiol, generated from the Maillard reaction and lipid oxidation, as the most vital chemical compound responsible for the meaty flavour of chicken broth. These compounds are obviously the major sources of chicken flavour Shi and Ho, ; Varavinit et al.


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Sasaki et al. Flavour precursors of chicken meat The major flavour precursors found in meat including chicken meat can be divided into two main groups: water soluble components and lipids Mottram, Although the flavour of meat from different species upon heat processing is expected to be similar due to the similarities of the free amino acids and carbohydrates in their meat Shahidi, , it was challenged by the lipid-derived species-specific notes, mainly from the intramuscular lipids Perez-Alvarez et al. In other words, differences in lipid-derived volatile components between species are mainly responsible for the species differences in flavour, whereas the precursors supplied by lean tissues generate the meaty flavour common to all cooked meats Mottram, Mottram reported free sugars, sugar phosphates, nucleotide bound sugars, free amino acids, peptides, nucleotides and other nitrogenous components, such as thiamine as the main water-soluble flavour precursors.

The reaction of cysteine and sugar can lead to characteristic meat flavour specially for chicken and pork Varavinit et al. This was further confirmed by a research where the quantities of carbohydrates and amino acids, in particular ribose and cysteine, are reduced during heating.

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The main carbohydrates with flavour-forming potential include ribose, ribosephosphate, glucose and glucosephosphate Meinert et al. Ribose which is associated with the ribonucleotides in the muscle is highly involved in flavour producing reactions during heating of meat Mottram, However, many flavour compounds may be formed by two or more possible mechanisms. The best example is 2-methylfuranthiol which is responsible for the meaty flavour of chicken broth Shi and Ho, ; Aliani and Farmer, Therefore, the relative importance of each precursor for flavour generation in cooked chicken meat is still unclear Aliani and Farmer, Lipid derived volatiles in chicken flavour The role of lipid-derived carbonyl compounds in poultry flavour has been assessed by many researchers over the years.

The lean meat contains intramuscular triglycerides and structural phospholipids. Therefore, desirable as well as undesirable flavours can be resulted in meat due to lipid oxidation. Mild thermal oxidative changes of lipids lead to generation of desirable flavour compounds and aromas in cooked meats Shahidi, Similar to other meats, the flavour development of poultry meat is partly attributed to its lipids Perez-Alvarez et al. Several hundred volatile compounds are generated in cooked meat through the lipid degradation, primarily the oxidation of the fatty acid components of lipids. Such compounds includes aliphatic hydrocarbons, aldehydes, alcohols, ketones, esters, carboxylic acids, some aromatic hydrocarbons Figure 1 , and oxygenated heterocyclic compounds such as lactones and alkylfurans Mottram, Forty-one out of total compounds reported in the flavour of roasted chicken are lipid-derived aldehydes.

Selected aldehydes identified in roasted and cooked chicken flavour are shown in Table 2 Shi and Ho, According to the Table 2 , hexanal and 2,4-decadienal are the most abundant aldehydes identified in chicken flavour which are known to be the primary oxidation products of linoleic acid Figure 3.

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However, 2,4-decadienal is considered as a more important odourant for chicken flavour compared to hexanal due to its much lower odour threshold Shi and Ho, Several studies have confirmed that phospholipids are much more important in the development of aroma volatiles during the cooking of meat than the triglycerides Mottram, This is attributed to the presence of much higher proportion of unsaturated fatty acids, including significant amounts of polyunsaturated fatty acids such as arachidonic acid in phospholipids Mottram and Edwards, Saturated and unsaturated aldehydes having green, fatty and tallowy aroma play a vital role in all cooked meat aroma Mottram, The higher levels of unsaturated fatty acids in chicken compared with red meat generate more unsaturated volatile aldehydes which are vital in determination of specific aromas of chicken meat Noleau and Toulemonde, Further, the aroma of fat-fried food is reported to be due to 2,4-decadienal Mottram, Therefore aliphatic aldehydes contribute to the fatty flavours of cooked meat including chicken meat.

A different composition of volatiles that is responsible for the desirable flavours gets formed due to quick reactions taken place in cooked meat Mottram, compare to those volatiles formed during long-term storage leading to rancid off-flavours. Lipid-derived compounds in meat volatiles have greater odour threshold values Table 3 as opposed to sulfur- and nitrogen-containing heterocyclic compounds which make them less aroma significant. Even at relatively low concentrations, heterocyclic compounds possess a significant effect on aroma due to their low odour threshold values Mottram, Volatiles from the Maillard reaction in chicken flavour The Maillard reaction is one of the main chemical reactions that take place during cooking of meat including chicken meat.

This typically occurs between amino compounds and reducing sugars and eventually results a large number of compounds responsible for the flavour of any meat Mottram, a. During the initial stages of this reaction, Amadori products are formed via glycosylamine as a result of condensation of the carbonyl group of a reducing sugar with amino compounds. Various sugar dehydration and degradation products such as furfural and furanone derivatives, hydroxyketones and dicarbonyl compounds are formed by rearranging and dehydration of the resulted product via deoxyosones.

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Subsequently these compounds interact with other reactive components such as amines, amino acids, aldehydes, hydrogen sulfide, and ammonia and the aroma compounds are formed Mottram, ; Calkins and Hodgen, Hydrogen sulfide, ammonia and acetaldehyde are also formed by Strecker degradation when cysteine is used as the amino acid Figure 4 ; Mottram, Maillard reaction-derived carbonyl compounds act with these compounds forming intermediates that further involve in flavour-forming reactions. This eventually produces many important classes of flavour compounds including furans, pyrazines, pyrroles, oxazoles, thiophenes, thiazoles and other heterocyclic compounds Figure 5 ; Melton, Shahidi and Mottram and Madruga reported that sulfur-compounds derived from ribose and cysteine, and carbonyl compounds are the principal contributors to meat flavour.

Ribose is considered as the most important flavour precursor in chicken Meinert et al. In addition, thiamine has also been proved as an important precursor that provides wide range of sulfur compounds Aliani and Farmer, Due to the degradation process of nucleotides such as IMP, ribose is formed and it is then involved in a number of secondary reactions yielding a large number of volatile compounds Kavitha and Modi, Hence, IMP is generally considered as the major nucleotide in muscle that imparts flavour to the meat Yamaguchi, Formation of 2-methylfuranthiol in chicken broth via Maillard reaction involves interaction between ribose and sulphur-containing amino acids cysteine or cystine or peptide glutathione.

Glutathione liberates hydrogen sulphide rapidly during the initial stages of cooking whereas cysteine does upon prolonged heating. Another important odourant in chicken broth, 2-furfurylthiol, is formed from the reaction between furfural and cysteine Shi and Ho, In addition, the Maillard reaction-derived volatiles are the major components in meat grilled under severe conditions Mottram, Volatiles compounds from lipid-Maillard interactions Whitfield and Mottram b reported that the interaction of lipid with the Maillard reaction leads to formation of a number of volatiles that have been identified in meat Figure 6.

Volatile compounds originating from lipid-Maillard interactions are given in Table 4 and 5. Several thiazoles with C 4 to C 8 n -alkyl substituents in the 2-position and some other alkylthiazoles with much longer 2-alkyl substituents C 13 to C 15 have been reported in fried chicken and heated chicken, respectively Tang et al. Melton reported that 2-octyl-4,5-dimethylthiazole found in chicken meat is also a product from interaction of Maillard reactions and lipids. Other compounds contributing to roasted, fried and boiled chicken flavour Heterocyclic compounds such as pyrazines, thiazoles and oxazoles are usually considered to be responsible for the roast flavours in foods including meat.

Melton reported that a large number of heterocyclic compounds are associated with roasted, grilled, fried or pressure cooked meats, but not boiled meat, due to higher temperatures used in those cooking methods. Different alkyl pyrazines and two classes of bicyclic compounds, 6,7-dihydro-5 H -cyclopentapyrazines and pyrrolopyrazines, were found in meat volatiles Mottram, It was noticed that both classes of compounds increased greatly with the increasing severity of heat treatment. However, Mottram reported that boiled meat contained higher levels of sulfur-containing heterocyclic compounds such as thiophenes, trithiolanes, trithianes that have low odour thresholds with sulfurous, onion-like and, sometimes, meaty aromas Fors, These compounds contributed to the overall flavour and aroma of boiled meat.

Thermal degradation of cysteine and glutathione results in two major volatile compounds of fried chicken. These volatiles were identified in cooked chicken as well Shi and Ho, In addition to 3,5-dimethyl-1,2,4-trithiolanes, three other alkyl-substituted trithiolanes, 3,5-diisobutyl-1,2,4-trithiolane, 3-methylbutyl-1,2,4-trithiolane and 3-methylpentyl-1,2,4-trithiolane, were identified in fried chicken flavour. The formation of latter 2 volatiles involves thermal and oxidative degradation of lipids, pentanal and hexanal Shi and Ho, Possible mechanisms for the formation of dithiazines and 3,5-diisobutyl-1,2,4-trithiolane are given in Figure 7.

According to Shi and Ho , flavour of deep-fat-fried foods such as fried chicken is attributed to lipid-derived aldehyde, 2,4-decadienal.

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Being an oxidation product of 2,4-decadienal, trans-4,5-epoxy-transdecenal having a low odour threshold plays a key role in the flavour of fried chicken. Heterocyclic compounds, mainly pyrazines, pyridines, pyrroles and thiazoles, found in fried and roasted chicken are listed in Table 6. Alkylpyrazines have a roasted, nut-like or toasted flavour notes and are present in the flavours of fried chicken and roasted chicken but not in chicken broth.

This confirms that the generation of pyrazines requires high temperature and low moisture.

Shi and Ho reported that fried chicken flavour was further intensified by 2-pentylpyridine strong fatty and tallow-like odour , 2-isobutyl-3,5-diisopropylpyridine roasted cocoa-like aroma , 2-pentylmethylethylthiazole strong paprika pepper flavour , 2-heptyl-4,5-dimethylthiazole strong spicy flavour and 2-octyl-4,5-dimethylthiazole sweet fatty aroma.

Generally WOF is generated within 24 h of refrigerated storage in precooked poultry Graf and Panter, A remarkable off-flavour problem in mechanically deboned chicken meat was reported by Shi and Ho and it was attributed to rancidity of fat. It was further stated by the same authors that haem pigments, being the catalyst of the above reaction, are largely responsible for the off-flavour formation. However, recent studies indicated that raw poultry meats are highly resistant to oxidative changes due to various antioxidants present in chicken meat Min et al.

Meanwhile, it has also been reported that increasing cooking temperature is associated with increased roasted, toasted, and bitter sensory notes Perez-Alvarez et al. Studies conducted to determine the critical changes in odour compounds of boiled chicken during refrigerated storage and reheating proved that refrigerated storage and reheating of boiled chicken showing WOF due to loss of meaty, chicken-like and sweet odour notes, and the formation of green, cardboard-like, and metallic off-odours by the secondary by-products of lipid oxidation.

These changes were caused primarily by an increase in hexanal sevenfold and six fold decrease in both E,E -2,4-decadienal and 2-furfurylthiol Kerler and Grosch, Rhee et al.

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Increased palatability of indigenous chicken compared to broilers is well documented by many authors. Additionally, chicken results in more unsaturated volatile aldehydes as compared to beef or lamb because their muscle contains higher levels of polyunsaturated fatty acids in the triglycerides than later species Calkins and Hodgen, These compounds contribute to the specific aromas of chicken Noleau and Toulemonde, ; Mottram, This confirms the effect of lipids on the flavour of chicken primarily via the differences in fatty acid composition and the resulting carbonyls Perez-Alvarez et al.

Further, positive correlations between the flavour of chicken meat and the intramuscular contents of amino acids, including glutamic acid and nucleotides, such as IMP were demonstrated by Kurihara , Rikimaru and Takahashi and Takahashi et al. Effect of sex on chicken meat flavour was demonstrated by many researchers, although the results were not consistent. Meat from male birds received higher scores for flavour as opposed to that from female birds Ramaswamy and Richards, ; Farmer, However, it was also shown that the breast and leg meat of female birds were preferred to those of male birds.

Many other researchers reported no significant relationship between the two parameters Farmer, In addition to these intramuscular compounds, the diet of the bird also plays a vital role towards the flavour of chicken meat Fanatico et al. Diet can either positively or negatively influence the flavour of chicken meat. Corn-fed chicken and arachidonic acid enriched diet fed chicken have produced tastier meat Lyon et al. Processing steps such as aging, cooking, irradiation and high pressure treatment also affect the flavour of chicken meat.

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Post-mortem aging results in many chemical flavour compounds including sugars, organic acids, peptides, free amino acids Yano et al. Cooking plays a vital role in flavour development and it affects the acceptability and volatile flavour components of poultry meat Sanudo et al. Cooking methods such as roasting, grilling, frying, and pressure cooking generates many pyrazines, pyridines, pyrroles and thiazoles compared to boiling of chicken meat Shi and Ho, Irradiation affects flavour and aroma of chicken meat primarily through the production of free radicals.

Aldehydes hexanal, pentanal, heptanal, octanal, and nonanal and sulfur volatiles mainly dimethyl trisulfide generated during irradiation result in the associated off-odour Patterson and Stevenson, ; Perez-Alvarez et al. However, very little negative effect was expressed by electron beam irradiation on the flavour of preheated chicken breast meat Rababah, Effect of high pressure treatment on flavour of meat including chicken meat had been variable over the years. Hayman et al. However, exposure of chicken meat to a pressure of MPa lead to a better flavour and taste as opposed to a MPa treatment Kruk et al.

According to Cheah and Ledward , pressure treatments at around MPa at room temperature initiated the changes which eventually lead to catalysis of lipid oxidation in pressure processed meat. However 2-methylfuranthiol that is produced from the reaction between ribose and cysteine or cystine, and from degradation of thiamin is considered as the most important compound in chicken flavour. A large number of heterocyclic compounds are formed during roasting, grilling, frying or pressure cooking of chicken meat due to higher temperature and low moisture conditions used in those cooking methods.

These compounds are absent in boiled meat.