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The key volatile components such as free fatty acids, their related esters, methyl ketones and lactones are produced during chemical and biochemical degradation of lipids. Oxidation/cleavage, esterification/hydrolysis are the kinds of reaction which leads to formation of aroma compounds from lipids. Generally, three types of oxidation occur after the hydrolysis by lipases. They can be degradative Î²-oxidation of fatty acid to CO2 and water, Î±-oxidation of alkanes to alcohol and Ï‰- oxidation of fatty acids to Î±,Ï‰-dicarboxylic acids(Waché et al., 2006).
WachÑ et al in his paper presented Ï‰- oxidation pathway, where in the first steps fatty acid is catalysed by enzyme complex (Ï‰-hydroxylase complex) with cytochrome P450 mono-oxygenase and NADPH-cytochrome reductase. Primary oxidation of terminal methyl group in fatty acid and alkanes are due to hydroxylase complex. Free fatty acids (flavour compounds) are formed latter in the pathway, which are two additional oxidation steps, catalysed by alcohol oxidase and aldehayde dehydrogenase.
Simplified process of lipid oxidation is shown in fig(). Auto-oxidation of fatty acids starts with initiation reaction, where initiator such as thermal dissociation, peroxidases, metal catalysis, photolysis leads to the breakdown of R-H bond and form a free radical compound R*. The reaction proceeds with the propagation steps, where free radical reacts with oxygen to form another free radical compound ROO*, which further reacts with basic RH to form peroxide(ROOH) and free radical R*. This chain reaction repeats till the availability of oxygen. The termination of the pathway occur when 2ROO* reacts to form non-radical product(Cadwallader and Singh, 2009). The amount of oxidation products varies with the nature of the food. The off-flavours produced as a result of oxidation of fatty acids are described as fatty, tallow, fried, plastic, fishy, metallic or cardboard-like(Cadwallader and Singh, 2009). Auto-oxidation of unsaturated fatty acids such as oleic acid gives rise to octanal, nonanal, decanal, 2-decanal, 2-undecanal etc, whereas linoleic acid gives hexanal, 2-octenal, 3-nonenal etc, similarly linoleic acid produces propanal, 3-hexnal, 2,4-heptadienal(Cadwallader and Singh, 2009).
Enzymatic hydrolysis by lipases and esterases known as lipolytic changes and oxidative chemical changes are likely to occur in high fat content foods. Dairy products are high fat containing foods. More than 98% of milk fat is triglyceride. Molecular weight of triacylglycerides is around 470-890 Da and have 24-54 acyl carbons. They have glycerol backbone (esters of glycerol) where three fatty acids are attached(Collins et al., 2003). The principal biochemical transformation of milk fat is hydrolysis of triacylglycerides. This reaction produces free fatty acids (FFAs), di- and mono glycerides and other compounds. In the other end phospholipids consist of < 1% of total milk lipids, they play major role in the milk fat globule membrane (MFGM). FFAs is the principle compounds leading to the flavour and aroma compounds in the milk and its products.
The flavour of cheese is generated due to the series of biochemical reaction and changes that occur during the formation of curd and its ripening. The biochemical changes depend on the starter bacteria, enzyme rennet, enzymes from milk accompanying lipases and other micro-organism. The three major metabolic pathways resulting in the production of the numerous compounds which are involved in cheese aroma and flavour are catabolism of lactate, protein and lipid. Hydrolysis of lipid result in the formation of FFA, that directly contributes to cheese flavour and also acts as the substrates for further reactions resulting in the production of highly catabolic end products(Collins et al., 2003, Adda et al., 1982).
Milk, starter, secondary starter and non-starter bacteria are the sources for lipases and esterases in cheese. Short and medium chain fatty acids as a result of FFA lypolysis contribute to cheese flavour. Flavour compounds such as methyl ketone, alkanes, esters, lactones and secondary alcohols are the product of catabolic reaction where FFA acts as precursor. Important fatty acids catabolites in blue cheese is methyl ketones (alkan-2-one). It's concentration increases up to 70d while ripening and decreases slowly. Mould lipases from Penicillium roqueforti and camembereti leads to production of methyl ketones in the cheese. Methyl ketone is produced from Î²-oxidation pathway in which FFA is released by lipases in the first step. FFA undergoes oxidation to produce Î±-ketoacids, followed by decarboxylation of keto acid to alkan-2-ones, which further reduced to corresponding alkan-2-ol fig(Collins et al., 2003, Adda et al., 1982). Long chain FFA (>12 carbon) have minor role in flavour because of their high perception thresholds, while short and intermediate even number fatty acid have major role in the flavour property. Botanoic acids have rancid and cheesy flavours, hexanoic acid gives pungent, blue cheese flavours, octanoic acid gives wax, soap, goat, fruity, rancid and musty flavours. The flavour effect in cheese due to FFA is controlled by pH. In the case of limburger cheese, butanoic and hexanoic acids relates to its strong aroma. In case of Italian varieties such as Romano it have higest concentration of FFA, Parmesan with lowest and provolone with intermidate. Butanoic, hexadecanoicacids and C18 congeners were reported to be in above three type of Italian variety. In the case of Swiss cheese flavour butanoic acid, methyl ketones and oct-1-en-3-ol(Collins et al., 2003).
Lactones aroma is not cheese like but they contribute to the overall cheese flavour. Generally it has been reported of possessing buttery character in cheese. Î´-Lactone have low flavour threshold in compare with other volatile compounds. They have fruity notes such as peach, coconut and apricot. Thioesters are generally found to impart aroma in foods such as onions, garlic and some fruits. The fruity flavour due to esters is not desirable in the case of cheddar cheese. Most of the esters separated in the chedder cheese odour headspace had "buttery" to "fruity" aroma (Collins et al., 2003, Adda et al., 1982). Thioester produced due to the reaction of esters of short-chain fatty acids with methional tends to have characteristic "cheesy" flavour in the case of cheddar cheese.
The lipolysis levels are measured as the function of released FFA. They vary between cheese variety from moderate kinds (e.g., Cheddar, Caerphilly, Cheshire) to extensive kinds (e.g., hard Italian, mould-ripened and surface bacterially ripened smear varieties. Fatty acids composition plays major role in the taste of all variety of the cheese. Fatty acid profile and its flavour depends on the nature of the foods. Fatty acids oxidation and the maillard reaction is generally the major background in the food aroma and flavour science.
Meat flavour is generally derived thermally. The uncooked meat has blood like taste and has little or no aroma. Flavour of cooked meat is affected by compounds attributing to its taste, whereas the aroma attribute is due to the volatile compounds formed during cooking which in generally tends to be major contributing factor. More than 1000 volatiles compounds have been identified in the meat (Mottram, 1998). Water soluble components and lipids are the two major precursors of meat flavour. Maillard reaction between amino acids and reducing sugars, and the thermal degradation of lipid result in the formation of volatile compounds. Free sugars, sugar phosphate, nucleotide bound sugar, peptides n other nitrogenous compounds are thought to be water soluble flavour precursors. Meat like flavour was generated when mixture of amino acid cysteine and sugar ribose was heated. More over meat flavouring reaction studies had been done in sulphur, generally as cysteine or sulphur containing amino acid and hydrogen sulphide(Elmore et al., 1999) . In the case of lipid derived volatile few examples are aliphatic haydrocarbons, ketones, alcohols, carboxylic acids and ester. Aromatic compounds mostly hydrocarbons and oxygenated heterocyclic compounds like lactones and alkylfurans have been reported. Unsaturated fatty acid undergoes autoxidation readily than the saturated form. Phospholipids contain a much more amount of unsaturated fatty acid than triglycerides and are also source of volatiles.
The characteristic flavour of the meat from different species is believed to be derived from lipid sources mainly aldehyde. There is high amount of unsaturated fatty acids in the triglyceride of chicken and pork in compare with lamb and beef. This leads to the production of more unsaturated fatty acid in the form of aldehyde. This amount of aldehydes is sought to have some relation in the distinct attribute in the different species of meat (Mottram, 1998). The sheep meat consist high amount of methyl-branched saturated fatty acids such as 4-nethyloctanoic and 4-methylonanoic acid. These acids have been known to posses characteristic flavour of mutton. In the case of beef 12-methyl-tridecanal has been associated to its tallow, beef-like aroma and believe to play important role in the characteristic aroma of the beef. Again iso- and anteiso-methly-branched aldehydes with the chain length of 11 to 17 carbons have been reported in relation with its characteristic beefy flavour. The formation of these methyl-branched aldehydes are sought to be from hydrolysis of plasmalogens. These plasmalogens are phosphoglycerides and in one position in its glycerol moiety, aldehyde is linked by enol-ether link.
The interaction of lipids and maillard reactions has been assumed to produce number of volatiles compounds that are identified in meat. There have been reports for various thiazoles with C4Â-C8 n-alkyl substituent in the second position for roast beef and fried chicken. Also in heated beef, chicken and mostly in beef heart muscle, alkylthiazoles with much longer 2-alkyl substituents C13Â-C15 have been reported. More than 50 alkyly-3-thiazoles have been reported from cooked beef especially from the cattle feed with fish oil supplements. Even the concentration of saturated and unsaturated aldehydes were
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