Classification And Origin Of Flavor Compounds

Classification of Flavor Compounds Based on Their Mode of Formation: Biogenetic and Thermogenetic

One way of classifying flavor compounds is based on their mode of formation; flavors can be generated either naturally (9,10) or by heat treatment during food processing (11,12). Natural flavors are mainly the secondary metabolites of the living tissue, formed during the natural growth cycle of the organism by the action of enzymes, whereas flavors produced by heat treatment are the result of thermal degradation and oxidation of various food ingredients and their complex interactions. The initial or primary precursors of flavors are the polymers found in food such as proteins, lipids, polysaccharides, and DNA, which can undergo either enzymatic or thermal hydrolysis to produce the intermediate precursors, mainly dimers and monomers. These, in turn, can undergo different biotransformation reactions during normal metabolic growth and produce metabolites that have specific flavor qualities that remain in situ when the plant is harvested and produce the perceived flavor effect when consumed. On the other hand, during thermal processing of food products, the intermediate precursors formed by thermal hydrolysis can undergo complex chemical transformations, and further

Figure 7. Examples of compounds causing pungency, (a) Piperine, component responsible for the sensation of black pepper; (b) capsaicin, the pungent principal of "hot" pepper.

Figure 7. Examples of compounds causing pungency, (a) Piperine, component responsible for the sensation of black pepper; (b) capsaicin, the pungent principal of "hot" pepper.

Figure 8. ( - )-Menthol causes a cooling sensation.

degradations, to generate cooked flavor (Fig. 9). The type of flavor effect depends on the conditions of processing and the type of the initial precursors found in the particular food product. In terms of composition, thermally generated flavors are richer in heterocyclic compounds compared with enzymatically produced flavors.

Precursors of Flavor Compounds in Food

Meat and Related Products. Raw meat has no particular appealing flavor. However, during various processes of cooking, the initial precursors in meat produce water-soluble intermediate precursors, such as glycopeptides, free nucleotides, peptides, amino acids, amino-sugars, free sugars, fatty acids, and so on. The meat flavor, therefore, is the combined effect of the chemicals produced from the thermal degradation of these intermediates and the products formed from the amino-carbonyl interactions, such as between the reducing sugars and amino acids; this reaction is known as the Maillard reaction or nonenzymatic browning as it is responsible also for the brown color produced when foods are heated (11-13). During thermal degradation amino acids and sugars produce complex mixture of compounds. Sulfur-containing amino acids are specially important for the generation of meat aroma; cysteine, cyctine, and methionine produce sulfur-containing small molecules, such as hydrogen sulfide, 3-(methylthio)-propionaldehyde, and 2-mercaptoethylamine, that play an important role in the aroma of meat products (Fig. 10). Thermal degradation of thiamine produces furans, thio-phenes, and thiazoles, important heterocyclic compounds with meatlike flavor. Thermal oxidation of unsaturated fatty acids leads to the formation of different aldehydes.

Flavour Compounds

Figure 9. Origin of flavor compounds in food.


Vs ch3

2-Methyl-3-(Methylthio)furan 2-Methyl-l,3-dithiolane

l,2,3-Trithiacylohept-5-ene Thiazole

Figure 10. Aroma compounds identified in cooked beef.

Nonalcoholic Beverages: Tea and Coffee. Tea and coffee are examples of food products in which both modes of flavor formation, thermal and enzymatic, play a role in the generation of flavor. During the initial fermentation of green tea leaves, for example, important intermediate precursors are produced by the action of endogenous enzymes to produce polyphenols, carotenoids, and unsaturated fatty acids, such as lenolenic acid, which under the action of lipoxygenase enzymes can produce important flavor aldehydes such as (Z)-3-hexenal and CE)-2-hexenal. In the heat-treatment stage tea leaves are dried at 85°C and the coffee beans are roasted at > 180°C to produce numerous volatile aroma compounds (Fig. 11) by thermal degradation and Maillard reaction processes. The main intermediate precursors involved during roasting of coffee are sugars, amino acids, fatty acids, peptides, amines, phenolic acids such as 3-(3',4'-dihydroxyphenyl)propenoic acid (caffeic acid), l-methylpyridinium-3-carboxylate (trigonelline), and so on. In addition, tea and coffee contain alkaloids such as 1,3,7-trimethylxanthine (caffeine) and3,7-dimethylxan-thine (theobromine) that impart a bitter taste to the beverage.

Alcoholic Beverages: Wine. There are three main sources of flavor compounds in wine; some are already present in the grape, some are formed during fermentation process, and some may form during aging by Maillard-type reactions. The type of wood in which the wine is stored can

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