H H

lated volatiles, whereas cruciferous vegetables generate isothiocyanates. The main precursor of disulfides in alliaceous plants are S-substituted l-cysteine sulfoxides, which on the action of allinases produce thiosulfinates (Fig. 19), the principal component responsible for the odor of fresh alliaceous plants. On heating, the thiosulfinates are converted into corresponding disulfides, which are responsible for the cooked flavor. The pungent taste of cruciferous vegetables, on the other hand, are caused by isothiocyanates (Fig. 20). The enzymes responsible for the release of these compounds are thioglucosidases, which act on different thioglucosides found in these vegetables to produce the corresponding isothiocyanates. The enzymes and the substrates in these vegetables are compartmentalized; the tissue of the plant should be disrupted to bring the substrate close to the enzyme for the generation of the flavor compounds.

Origins of Enzymatically Produced Flavors in Fruits

About 2000 distinct fruit volatiles have been isolated till now. They consist mainly of esters, aromatic aldehydes, lactones, alcohols, terpenoids, and some thioesters. Unlike fresh vegetables, the aroma is more frequently preformed and arises directly from the intact fruit.

Esters and Aromatic Aldehydes. Esters make the most important contribution to what we usually perceive as fruit flavors. They are present in higher levels than any other class of compound, and also they are present in a greater variety compared with any other class. Most of the esters contribute to the character impact of the resultant aroma. Certain esters have been associated with the aroma of specific fruits, such as methyl butyrate with apple, isopentyl acetate with banana, and ethyl butyrate with orange (Fig. 21). Esters are biosynthesized by enzymatic esterification of carboxylic acids with alcohols through the action of such enzymes as esterases and acyl CoA-alcohol transacetylase.

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Allicin - fresh garlic flavor cook cook

Diallyldisulfide - cooked garlic flavor Figure 19. Components responsible for the garlic flavor.

Diallyldisulfide - cooked garlic flavor Figure 19. Components responsible for the garlic flavor.

Figure 20. Allylisothiocyanate, principal "hot" component in mustard.

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