2.4. Flavours formed by enzymatic action
Enzymatic action is responsible for the formation of flavouring components in many plant tissues. As examples, in the alliaceous vegetables, the enzyme and the flavour precursors only react when the cellular tissues are ruptured; in mustard, the reaction only takes place when the milled seeds are mixed with water.
Alliaceous vegetables
Such vegetables as onion (Allium cepa L.), garlic (Allium sativum L.), chive (Allium schoenoprasum L.) and leek (Allium porrum L.) have characteristic flavours which not exist in the bulb before processing, but are produced when the cellular tissues are ruptured by cutting or chewing. Under these conditions the flavour is produced very rapidly by the action of an enzyme on the odorless precursors which coexist in the cells.
The distinct flavours of onions, garlics, leeks and chives are due to s-alkenyl derivatives of L-cysteine sulfoxide which is rapidly hydrolysed by the enzyme alliinase to give an unstable sulfenic acid derivative plus pyruvic acid and ammonia. The sulfenic acid derivative breaks down and rearranges to form the more stable thiopropanal s-oxide, which has lachrymatory properties; or it reacts with other compounds to produce a series of di-or trisulfides.The organoleptic differences between onion, garlic, leeks and chives are explained in terms of the quantitative and qualitative differences in the occurrence of the four precursors: 1-propenyl-, 2-propenyl-, 1-propyl- and methyl-L-cysteine sulfoxide (Freeman and Whenham, 1975), the most important being:
The action of alliinase on the onion precursor leads to a distinctive odour and a marked pungency.
The garlic precursor leads to the formation of allyl thiosulfinate, which characterizes the freshly crushed garlic. This compound forms allyl dissulfide, thiosulfonate and traces quantities of allyl trisulfide.
Mustard
The characteristic flavour and pungency of this condiment is also dependent upon enzymatic reaction involving odorless precursors in the presence of water.
When freshly ground, the seeds exhale a very slight odor, but, when mixed with water the odour level increase rapidly. This is due to enzymatic action of myrosinase on thioglucoside precursors.
In white mustard the odour is produced by the p-hydroxybenzyl isothiocyanate. In the black mustard, the allyl isothiocyanate is the responsible of the pungent characteristics of this condiment.
Flavour degradation
Enzymes also produce flavour deterioration. This is important to quick frozen food processors. If the enzymes present in the unfrozen foods are not destroyed they remain active and, even at very low temperatures, may result in the formation of off-odours and off-flavours as well as undesirable textural and other changes, as discoloration.