In the natural products so far considered, the aromatic components exist per se in the plant material and can be recovered by simple distillation or extraction. The flavor of onions, on the other hand is an excellent example of the action of enzymes in flavor development as the characteristic notes are created spontaneously only when the onion is cut and the cells disrupted. The study has established that much of the odor and hence, flavor arise as a result of the conversion of odorless, nonvolatile, S-substituted cysteine sulfoxide derivatives, first to an unstable alkyl sulfenic acid intermediate by the action of an alliinase enzyme system.
The sulfenic acid moiety is very unstable and the reaction proceeds directly to a mixture of mercaptans, di-, tri-, and poly-sulfides as well as thiosulfonates. All possible combinations of methyl and prophyl derivatives have been isolated but little or no allyl compounds are present in onion. A similar system in garlic results in the formation of mainly allyl compounds, the difference being due to the nature of the precursor present.Most onion flavor components are the result of the degradation of the thiosulfonates, resulting in a complex mixture of sulfides plus ammonia and pyruvic acid which is produced in the first stage of the reaction. A determination of pyruvic acid has been suggested as a measure of the flavoring strength of onion and onion products even though the acid itself does not contribute to the flavor profile.
The Flavor of Onion
