More than 400 volatile components have been identified in tomato fruit, of which only 10–16 are likely to be important contributors to tomato flavor/aroma based on odor threshold data. Tomato volatiles are grouped as lipid-derived, carotenoid-related, amino acid-related, lignin-related, or of uncertain origin. These flavor components are either present in intact fruit or formed after blending due to mixing of previously compartmentalized enzymes and substrates. Lipid-derived volatiles are the biggest group containing cis-3-hexenal and hexanal, which are quantitatively the major volatile compounds in tomato fruit. cis-3-Hexenal and -ionone have the highest odor thresholds among tomato volatile compounds so far identified. Most of these compounds increase during ripening (or the enzymes, substrates and conditions develop that result in increased levels after blending) and appear to be related to ethylene production. Biosynthetic pathways have been established or suggested for most of the important flavor components, of which lipid degradation is the best-understood. Linoleic and linolenic acids are oxidized to hydroperoxides by lipoxygenase, which are then cleaved to volatile C6 aldehydes (hexanal and cis-3-hexenal, respectively). There are two membrane-associated lipoxygenases (tomloxA and B), of which tomloxB appears to be fruit-specific and increases during ripening. Alcohol dehydrogenase (ADH) has been demonstrated to catalyze the interconversion of trans-hexene-2-al and -2-ol and of trans-hexene-2-al, hexanal and hexanol. The enzyme product of the Adh2 gene is induced by 3% O2, and is developmentally expressed in fruit aside from anoxic induction. Naturally occurring mutants and genetically engineered tomatoes with reduced ethylene production, color and/or retarded ripening patterns show changes in volatile concentrations.
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