Fruit flavors and aromas

Fruit flavor is a complex trait determined by the mixture and balance of sugars, acids and a large number of aldehyde, ketone and alcohol volatiles (Buttery, 1993; Baldwin et al., 2000). Lipoxygenases are key enzymes in fatty acid metabolism, producing the hexanal aldehyde and alcohols that contribute substantially to the volatile tomato aromas and flavors. At least three lipoxygenase genes (LOX) are expressed during tomato fruit ripening (Ferrie et al., 1994; Kausch and Handa, 1997; Griffiths et al., 1999a). The expression of the LOX genes in ripening tomato fruit is regulated by both ethylene and developmental factors. In order to modify the C6 aldehyde and alcohol composition of tomatoes, antisense transgenic tomatoes were developed using a conserved region of lipoxygenases to potentially suppress expression of all LOX activities (Griffiths et al., 1999b). In ripe fruit of these transgenic plants, the expression of tomloxA and tomloxB, but not tomloxC, was substantially reduced. However, no changes in the flavor and aroma constituents resulted, suggesting that regulation of their biosynthesis may be complex or that tomloxC expression was sufficient for these pathways to proceed unimpeded.

Another approach to altering the flavor composition of tomato fruit was engineered by the expression of a yeast Δ9 desaturase gene (Wang et al., 1996), resulting in changes in the fatty acid content of fruit. Concentrations of palmitoleic acid, 9,12-hexadienoic acid, and linoleic acid increased and concentrations of palmitic acid and stearic acid were reduced. Concentrations of flavor compounds derived from these fatty acids were changed and resistance to powdery mildew was enhanced (Wan et al., 1998). Another approach to changing the composition of flavor aldehydes in tomato fruit was taken by altering the expression of alcohol dehydrogenase adhII (2) (Speirset al., 1998; Prestage et al., 1999). The amount of Z-3-hexanol decreased and the amount of 3-methylbutanal increased in plants with reduced expression of adhII. Fruit from plants with increased expression of adhII had a more intense ‘ripe fruit’ flavor and had increased amounts of hexanol and Z-3-hexanol.