Flavor and Volatile Metabolism in Produce 161
Ethyl 3-methylthiopropanoate, methyl 3-methylthiopropanoate, and 3-methylthio-
propyl acetate are important thio-esters found in pineapple [11] and passion fruit [44].
Thio-esters are formed from sulfur-containing amino acids as described above for the
branched-chain esters. Various other types of sulfur aroma compounds, such as thiols,
mercapto-alcohols, and mercapto-ketones, are aroma constituents of several fruits, such
as durian [45], passion fruit [46,47], black current [48], and orange [49].
6.2.3.7 Aromatic Compounds
L-Phenylalanine, derived via the shikimic acid pathway, serves as an important
precursor for the biosynthesis of aromatic compounds. This amino acid can give
rise to 2-phenylacetyl-CoA in a reaction analogous to that described above for
leucine metabolism to form esters. 2-Phenylacetyl-CoA is further converted to 2-phe-
nylacetate esters or is reduced to 2-phenylethanol and 2-phenylethyl esters [3]. Other
compounds derived from the shikimic acid pathway include cinnamic aldehyde,
cinamyl alcohol, and esters. Reduction of cinnamyl alcohol leads to formation of
propenyl- and allylphenols including eugenol, methyleugenol, and elemicin found
in bananas [3].
6.2.3.8 Lipoxygenase-Derived Alcohols and Aldehydes
Tissue disruption in green plants releases a group of enzymes that act upon the
available polyunsaturated fatty acids (PUFAs) to yield C 6 , C 8 , and C 9 carbonyls and
alcohols with distinctive green, cut-leaf-like, and melon-like aromas [50]. These
compounds are derived from PUFAs, such as linoleic and linolenic acids, through
the action of endogenous lipoxygenase, hydroperoxide lyases, Z-3/E-2-enal
isomerases, and alcohol dehydrogenases [51,52]. The 9-lipoxygenase acting on
linolenic acid produces a n-9 hydroperoxide. Hydrolysis of this hydroperoxide by
a specific hydroperoxide lyase leads to formation of mainly (Z,Z)-3,6-nonadienal
(watermelon-like aroma), which can undergo spontaneous or enzyme-catalyzed
isomeration to (E,Z)-2,6-nonadienal (cucumber-like aroma). These aldehydes may
undergo reduction to their corresponding alcohols. The conversion of the aldehydes
to their corresponding alcohols is a significant step since it leads to a general decline
in aroma intensity due to alcohols having somewhat higher odor detection thresholds
than the aldehydes. In tomato, for example, linoleic acid and α-linolenic acid are
converted to the n-13 hydroperoxides, which undergo hydrolysis via hydroperoxide
lyase to yield the C 6 aldehydes hexanal and (Z)-3-hexenal, respectively [53,54].
(Z)-3-Hexenal can then undergo isomerization to form (E)-2-hexenal. These alde-
hydes, and their corresponding alcohols formed by action of alcohol dehydrogenase,
are often found among the volatiles of tomato homogenates [54].
6.3 FRUIT AND VEGETABLE FLAVOR SYSTEMS
Fruit and vegetable flavors are composed of complex mixtures of volatile chemicals
derived from numerous biochemical pathways. In addition, some nonvolatile com-
pounds, such as sugars and organic acids, impart sweet and sour tastes, respectively.