158 Produce Degradation: Reaction Pathways and their Prevention
ripeness. Many volatile compounds are generated in plants through biosynthetic
processes or during processing of these materials. During fruit and vegetable ripening
the plant undergoes many catabolic reactions to form volatiles as secondary products.
Acyl pathways play a central role in the metabolism of fatty acids, terpenes,
amino acids, and carbohydrates [3,4]. β-Oxidation of linolenyl-CoA results in the
formation of volatiles important in the aroma of pears and other fruit. Hydroxylated
fatty acids (C 8 –C 12 ) are also formed via β-oxidation and may cyclize (spontaneously)
to form γ- and δ-lactones. Branched-chain esters are formed via amino acid metab-
olism to give typical fruity aromas. In strawberries, the character-impact compound
2,5-dimethyl-4-hydroxy-3(2H)-furanone is formed from metabolism of carbohy-
drates. Metabolism of carotenoids leads to the formation of aroma-significant
C 10 and C 13 norisoprenoids. Lipoxygenase-derived compounds play an important
role in the generation of aromas in green plants. Aromas of some vegetables such as
onions are formed by the action of specific enzymes released after tissue disruption.
It is not possible to cover all of the possible compounds and pathways involved
in the generation of fruit and vegetable aromas. The biogenesis of aroma in plants
has been the subject of extensive review [3–6]. This discussion will present only a
few examples of mechanisms involved in the biogenesis of important volatile classes
involved in the aromas of fruits and vegetables.
6.2.3.1 Esters
Esters are one of the largest and most important classes of aroma compounds,
especially in fruit flavors. Most of the intense (low threshold) esters are synthesized
during ripening from free amino acids in the fruit (or from alcohols). For the most
part, biosynthesis of esters in plants is mediated via biochemical processes involving
acetyl CoA transferase [7]. Acetyl CoA transferase is a coenzyme-A-dependent
enzyme that catalyzes the transfer of an acyl moiety residing on an acyl CoA onto
a corresponding alcohol [8]. Ester production is, therefore, dependent on alcohols
derived from lipids, free amino acids, lipoxygenase pathways, or fermentation
(e.g., ethanol). Unsaturated esters, such as methyl and ethyl decadienoates of Bartlett
pears, are derived via (E,Z)-2,4-decadienoyl-CoA by β-oxidation of unsaturated fatty
acids [3]. In apples, β-oxidation of fatty acids is responsible for generation of some
straight-chain esters from acetic, butanoic, and hexanoic acids, which also may
undergo reduction to alcohols prior to esterification [9,10]. Amino acid catabolism
accounts for methyl branched-chain esters as well as alcohols, acids, ketones, and
sulfur-containing and aromatic compounds [3,9]. For example, transamination of
L-leucine leads to the formation of α-ketoisocaproate, which undergoes decarboxy-
lation to form 3-methylbutanoyl-CoA. Further catabolism of 3-methylbutanoyl-CoA
gives rise to 3-methylbutanoate esters, 3-methyl-1-butanol, and 3-methylbutyl esters
[3,5]. Likewise, methionine may be transformed into thio-esters important in some
fruit aromas, such as pineapples [11]. Fatty acid degradation via the lipoxygenase
pathway is responsible for formation of hexanal, (E)-2-hexenal, and (Z)-3-hexenal,
which after further isomerization, oxidation, and/or reduction can participate in ester
synthesis or other metabolic reactions [10].