168 Produce Degradation: Reaction Pathways and their Prevention
contribute to raw bell pepper aroma include (E,Z)-2,6-nonadienal, (E,E)-2,4-deca-
dienal, and 2-sec-butyl-3-methoxypyrazine [144,155].
6.3.2.5 Broccoli
Broccoli (Brassica oleracea var. itallica) is one of the cruciferous vegetables, which
include cabbage, cauliflower, mustard, and horseradish. These products often have
characteristically irritating, pungent, and lachrymatory odors. As explained earlier,
the flavor constituents of crucifers are primarily formed through enzymatic processes
in disrupted tissues or may form during cooking. In raw broccoli, tissue disruptions
lead to formation of numerous isothiocyanates from glucosinolate precursors. The
isothiocyanates are unstable and are destroyed during cooking to allow nitriles and
other sulfur-containing degradation products and rearrangement products to predom-
inate. In a recent study, the major volatiles in blanched broccoli were identified,
including 3-methyl-2-pentanone, hexanal, heptanal, cyclopentanecarboxaldehyde,
ethyl acetate, 3-methylbutanal, 3-butenenitrile, 2-methylbutanal, dimethyl trisulfide,
and dimethyl disulfide [157]. In raw broccoli, cysteine lyase action on sulfur-con-
taining amino acid, S-methylcysteine S-oxide, can generate methanethiol, dimethyl
sulfide, dimethyldisulfide, and dimethyltrisulfide [158]. These same volatiles may be
formed by thermal degradation of S-methylcysteine and its S-oxide derivative [159].
6.3.2.6 Onion
Members of the genus Allium, such as onions, garlic, leeks, chives, and shallots,
have characteristically strong, pungent, and penetrating odors that are formed fol-
lowing tissue damage as discussed above. Onion (A. sepa) flavor is dominated by
volatile sulfur-containing compounds that arise from the enzymatic hydrolysis of
S-alk(en)yl-L-cysteine S-oxide [42]. For example, breakdown of the major flavor
precursor, S-1-propenyl-L-cysteine S-oxide, leads to formation of lachrymatory com-
pounds, such as propanethial S-oxide [160–162]. A series of sulfinates are then
formed from various sulfenic acid intermediates to give the characteristic aroma of
onion [161].
6.4 FACTORS INFLUENCING FLAVOR AND FLAVOR
STABILITYThe aroma profiles can change rapidly during postharvest storage of fresh fruits and
vegetables, particularly for climacteric fruits, in which the dominant volatiles may
be quite different in the unripe fruit, ripe fruit, or overripe fruit [163]. Some produce,
especially those designed for vegetative reproduction (carrot, potato, and onion) and
generative reproduction (seeds and fruits) are well-equipped to live detached from
the host plant. These products contain relatively large amounts of carbohydrate
reserves that enable the maintenance of respiration and energy production. Other
harvested produce, such as leaves (spinach) or whole plants (lettuce or endive) do
not contain sufficient storage reserves and are susceptible to rapid senescence and
wilting [164].