Flavor and Volatile Metabolism in Produce 167
6.3.2 VEGETABLE FLAVORS
Whitfield and Last [59] reviewed the subject of volatile constituents of vegetables
and covered the published literature up to about 1991. Since that time much progress
has been made in our understanding of vegetable flavors. Most studies have dealt
with the flavor and aroma composition of cooked or prepared (e.g., fermented)
vegetables. This seems reasonable since most vegetables are consumed after cooking
or further processing. A discussion of the flavor and volatile components of all major
vegetable crops is beyond the scope of this chapter. Instead, this section will present
recent findings on some selected vegetables that are commonly consumed in their
raw (uncooked) state, such as lettuce, celery, carrot, bell pepper, broccoli, and onion.
6.3.2.1 Lettuce
The green leafy portion of lettuce (Lactuca sativa L.) is mostly consumed in its raw
state as a salad component. To date, very few studies have been conducted on the
volatile components of lettuce. Murray and Whitfield [144] reported the occurrence
of earthy/bell pepper-like smelling 2-isopropyl-, 2-sec-butyl-, and 2-methylpropyl-
3-methoxypyrazines in the headspace of raw lettuce. Volatile emissions of (Z)-3-
hexenal, (Z)-3-hexen-1-ol, and (Z)-3-hexenyl acetate from lettuce have been reported
[145].
6.3.2.2 Celery
The main volatile constituents of raw celery (Apium gaveolens L.) stalk are monot-
erpene and sesquiterpene hydrocarbons, accounting for over 80% of the volatile oil.
Additional major components include monoterpene alcohols, phthalides, and terpene
carbonyls [146]. Among these, the phthalides are considered the most important
with respect to aroma [147]. 3-Butylphthalide and some hydrogenated derivatives,
sedanolide (3-butyl-3α,4,5,6-tetrahydrophthalide), 3-isobutylidene-3α,4,5,6-
tetrahydrophthalide, and 3-isobutylidene-3α,4-dihydrophthalide, were reported as
character-impact aroma components of celery [148,149].
6.3.2.3 Carrot
More than 90 volatile compounds have been identified in carrot (Daucus carota L.)
[150]. Mono- and sesquiterpenes are the predominant volatile compounds, compris-
ing over 98% of the total volatile composition [150,151]. Characteristic aroma
components of carrot include α- and β-pinene, β-myrcene, γ-terpinene, terpenolene,
6-methyl-5-hepten-2-one, β-bisabolene, β-ionone, and myristicin [150,152–154].
6.3.2.4 Bell Pepper
Bell or sweet pepper (Capsicum annuum, var. grossum Sendt) is often consumed in
the fresh state in salads or as cut vegetables in vegetable platters. Fresh bell pepper
aroma is best characterized by 2-methylpropyl-3-methoxypyrazine, which has an
earthy aroma reminiscent of bell peppers [155,156]. Other compounds reported to