540 Part V: Fruits, Vegetables, and Cereals
other compounds. Of more than 400 volatiles identi-
fied in tomato, the following have been reported to
play important roles in fresh tomato flavor: hexanal,
trans-2-hexenal, cis-3-hexenal, cis-3-hexenol, trans-
2-trans-4 decadienal, 2-isobutylthiazole, 6-methyl-5-
hepten-2-one, 1-penten-3-one, and -ionone (Petro-
Turza 1986–87).
WATER
In general, water is the most abundant single com-
ponent of vegetables (up to 90% of total weight).
The maximum water content of vegetables varies be-
tween individuals due to structural differences. Ag-
ricultural conditions also influence the water content
of plants. As a major component of vegetables,
water impacts both quality and rate of deterioration.
Harvest should be done during the cool part of the
day in order to keep turgidity at its optimum. Loss of
turgor under postharvest storage is a major quality-
reducing factor in vegetables (wilting of leaves such
as spinach).
ORGANICACIDS
Organic acids are important contributors to the taste
and flavor of many vegetables (tomato). Total titrat-
able acidity and the quantity and specificity of or-
ganic acids present in vegetables influence the buf-
fering system and the pH. Acid content decreases
during maturation, because part of it is used for res-
piration and another part is transformed into sugars
(gluconeogenesis).
PIGMENTS
Pigments are mainly responsible for the skin and
flesh colors in vegetables. They undergo changes
during maturation and ripening of vegetables, in-
cluding loss of chlorophyll (green color), synthesis
and/or revelation of carotenoids (yellow and orange),
and development of anthocyanins as in eggplant (red,
blue, and purple). Vegetables such as carrots are es-
pecially rich in carotene, and red beets owe their red
color to betacyanins. Anthocyanins belong to the
group of flavonoids, occur as glycosides, and are
water soluble. They are unstable and easily hydro-
lyzed by enzymes to free anthocyanins. The latter are
oxidized to brown products by phenol oxidases. The
colors of anthocyanins are pH dependent. In a basic
medium they are mostly violet or blue, whereas in an
acidic medium they tend to be red.PHENOLICSPhenolic compounds found in vegetables vary in
structure from simple monomers to complex tan-
nins. Under most circumstances they are colorless,
but after reaction with metal ions, they assume red,
brown, green, gray, or black coloration. The various
shades of color depend on the particular tannins, the
specific metal ion, pH, and the concentration of the
phenolic complex. Phenolics, which are responsible
for astringency in vegetables, decrease with maturi-
ty, because of the conversion of astringent phenolics
from soluble to insoluble form (eggplant, plantain).CARBOHYDRATESThe carbohydrate content of vegetables is between 3
and 27% (Table 23.3). Carbohydrates vary from low
molecular weight sugars to polysaccharides such as
starch, cellulose, hemicellulose, pectin, and lignin.
The main sugars found in vegetables are glucose, fruc-
tose, and sucrose. Vegetables contain higher amounts
of polysaccharides (starch), whereas fruits contain
more mono- and disaccharides instead.TURGOR ANDTEXTUREThe predominant structural feature of vegetables is
the parenchyma cell. The texture of vegetables is
largely related to the elasticity and permeability of the
parenchyma cells. Cells with a high content of water
exhibit a crisp texture. The cell vacuoles contain
most of the water of plant cells. The vacuolar solution
contains dissolved sugars, acids, salts, amino acids,
pigments, and vitamins, as well as several other low
molecular weight constituents. The osmotic pressure
within the cell vacuole, and within the protoplast
against the cell walls, causes the cell walls to stretch
slightly in accordance with their elastic properties.
These processes determine the specific appearance
and crispness of vegetables. Damaged vegetables,
after processing, lose their turgor and tend to become
soft unless precautions are taken in packaging and
storage.