Food Biochemistry and Food Processing

23 Biochemistry of Vegetable Processing 539

drates, protein, and fat as their major constituents,
and water as a minor constituent. Soybeans, for ex-
ample, are composed of 36% protein, 35% carbohy-
drate, 19% fat, and 10% water. Wet crops tend to
perish more rapidly than dry crops.

CHEMICAL COMPOSITION OF
VEGETABLES

Fresh vegetables contain more than 70% water, and
very frequently greater than 85%. Beans and other
dry crops are exceptions. The protein content is often
less than 3.5% and the fat content less than 0.5%.
Vegetables are also important sources of digestible
and indigestible carbohydrate, as well as of minerals
and vitamins. They contain the precursor of vitamin
A, beta-carotene, and other carotenoids. Carrots are
one of the richest sources of beta-carotene (provita-
min A).

VITAMINS

Vegetables are major contributors to our daily vita-
min requirements. The nutrient contribution from
a specific vegetable is dependent on the amount of
vitamins present in the vegetable, as well as the
amount consumed. The approximate percentage that
vegetables contribute to daily vitamin intake is vita-
min A, 50%; thiamine, 60%; riboflavin, 30%; nia-
cin, 50%; vitamin C, 100%. Vitamins are sensitive
to different processing conditions including expo-
sure to heat, oxygen, light, free water, or traces
of certain minerals. Trimming, washing, blanching,
and canning can cause loss in the vitamin content of
fruits and vegetables.

MINERALS

The amount and types of minerals depends on the
specific vegetable. Not all minerals in plant materi-
als are readily available, and they are mostly in the
form of complexes. An example is calcium found in
vegetables as calcium oxalate.

DIETARYFIBER

The major polysaccharides found in vegetables
include starch and dietary fiber, cellulose, hemicel-
lulose, pectic substances, and lignin. Cell walls in
young vegetables are composed of cellulose. As the

produce ages, cell walls become higher in hemicel-

lulose and lignin. These materials are tough and fib-

rous, and their consistency is not affected by pro-

cessing.

PROTEINS

Most vegetables contain less than 3.5% protein. Soy-

beans are an exception. In general, plant proteins are

major sources of dietary protein in places where ani-

mal protein is in short supply. Plant proteins, howev-

er, are often deficient or limiting in one or more

essential amino acids. Wheat protein is limiting in

lysine, while soybean protein is limiting in methion-

ine. That is why multiple sources of plant proteins

are recommended in the diet. Enzymes, which cat-

alyze metabolic processes, are important in the reac-

tions involved in fruit ripening and senescence.

LIPIDS

The lipid content of vegetables is less than 0.5% and

is primarily found in the cuticles, as constituents of

the cell membrane, and in some cases, as a part of

the internal cell structure (oleosomes). Even though

lipids are a minor component of vegetables, they

play an important role in the characteristic aroma

and flavor of the vegetable. The characteristic aro-

mas of cut tomato and cucumber result from com-

ponents released from the lipoxygenase pathway,

through the action of lipoxygenase upon linoleic and

linolenic acids and the action of hydroperoxide

lyase on the peroxidized fatty acids to produce vol-

atile compounds. This action is accentuated when

the tissue is damaged. The results of the action of

lipoxygenase are sometimes deleterious to quality;

for example, the action of lipoxygenase on soybean

oil leads to rancid flavors and aromas.

VOLATILES

The specific aroma of vegetables is due to the amount

and diversity of volatiles they contain. Volatiles are

present in extremely small quantities (
 100 
g/g

fresh weight). Characteristic flavors and aromas are a

combination of various compounds, mainly short-

chain aldehydes, ketones, and organic acids. Their

relative importance depends upon threshold concen-

tration (sometimes
1 ppb) and interaction with