BLBS102-c29 BLBS102-Simpson March 21, 2012 13:27 Trim: 276mm X 219mm Printer Name: Yet to Come
572 Part 5: Fruits, Vegetables, and CerealsVitaminsVegetables are major contributors to our daily vitamin require-
ments. 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: vitamin
A-50%, thiamine-60%, riboflavin-30%, niacin-50%, and vita-
min C-100%. Vitamins are sensitive to different processing con-
ditions including; exposure to heat, oxygen, light, free water,
and traces of certain minerals. Trimming, washing, blanching,
and canning can cause loss in vitamin content of fruits and
vegetables.MineralsThe amount and types of minerals depend on the specific veg-
etable. Not all minerals in plant materials are readily available
and are mostly in the form of complexes. An example is calcium
found in vegetables as calcium oxalate. Green leafy vegetables
are rich in magnesium and iron.Dietary FiberThe major polysaccharides found in vegetables include starch,
and dietary fiber such as cellulose, hemicellulose, pectic sub-
stances, and lignin. Cell walls in young vegetables are composed
of cellulose. As the produce ages, cell walls become higher in
hemicellulose and lignin. These materials are tough and fibrous
and their consistency is not affected by processing. Some veg-
etables such as potato also contain varying amounts of starch
resistant to hydrolysis (resistant starch). Resistant starch is not
digested as efficiently as regular starch, and reaches the colon
where they undergo microbial fermentation. The short chain
fatty acids liberated during the digestion of resistant starch is
considered to be beneficial to the health. Roots of vegetables
such as endive are rich in fructooligosaccharides such as inulin,
which are considered to possess health regulatory function.ProteinsMost vegetables contain less than 3.5% protein. Soybeans are
an exception. In general, plant proteins are major sources of di-
etary protein in places where animal protein is in short supply.
Plant proteins, however, are often deficient or limiting in one or
more essential amino acids. Wheat protein is limiting in lysine,
while soybean protein is limiting in methionine. Leafy green
vegetables are also rich in proteins, especially photosynthetic
proteins such as ribulose-bis-phosphate carboxylase oxygenase
(RubisCO) and other chloroplast proteins. Multiple sources of
plant proteins are recommended in the diet because of the
absence of key amino acids.LipidsThe lipid content of vegetables is less than 0.5% and primarily
found in the cuticles, as constituents of the cell membrane, andin 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 aroma of cut tomato and
cucumber results from components released from the lipoxyge-
nase pathway, through the action of lipoxygenase upon linoleic
and linolenic acids and the hydroperoxide lyase action on the
peroxidized fatty acids to produce volatile compounds. This ac-
tion is accentuated, when the tissue is damaged. The results
of the action of lipoxygenase are sometimes deleterious to the
quality—for example, the action of lipoxygenase on soybean oil
leads to rancid flavors and aromas.VolatilesThe specific aroma of vegetables is due to the amount and diver-
sity of volatiles they contain. Volatiles are present in extremely
small quantities (<100 ug/g fresh wt). Characteristic flavors
and aromas are a combination of various compounds, mainly
short chain aldehydes, ketones, and organic acids. Their relative
importance depends upon threshold concentration (sometimes
<1 ppb), and interaction with other compounds. Of more than
400 volatiles identified in tomato, the following have been re-
ported 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, andβ-ionone (Petro-Turza 1986–1987).WaterIn general, water is the most abundant single component of
most vegetables (up to 90% of total weight). The maximum
water content of vegetables varies between individuals due to
structural differences. Agricultural conditions also influence the
water content of plants. As a major component of vegetables,
water impacts both on the quality and the rate of deterioration.
Harvest should be done during the cool part of the day in order
to keep the turgidity to its optimum. Loss of turgor under post
harvest storage is a major quality-reducing factor in vegetables
(wilting of leaves such as spinach).Organic AcidsOrganic acids are important contributors to the taste and flavor
of many vegetables (tomato). Total titratable acidity, the quantity
and specificity of organic acids present in vegetables influence
the buffering system and the pH. Acid content decreases during
maturation, because of its use for respiration and transformation
into sugars through gluconeogenesis. Certain fruits are used
as vegetables when immature. For example, immature mango
fruits, which are rich in organic acids, are used as vegetables.PigmentsPigments are mainly responsible for the skin and flesh colors
in vegetables. The vegetables undergo changes during matura-
tion and ripening of vegetables, including loss of chlorophyll