Nutrient Loss 227
However, some vegetables such as potatoes have highly digestible proteins, although
the total protein content is low (2%) (Salunkhe and Kadam, 1991). Compared to
other fresh vegetables, mushrooms have a high protein content. The protein content
of mushrooms approaches that of beans, which ranges from 16 to 25% on a dry
weight basis (Bano and Rajarathnam, 1988).
Protein in fruits and vegetables is also present in the form of simple nitrogenous
substances that may be present in the uncombined form. For example, asparagine
and glutamine and their acids aspartic and glutamic acids are abundant in fruits and
vegetables such as strawberries, tomatoes, and citrus fruits. Asparagine is also very
high in apples and pears, and oranges are rich in proline (Salunkhe et al., 1991).
The protein content in these fruits and vegetables can be estimated by multiplying
the total nitrogen content by a factor of 6.25. This figure is based on the fact that
protein contains about 16% nitrogen.
8.2.1.3 Carbohydrates
Carbohydrates play a pivotal role in human energy modulation (especially blood
glucose, can affect lipid and vitamin and mineral absorption, have an impact on
fecal bulk and colonic motility, and can have protective influences (e.g., against
cancer) (Mahan and Escott-Stump, 2000). Vegetables are generally lower sources
of carbohydrates than are fruits. Carbohydrates consist of polysaccharides such as
starch and fiber (cellulose, hemicellulose, and pectic material), and also of disac-
charides and monosaccharides such as the sugars sucrose, fructose, and glucose
(Wardlaw and Kessel, 2002). The amount of each of these constituents can change
significantly, especially during ripening of fruits. Sugars are usually abundant when
a fruit reaches full maturity. In fruits containing starch, all the starch is usually
hydrolyzed to glucose and fructose, but in some fruits such as peaches, apricots, and
nectarines sucrose is the main sugar (Wills et al., 1983). Traces of other mono- and
disaccharide sugars such as xylose, arabinose, mannose, galactose, and maltose may
also be present in fruits in small quantities. Sorbitol, a polyol related to sugars in
structure, known for its laxative effect, is also present in relatively high concentrations
in pears and plums (Wrolstad and Sallenberger, 1981). However, there are fruits, such
as strawberries, that do not have sorbitol at all (Wrolstad and Shallenberger, 1981).
Fruits and vegetables are excellent sources of dietary fiber. Dietary fiber is the
structural material of plant cells that is resistant to the digestive enzymes in the
human stomach or intestines. It includes the structural polysaccharides of the cell
wall such as cellulose, hemicelluloses, lignin, gums, pectins, and mucins with dif-
ferent chemical, physiochemical, and physiological properties (Ross et al., 1985;
Dreher, 1987). Cellulose and hemicellulose, which have numerous functional uses
in foods, are derived from the cell walls of fruits and vegetables. Pectin, which is
also found in cell walls of fruits, is used commercially for the manufacture of jams
and jellies because of its gelatinous or thickening properties. Pectin is also commonly
extracted from the white spongy layers of citrus fruit skins, especially grapefruits
and lemons.
The dietary and health benefits of fiber vary widely depending on the charac-
teristics of polysaccharides. Total dietary fiber can be further classified as soluble