230 Produce Degradation: Reaction Pathways and their Prevention
spinach, and tomatoes, are rich in provitamin A, while most green leafy vegetables are
good sources of folate. On average, green leafy vegetables provide about 40 to 80
μg of folic acid per 100 g of edible portions (Salunkhe et al., 1991). Folate is also
present in significant amounts in fruits such as strawberries, oranges, and grapefruits.
Other fruits such as strawberries, black currants, and citrus fruits are moderate to
good sources of ascorbic acid (Salunkhe et al., 1991). Plums and tomatoes are good
sources of niacin and pantothenic acid.
Some vitamins, especially vitamins B 12 , D, and E, are not present in significant
amounts in fruits. Vitamin B 12 is found in moderate amounts in vegetables, but it is
not present in fruits. Vitamin D is also absent in fruits, while vitamin E occurs only
in small quantities in fruits (Holland et al., 1992). Considerable differences in vitamin
content are reported between fruit and vegetable species and varieties as well as
within the same variety grown under different environmental conditions. Climate,
soil, and agronomical practices affect the level of vitamins in fruits and vegetables.
The application of fertilizer to plants, for example, affects the concentration of
provitamin A in the leaves and fruit.
8.3 HEALTHY NONNUTRIENT COMPOUNDS
8.3.1 FLAVOR COMPOUNDS
Apart from the major nutrients, there are compounds in fruits and vegetables that
play significant roles in food consumption and health. These compounds influence
the appearance, color, taste, and aroma of fruits and vegetables, thus affecting
consumption and subsequent nutrient benefit from these foods significantly. Flavor
compounds in vegetables include sugars, amino acids, organic acids, aromatic com-
pounds, hydrocarbons, aldehydes, acetals, ketones, alcohols, esters, and sulfur com-
pounds (volatiles). The aromatic compounds in onions and garlic, for example,
explain the common use of these vegetables as flavoring agents in a variety of soups,
sausage, and curries. With respect to garlic, the volatile sulfur compound diallyl
disulfide is responsible for the garlicky aroma. Diallyl disulfide is produced by the
action of allinase on the amino acid allin present in garlic (Roy and Chakrabarti,
1993). In addition to flavor, garlic has been investigated as a potential health sup-
plement for cancer prevention (Fleischauer and Arab, 2001; Hsing et al., 2002).
In fruits, organic acids play a role in taste through the sugar/acid ratio. Sugars
provide the sweetness and organic acids the sourness in fruits. The main organic
acids present in fruits are citric and malic acids. Citrus fruits, strawberries, pears,
and tomatoes predominantly contain citric acid, while apples, plums, cherries, and
apricots primarily contain malic acid. In peaches, these two acids are present in
equal amounts. Grapes differ from the other fruits because they contain high levels
of tartaric acid. In general, there is a decrease of acidity during ripening in all fruits.
Bitterness in fruits, though not common, is a characteristics of flavonoids, such as
naringin in grapefruits and limonene in citrus fruits. The aroma of fruits is a key
factor in assessing quality as well as identity (Salunkhe et al., 1991). The major
chemical compounds associated with aroma are esters of aliphatic alcohols and short-
chain fatty acids.