228 Produce Degradation: Reaction Pathways and their Prevention
or insoluble. Soluble fiber includes noncellulosic polysaccharides, pectins, gums,
and mucilages (Dreher, 1987). They possess the capacity to imbibe water and swell,
and thus they contribute to dietary bulkiness and viscosity in foods and may improve
fecal incontinence (Prakash, 1995; Bliss et al., 2001). Soluble dietary fiber, partic-
ularly pectins and gums, also causes distension of the stomach, which results in
feelings of satiety. Soluble fiber increases the activity of gastric enzymes by pro-
tecting them from degradation. It also delays absorption of glucose and fat after a
meal, and it increases fecal loss of bile acids by binding and promoting their loss
through fecal matter. In this respect, soluble fiber has been shown to have a poten-
tially important role in glucose and fat metabolism and hence cardiovascular disease
and diabetes (Florholmen et al., 1982; Anderson et al., 2000; Burke et al., 2001;
Clark et al., 2002; Jenkins et al., 2002). Among the different fractions of fiber, lignin
has the strongest binding capacity. Insoluble dietary fiber provides a carbohydrate
substrate, which stimulates growth of bacteria in the large bowel. It increases fecal
bulk and reduces transit time, both of which are important in cancer risk reduction
(Chavan and Kadam, 1989; Dickerson, 1993).
In summary, vegetables and fruits provide the dietary fiber essential for bowel
movement and possibly for decreasing the risk of health problems such as consti-
pation and fecal incontinence, coronary heart disease, appendicitis, colon cancer,
diabetes, diverticulosis, gallstones, and obesity as well as assisting in improving
blood glucose control and insulin utilization in individuals with diabetes and decreas-
ing blood pressure (Florholmen et al., 1982; Roy and Chakrabarti, 1993; Anderson
et al., 2000; Bliss et al., 2001; Burke et al., 2001; Fleischauer and Arab, 2001; Clark
et al., 2002; Hsing et al., 2002; Jenkins et al., 2002; Southon and Faulks, 2002).
8.2.1.4 Lipids
The lipid/fat content of fruits and vegetables is generally low and varies with the
type of fruits or vegetables (olives and avocados are especially high in fat content).
Lipids/fats and carbohydrates are the major sources of energy for humans. However,
since fruits and vegetables are not good sources of fats, they contribute to total
energy mostly through their carbohydrate content. To a limited extent, the proteins
and organic acids found in fruits and vegetables can also serve as a source of energy
(Holland et al., 1992).
8.2.2 MICRONUTRIENTS
Fruits and vegetables contain significantly more micronutrients than proteins and
fat (as depicted in Table 8.1). Generally, vegetables have a much higher content of
minerals than fruits, while some fruits have a higher content of some vitamins relative
to vegetables (e.g., vitamin C or ascorbic acid in citrus fruit). When viewed within
the context of the total food supply, fruits and vegetables contribute a significant
amount of the micronutrients compared to macronutrients (Figure 8.1): 90% of the
dietary vitamin C, more than 50% of the vitamin A, and more than 35% of vitamin
B 6. Hence, the importance of fruit and vegetables in human nutrition is clearly
evident.