226 Produce Degradation: Reaction Pathways and their Prevention
compounds. However, apart from one or two exceptions, these compounds are also
present (in varying amounts) in most other fruits and vegetables. Most of the nutrients
are essential for the human body. The amount of each of the nutrients required by
the body depends on factors such as age, body mass, gender, physiological state,
health status, and level of physical activity of individuals (National Academy of
Sciences, 1999). Some nutrients are required only in small quantities, while others
are required in relatively large amounts. Some of the key metabolic roles of these
nutrients or nutrition-related components include healthy vision, healthy red blood
cells, skin and bone strength and maintenance, enhanced immune system (some
components in fruits and vegetables have the capacity to act as antioxidant, antiviral,
and antibacterial agents), healthy neuromuscular and gastrointestinal systems, anti-
inflammatory and anticancer agents, and cardiovascular health (Attaway, 1994; Mid-
dleton and Kandaswami, 1994; Meydani et al., 1997; Groff and Gropper, 2000;
Volpe, 2000; Jongen, 2002; Fragakis, 2003).
8.2.1 MACRONUTRIENTS
8.2.1.1 Water
Water is an essential component of all body tissues and critical to the physiologic
processes of digestion, absorption, and excretion. It is key to the structure and
function of the circulatory system, maintains physical and chemical constancy of
intracellular and extracellular fluids, and has a direct role in the maintenance of body
temperature. Survival without water is limited to 10 days in moderate weather
(Mahan and Escott-Stump, 2000). It is the most abundant component of fruits and
vegetables. On average, fruits and vegetables contain 80 to 85% water. Grapes,
strawberries, and tomatoes, for example, have a water content as high as 82, 90, and
93%, respectively (Holland et al., 1992). The maximum water content in fruits and
vegetables may vary between fruits and vegetables of the same kind due to structural
differences. These structural differences arise from the influence of agronomic condi-
tions on structural differentiation of fruit and vegetable tissues (Salunkhe et al., 1991).
8.2.1.2 Proteins
The structure of humans is based on proteins, which are also a source of energy
(Mahan and Escott-Stump, 2000). The quality of a protein in food is largely deter-
mined by the number and amount of essential amino acids, which cannot be syn-
thesized by the body. A protein that contains all 10 essential amino acids (valine,
threonine, tryptophan, isoleucine, leucine, methionine, lysine, phenylalanine, histi-
dine, and arginine) is referred to as a complete protein (Potter, 1986). Apart from
being a complete protein, a high-quality protein must have all the amino acids fully
bioavailable to the body in the correct proportions. Incomplete proteins may be
supplemented with the essential amino acids either in the form of synthetic com-
pounds or as protein concentrates from natural sources.
Proteins usually constitute less than 1% of the fresh weight of fruits and less
than 5% of the weight of fresh vegetables. The protein content of fresh fruits and
vegetables is generally of low biological value compared to that of animal products.