240 Produce Degradation: Reaction Pathways and their Prevention
causes loss of weight (as some of the food components are oxidized), loss of
sweetness, and change in texture. Generally, during storage of fruits and vegetables,
there is an initial increase in sugar concentration, followed by a decrease (Potter,
1986). This is typical of climacteric fruits; the increase is due to the breakdown of
polysaccharides during respiration. For example, starch hydrolysis in bananas results
in equal concentrations of glucose and fructose and a little sucrose. When bananas
are stored for a long time, the three sugars decrease in amount. Mangoes also have
shown a large increase in sucrose, which is later converted to reducing sugars
(Salunkhe and Kadam, 1998). In nonclimacteric fruits, changes in sugar content are
slight and slow.
8.8 STABILITY OF NUTRIENTS AND OTHER HEALTHY
COMPOUNDS IN FRUIT AND VEGETABLES
DURING STORAGEBoth nutrients and nonnutrient compounds may be adversely affected during storage,
as follows.
8.8.1 PROTEINS
The storage of fruit and vegetables influences the stability of proteins, amino acids,
and other nitrogenous compounds. Generally, proteolysis occurs early in storage
with a corresponding increase in free amino acids in fruits and vegetables (Salunkhe,
1974; Salunkhe et al., 1991). The free amino acids are then metabolized into nitro-
gen-storage compounds. These nitrogen-storage compounds are degraded at high
temperature to liberate ammonia, which is accompanied by a rapid decrease in amino
acid concentrations. Monitoring of this sequence of reactions has been used to
characterize the degree of freshness of fruits and vegetables when they are stored
at low temperatures. The actual storage conditions also affect the stability of amino
acids in fruits and vegetables. For example, storage of potatoes in a high-nitrogen
or in a low-oxygen and high-CO 2 atmosphere increases the level of free amino acids
(Salunkhe and Kadam, 1998). The quantity of proteins, nitrates, and nitrites and free
amino acids in fruits and vegetables are mostly governed by preharvest conditions
such as climate, agronomic practices, varieties, soil fertility, maturity, and photo-
synthesis. Nitrogenous compounds, nitrates and nitrites, have received much atten-
tion in recent years because they have been associated with the methemoglobinemia
condition in humans (Salunkhe et al., 1991). The conversion of nitrates to nitrites
is the hazardous principle of nitrate accumulation in fruits and vegetables (Salunkhe
et al., 1991). Nitrate formation is usually a postharvest process. During storage,
nitrate may be reduced to nitrite, but the conversion rate depends mostly on the
storage conditions.
The higher the storage temperatures, the higher the quantity of nitrites in fruits
and vegetables. Nitrate reductase activity declines quickly or remains steady during
storage of produce. Treatment with high CO 2 does not affect the nitrate concentra-
tion; however, packaging in nitrogen has been shown to reduce the nitrate content