234 Produce Degradation: Reaction Pathways and their Prevention
to increase consumption of phytonutrient-rich foods including fruits, vegetables, and
whole grains.
8.5 ENZYMES AND HORMONES IN FRUITS AND VEGETABLES
Enzymes are important biological compounds that catalyze chemical reactions in
both fruits and vegetables. Common enzymes in fruits and vegetables include phe-
nolase, lipases, oxidases, and phenoloxidases found in apples, pears, grapes, straw-
berries, avocados, and figs (Salunkhe, 1991). They cause quality/structural deterio-
ration of produce, which in turn affects acceptability and consumption. These
enzymes are responsible for the discoloration of cut surfaces of fruits and vegetables
when exposed to air. Other enzymes include polygalacturonase in tomatoes and
peptic esterase in citrus fruits and tomatoes. These pectolytic enzymes are respon-
sible for softening of fruits during ripening.
The ethylene hormone produced or used in the fruit ripening process has shown
some effects on the nutritive value of fruits and vegetables. Ethylene has been
reported to increase concentration of vitamin C/ascorbic acid in fruits when used to
aid ripening. For example, the average content of ascorbic acid was different in
tomatoes harvested mature-green compared to those harvested red-ripe (Saltveit,
1999). Variations in ascorbic acid content were large. The differences among culti-
vars were greater between ripe fruits harvested as mature-green or red-ripe. The
ascorbic acid content of fruits harvested mature-green and ripened with the aid of
ethylene was higher than for untreated fruit. Similarly, the content of ascorbic acid
was significantly higher in papaya fruit ripened with the aid of ethylene than in
controls left to ripen on their own (Bal et al., 1992). However, this effect was not
consistent among fruits from different growers or fruits harvested at different times
of the year. In both cases, the effect of ethylene was not directly on ascorbic acid
only, but also through the stimulation of the other ripening parameters. As a result,
the fruit ripened quicker, and there was less time for the loss of ascorbic acid.
Therefore, enzymes and hormones in fruits and vegetables might have an impact
on nutrient content or losses. It is important to consider the extent to which these
are present or used when determining nutrient differences or content in fruits and
vegetables and processed products.
8.6 PATHWAYS OF NUTRIENT LOSS
The major pathways for potential nutrient loss in fruits and vegetables are summa-
rized in Figure 8.2.
8.6.1 PREHARVEST
The postharvest storage life of fresh produce or processed products depends on
preharvest agronomical and environmental conditions. These are important determi-
nants of quality and nutritional value of fresh produce. Procedures such as early or
late harvesting of fruits and vegetables have significant effects on the nutritional
quality of produce (Salunkhe and Kadam, 1998).