Nutrient Loss 241
and keep the enzyme nitrate reductase activity high (Salunkhe, 1974; Salunkhe et
al., 1991). Frozen and canned fruits and vegetables do not form nitrites (Salunkhe
et al., 1991; Salunkhe and Kadam, 1998).
8.8.2 CARBOHYDRATES
During storage, reduction in firmness of fruits and vegetables may be caused by the
degradation of insoluble protopectins to the more soluble pectic acid and pectin. If
no ripening occurs in storage, changes in the pectic substances are minimal. For
example, in grapefruits, no detectable changes are observed in soluble pectin or
protopectin substances during storage for up to 6 weeks because no ripening occurs
during storage (Salunkhe et al., 1991).
In produce, storage temperature and treatments also influence the production of
volatile materials. Controlled-atmosphere storage lowers the production of volatile
compounds, and fruits usually evolve volatiles fairly late after removal from a
controlled atmosphere. Under very low oxygen conditions, off-flavors may develop
in fruits due to accumulation of acetaldehydes. During storage, some enzymes also
increase their activity. For example, catalase, pectinesterase, cellulase, and amylase
increase their activity during storage. Oxidases usually exhibit a reduction in activity.
The activity of most enzymes is dependent on storage temperature and maturity of
the stored fruits. Fully mature fruits usually show higher activities of catalase and
pectinesterase and a lower oxidase activity during storage compared to fruits picked
at a relatively less mature stage. Conversely, starch hydrolytic enzyme activity is
greater in immature fruits than in fully mature ones (Salunkhe et al., 1991).
8.8.3 LIPIDS
Storage also affects the lipid fraction in fruits and vegetables. Fruits usually become
greasy in storage. The increase in the amount of waxy constituents of the cuticle is
most noticeable in apples. Wax analysis shows that the oily fraction increases at a
faster rate than the ursolic acid (Salunkhe, 1974). The nonvolatile esters are produced
most rapidly during the early stages of storage, while the volatile esters appear much
later. With prolonged storage, the soft wax fraction of the cuticle accumulates,
increasing amounts of unsaturated compounds. The increase in wax, however,
depends on the variety of fruit. For example, the surface wax of Cox’s Orange Pippin
apple variety remains constant in quantity and composition, while in the Bramley
apple variety there is an increase in the amount of surface wax and esters (Salunkhe,
1974). As a whole, saturated fatty acids increase as the storage period increases and
higher unsaturated fatty acids such as linolenic, linoleic, and oleic acids are metab-
olized rapidly during the early stages of the storage period (Salunkhe, 1974; Salunkhe
et al., 1991). This may have implications for some products such as dehydrated
potato products, because a high percentage of low-molecular-weight polyunsaturated
fatty acids may cause oxidative off-flavors. Controlled-atmosphere storage increases
the content of palmitic and palmitoleic acids and decreases oleic acid (Salunkhe et
al., 1991). The percentage of polyunsaturated fatty acids in avocados is therefore
higher in a more oxygenated atmosphere (Salunkhe et al., 1991).