Fruit and Vegetable Quality

Mechanisms to prevent peroxidation of membrane lipids are linked
to the activity of antioxidant enzymes and endogenous antioxidants
(Leshem, 1988). According to this hypothesis, senescence is a conse-
quence of the increasing inability of the tissue to maintain a total an-
tioxidant level necessary for a simultaneous elimination of oxygen
radicals. This means that freshness should have a bearing not only on
an attractive appearance of the vegetable but also for the food value due
to a higher level of antioxidants in the tissue. During recent years sev-
eral reports have reviewed the epidemiological evidence that increased
consumption of fruits and vegetables reduces the risk of certain cancer
diseases (Block et al., 1992; Williamson, 1996). Development of meth-
ods aimed at preserving the high level of antioxidants present at harvest
during the postharvest handling will present new challenges for posthar-
vest and food scientists.
It is hard to believe that wound ethylene is the only senescence-
promoting factor in all harvested vegetables, especially in those har-
vested immature or in storage organs with a low metabolic rate. Veg-
etables harvested in a period of rapid growth, a common situation for
broccoli, cauliflower and asparagus spears, constitute suitable plant ma-
terial for investigations into harvest-mediated processes other than degra-
dation of chlorophyll associated with senescence. Early responses of
asparagus and broccoli tissues following harvest have been investigated
to elucidate processes regulating harvest-induced senescence (Irving and
Hurst, 1993; King and Morris, 1994). Within two to four hours after har-
vest, the tips of asparagus and broccoli florets lose large amounts of su-
crose and undergo major changes in gene expression (King et al., 1995).
These cellular responses lead to a markedly altered metabolism. Proteins
and lipids are lost, free amino acids accumulate and a general loss of
cellular compartmentation leading to tissue breakdown occurs. This
seems to be a situation of starvation with progressive deprivation of sug-
ars used as fuel for the rapid respiration. Such physiological changes are
similar to starvation responses found in dark treated leaves (Peeters and
van Laere, 1992). The termination of photosynthesis is accompanied by
a disappearance of starch granules in the chloroplasts paralleling the rate
of chlorophyll degradation.
Rapid physiological changes that occur immediately after harvest in
immature vegetables require strategies to maintain quality during the
postharvest chain. Rapid precooling immediately after harvest will re-
duce the metabolic rate and limit depletion of sugars and synthesis of
undesirable compounds.

110 POSTHARVEST HANDLING AND STORAGE OF VEGETABLES