erated vegetables (MPR). Besides reducing respiration, MA can reduce
microbial spoilage, the incidence of physiological disorders, degradation
of chlorophyll and enzymatic browning.
Preparation of minimally processed, refrigerated vegetables entails
wounding of the plant material. Slicing, chopping, dicing or shredding
result in an increased respiration and ethylene production together with
exposure of leakage cell content. MAP may reduce enzymatic peroxi-
dation of unsaturated fatty acids catalyzed by lipoxygenase with the for-
mation of aldehydes and ketones resulting in off-flavor. Ethylene
production is dependent on oxygen and carbon dioxide is reported to in-
hibit ethylene action by blockage of the receptors. MAP will therefore
delay senescence and chlorophyll degradation induced by ethylene. Dis-
coloration due to formation of brown polymers from oxidation of
o-diphenols to o-quinones catalyzed by the enzyme polyphenol oxidase
may also be inhibited by MAP. This enzyme has a low affinity for oxy-
gen but markedly dependent on pH, which is why added citric acid or
ascorbic acid to MPR will inhibit enzymatic browning more than MAP
alone.
PHYSIOLOGY OF STORAGE
Senescence and Ripening
In plants showing a monocarpic behavior, the ultimate stage of de-
velopment implies a massive mobilization of nitrogen, carbon and min-
erals from leaves, roots and stems to the developing seeds, resulting in
whole-plant senescence with subsequent death of the remaining tissue.
The changes taking place form a genetically programmed sequence with
close coordination at the cell and tissue level (Buchanan-Wollaston,
1997). During growth, similar endogenously controlled deteriorative
changes may also take place in the older leaves due to unfavorable con-
ditions such as nitrogen deficiency, light limitation or drought. In the
senescing leaf, degradation of macromolecules is followed by remobi-
lization of the components to developing parts of the plant. In vegeta-
bles composed of both old leaves and young developing leaves, e.g.,
cabbage, onions and lettuce, postharvest senescence of the outermost
leaves will result in a translocation of carbohydrates and amino acids to
the younger leaves and the stem but for many harvested vegetables a
similar remobilization is not possible. The senescence process seems