Microstructure of Produce Degradation 555
indicated that the primary mode of tissue failure was cell-to-cell debonding, not cell
breakage, in both air- and CO 2 -treated fruit [60]. An increase in firmness over harvest
is common in fruit that is stored at 0°C. The mechanism is not well understood but
is theorized to be due to an increase in viscosity of pectin because no change in
structure or chemistry is observed [61,62].
18.7 SPECIFIC DISORDERS AND THEIR IMPACT
ON PRODUCE QUALITYProduce shows an increase in susceptibility to postharvest diseases and infestation
during prolonged storage partly due to ongoing physiological changes that enable
pathogen development on or in the fruits [63]. Electron microscopy exhibited the
“withered juice sac” or dry pulp disorder, where the soluble solids content is higher
in the rind than in the pulp, in Ponkan mandarin orange at the cellular level. Prior
to the onset of symptoms of the disorder, nuclear divisions occurred in the cells of
the flavedo (the colored part of the rind). The appearance of symptoms corresponded
to further changes in the flavedo cells, the enlargement of the cells and their vacuoles,
and a decrease in the cytoplasm content. The tonoplast disappeared in latter stages
of the disorder and only a nucleus and small amounts of cytoplasm remained. Thus,
the disorder was caused by changes occurring in the rind: cell division, growth, and
senescence, which caused water to be translocated to the rind due to a water potential
gradient. Prestorage treatment with gibberellic acid, a plant hormone that slows
senescence, delayed the onset of the disorder [64].
Water loss, determined by weight loss, was found to be a nondestructive predictor
of chilling injury in grapefruit and lemon. Chilling injury appears as distinct, swollen
areas with pitting and cuticular damage. Prior to the appearance of the gross symp-
toms of chilling injury, scanning electron microscopy revealed calcium oxalate
crystals growing inside cracks that had developed around the stomata [65].
Electron microscopy and electrophoresis were useful in determining the reason
for degreening inhibition of bananas. In bananas, degreening is inhibited above 24°C;
ripening at higher temperatures results in retained thylakoid membranes, a delayed
breakdown in chlorophyll b, and a reduced breakdown of pigment-protein com-
plexes. Retention of thylakoid membranes is an important factor in the failure of
Cavendish bananas to degreen when ripened at tropical temperatures [66]. Fruit
softening in bananas is the result of the coordinated degradation of pectin, hemicel-
lulose, and starch (Figure 18.17a and b) in banana pulp [67]. Stress-relaxation probe
measurements were used to describe the changes in physical properties during
softening of the banana fruit pulp.
Eggplants had a higher average gloss reading than mature green tomatoes or
apples. Scanning electron microscopy showed that the epicuticular wax covering of
eggplants was smoother than that on tomatoes or apples, thus, giving a more effective
light scattering surface. The removal of wax from eggplants resulted in decreases
in gloss. The results indicate how wax influences shininess; however, the roughness
measurements of flattened peel after wax removal suggest that surface topography
also influences gloss [68]. The tomato has a dimpled surface (Figure 18.18a) with