Microstructure of Produce Degradation 539
apparent in the voided areas (Figure 18.8c and d). The exocarp, which has thick,
sclerified cell walls, shows relatively little change (Figure 18.8e). The cell walls of
the inner mesocarp also do not show much change in structure at this point of
degradation (Figure 18.8f).
In contrast to produce with a short shelf life, the carrot, a root, has a very long
shelf life unless it is grown in the presence of Chalara elegans, which can cause
postharvest black root rot, common in the Fraser Valley of British Columbia, Canada
[15]. Carrots can be harvested at any time after a carrot has formed, although older
carrots tend to be “woody” and are less acceptable to consumers. Small, ready-to-
eat carrots were chosen for SEM study because of their convenience to consumers.
The carrots are peeled and scrubbed prior to packaging, so their surfaces differ from
those that are freshly harvested. The outer surfaces of fresh or aged prepackaged
carrots are similar and consist of crushed outer cortex (Figure 18.9a and b). The
surfaces of carrots that have been peeled and stored often have a white, translucent
appearance. The white appearance was shown to be due the shredding of superficial
layers of the carrot that then dehydrate, giving a white appearance to the carrot skin
[16]. Slicing of carrots causes physical damage, stress, and the increased risk of
microbial growth. The risk and severity of the effects were found to be less with
gentle handling and the use of extremely sharp knives for cutting [17] and were
even less following vacuum packaging [18].
Both fresh (Figure 18.9c) and aged (Figure 18.9d) carrots are similar in cross
section even though the aged carrot had been aged to the point of wilting. Carrots
are resilient and they nearly fully recovered their original structure due to the
imbibition of water during aqueous fixation for scanning electron microscopy; there-
fore, most of the cell walls in the aged carrot appear turgid. Some cells in the aged
carrot were found to have broken cell walls and the cells had joined with nearby
cells to form larger gaps in the tissue (Figure 18.9e) and some cells appeared to be
collapsed (Figure 18.9f). Broken cell walls and collapsed cells were not apparent in
the fresh sample (Figure 18.9g).
FIGURE 18.6 (Opposite page)Green bean. Scanning electron micrographs (SEMs) show-
ing cross sections of pericarp of fresh (a) and aging (b) green bean. (a) Fresh green bean
showing regular, thin-walled parenchyma cells of the mesocarp. (b) Aging green bean showing
collapse of cells in the outer mesocarp (circle) and gaps that have formed by cell wall breakage
have joined multiple cells (arrows). SEMs of the surface of the epidermis of green bean of
fresh (c, e, f) and aging (d, g, h) tissue. (c) Fresh green bean showing fine hairs (papillae)
and stomata and an unwrinkled surface. (d) Aging bean showing the very disrupted, wrinkled
surface. (e) Fresh epidermis showing hooked papillae and stomatal complexes with linear
epidermal cells and debris (*). (f) Fractured papilla of fresh green bean. (g) Aging green bean
epidermis showing a dehydrated and constricted papilla, sunken stomata (arrows) and irreg-
ular, pinched epidermal cells. (h) Aging green bean epidermis showing fungal hyphae that
have begun growing into a stomate. Magnification bars: a, b, 250 μm; c, d, 500 μm; e, g, 100
μm; f, g, 25 μm.