Microstructure of Produce Degradation 537
of the pH and the conversion of chlorophyll to pheophytin [14]. The epidermis of
fresh broccoli has the characteristic turgid epidermal cells and smooth appearance
to the skin (Figure 18.5c). The aged epidermis shows signs of dehydration and
superficial mold growth (Figure 18.5d). Stem cross sections of both fresh (Figure
18.5e) and aged (Figure 18.5f) tissue are virtually identical at low magnification. At
higher magnifications, minor differences may be seen between the fresh (Figure
18.5g) and aged (Figure 18.5h) tissue. Parenchyma cell walls tend to be thicker and
intercellular spaces tend to be smaller in the fresh (Figure 18.5g) than in the aged
(Figure 18.5h) sample. The cells in the aged sample also seem to be less turgid and
more rounded than comparable cells in the fresh sample.
Green beans, botanically fruits, are harvested and consumed in the immature,
green state. Fresh green beans are bright green in color and have a characteristic
“snap” when broken with the fingers. The green bean consists, in large part, of the
pod, most of which is fruit coat, or pericarp. The bean pod has a number of immature
seeds in the center. The bulk of the pericarp is mesocarp, which consists of thin-
walled parenchyma cells (Figure 18.6a). Upon aging, mesocarp cells start to collapse
in the outermost layer (Figure 18.6b). In the inner mesocarp, cell walls break and
several cells or cell layers conjoin and form huge gaps (Figure 18.6b). The epidermis
in the fresh bean (Figure 18.6c) is flat and covered with hairs or papillae and stomata.
The unprotected green bean loses turgor or dehydrates as it ages and large folds
occur (Figure 18.6d). At higher magnification, the epidermal cells in the fresh green
bean (Figure 18.6e) have a linear appearance, the stomata are evident, and the fine
papillae are slightly hooked on the ends. Debris can also be found on the surface
of the fresh bean. Epidermal papillae are hollow, evident once they have been
fractured (Figure 18.6f). The papillae of the aging bean are dehydrated and may be
constricted at points. Epidermal cells have begun to pucker (Figure 18.6g) due to
the effects of dehydration and fungal hyphae may be seen on the surface (Figure
18.6g) or growing into stomata (Figure 18.6h).
Cucumber is another fruit that is consumed in its immature, green state. The
epidermis in the fresh fruit is covered with epicuticular wax and is completely smooth
(Figure 18.7a). In the aging cucumber, the epidermis begins to have irregular “pools”
of differing electron densities (Figure 18.7b). Closer inspection of the darkened
“pools” reveals areas where the epicuticular wax has begun to be disrupted and the
rounded epidermal cells become exposed (Figure 18.7c and d). Large areas that have
FIGURE 18.5 (Opposite page)Scanning electron micrographs of broccoli florets. (a, b)
Aged broccoli flower buds showing some growth of fungi on the epidermis and dehydration.
(c) Fresh floret stalk epidermis showing smooth, hydrated epidermal cells. (d) Aged floret
stalk epidermis showing the effects of dehydration and fungal growth. (e) Fresh cross section
of floret stalk showing the organization of the various tissues. (f) Aged cross section of floret
stalk with structure similar to that of the fresh tissue shown in e (g) Fresh cells of the cortex
of the floret stalk showing thickened cell walls, somewhat angular cell shapes, and small
intercellular spaces. (h) Aged cells of the cortex of the floret stalk; compared to f, cells are
more rounded in shape, have less apparent turgor, and have thinner cell walls and larger
intercellular spaces. C, cortex; CW, cell wall; IS, intercellular space; P, pith; VB, vascular
bundles. Magnification bars: a, e, 1 mm; b, 500 μm; c, d, f, 200 μm; g, h, 10 μm.