Microstructure of Produce Degradation 533
Fungi, unlike plants, are nonphotosynthetic organisms. Edible mushrooms are
essentially a mass of hyphae that has joined together to form a basidiocarp, or fruiting
body. A high-quality, fresh, white button mushroom has a tightly closed cap, or
pileus, the gills are covered by a membrane or velum, the stipe (stalk) is short, and
the mushroom is white. Discoloration or browning is an unacceptable quality of
mushrooms that might develop upon storage. Several mechanisms may be associated
with the browning reaction, ranging from rough handling to the action of bacteria
or mold on the mushroom tissues, causing brown spots. To inhibit browning, low-
dose irradiation was shown effective as a preventive measure to browning both as
an antioxidant and a microbial inhibitor. A high dose rate was found to be less
effective than the low dose rate in reducing the onset of browning. The high dose
rate induced changes in the pileus, including cell wall thickening, which was shown
to change permeability rates in the cells [8].
Another unacceptable quality parameter has to do with normal mushroom devel-
opment, or aging. The stipe grows, the velum stretches and breaks, and the pileus
opens to expose and release the spores on the surfaces of the growing gill tissue.
The broken velum leaves a ring of tissue on the stipe, which is then termed the
annulus [9]. Part of the pileus structure is shown in Figure 18.3a and b. Spores form
on the inner portions of the gills shown by the arrows (Figure 18.3a and b); the rest
of the pileus and the stipe are composed of masses of hyphae. Hyphae in the fresh
mushroom are tightly compacted and have smooth, plump, hydrated cell walls and
evident cytoplasm (Figure 18.3c). In the aged mushroom, the cell walls are somewhat
collapsed and wrinkled (Figure 18.3d), indicative of dehydration stress, and cyto-
plasm is no longer evident. The apparent disappearance of cytoplasm agrees with
findings of Braaksma et al. [9]; growth during aging of the pileus tissue was reported
to occur by vacuolar expansion. Spores in the fresh mushroom are smooth, rounded,
and immature (Figure 18.3e, f, and h) and some of them are covered by a smooth
membranous substance. The aged mushroom (Figure 18.3g and i) has spores that
are closer to maturity, are ovoid in shape and are covered with irregular, sticky-
looking deposits on their surfaces, which may be the remnants of the smooth
membranous substance that has undergone digestion in the formation of the spores
and dehydration.
FIGURE 18.3 (Opposite page)Scanning electron micrographs of a white button mush-
room. (a) Cross section of the cap showing the cap body and the gills, on the surfaces of
which spores are formed (arrow). The boxed area is shown in higher magnification in b. (b)
Close view of two gill fingers; arrow indicates the gap between the gill fingers; spores form
on the surfaces on either side of the gap. (c) Fresh hyphae showing the smoothness of the
cell walls of the fully hydrated mushroom. (d) Aged hyphae showing collapsing and wrinkling
indicative of dehydration. (e) Immature spores from a fresh sample. (f) Immature spores from
a fresh sample; these look as if they are more developed than those shown in e. (g) Mature
spores from an aged sample. (h) Spores from a fresh sample; it appears as if the spores are
covered with a thin membranous substance. (i) Spores from an aged sample; it appears that
the thin membranous substance of h has undergone digestion and dehydration and that
remnants remain on the surfaces of the mature spores. Magnification bars: a, 500 μm; b, 50
μm; c, 10 μm; d–g, 20 μm; h, i, 5 μm.