40 Produce Degradation: Reaction Pathways and their Prevention
CHCl 3 /methanol, water permeation increased 3.6- and 48.6-fold in pericarp seg-
ments, respectively. The results clearly indicate that the waxes are critical for decreas-
ing water permeation.
Microscopy techniques have been used to document the sequence of events that
lead to cuticular cracking and, subsequently, flesh cracking in cherries [100]. The
first step involves moisture absorbance by the fruit. This may be accomplished by
simply incubating fruit in aerated, distilled water [111]. As previously discussed,
water absorption may occur either directly through the cuticle or through surface
structures such as the stylar scar, pedicel, and possibly the stomates [100]. The high
soluble solids content and high osmotic potential of the epidermal cells serve as a
driving force to draw water into the cells, resulting in an increase in cell turgidity.
The turgid cells give the once-planar surface a stippled texture. This is shown in
Figure 2.7. The continued swelling of the epidermal and subepidermal cells places
sufficient tensile stress on the cuticle to form small cuticular tears or cracks. The
cuticular cracking can be severe and may be accompanied by a bursting or rupturing
of the epidermal cells (Figure 2.8). In some cases, cuticular cracks may continue to
widen and extend into the fruit’s flesh, resulting in a large visible flesh crack. In
other cases, the cuticular fractures sufficiently relieve the stress created by the
swelling tissue so that flesh cracks do not form (see Figure 2.9). Cuticular cracking
appears to be a precursor to flesh cracking [100]. As such, cuticular cracks may
appear alone or in conjunction with flesh cracks, whereas flesh cracks seldom appear
alone. In one study, Sekse and Ystaas [112] showed a positive correlation between
the degree of flesh cracking and the number of cuticle cracks in cherry fruit.
Severe cuticular fracturing and epidermal cell damage can apparently occur in
some fruit without showing any visible damage (Figure 2.9). In extreme cases, the
hydraulic pressure developed in fruit exposed to water can cause small beads of
water and cellular contents to exude onto the fruit surface (Figure 2.9). In addition,
the cuticular cracks sometimes become oriented perpendicular to the long axis of
the fruit (Figure 2.10). When fruit containing severe cuticular cracking are stored at
room temperature and 50% relative humidity, they quickly lose weight and shrivel
in areas where localized cuticular cracking has occurred (Figure 2.11). It is not
surprising that fruit desiccation occurs as a result of cuticular cracking, because the
cuticle provides the major vapor barrier to water loss.
In years when fruit cracking occurs due to heavy rains, it is a common practice
to hand-sort damaged fruit. The assumption is that only severely damaged fruit
contains visible flesh cracks. However, as indicated in Figures 2.8–2.10, there may
be considerable damage to the cuticle and to the epidermal and subepidermal cell
layers before a visible flesh crack ever develops. It is no wonder that rain-damaged
fruit sorted for visible flesh cracks typically has poor shelf life and often becomes
soft or infected by fungi. Borve et al. [102,113,114] studied the effect of cuticular
cracking on postharvest quality of cherry fruit. They reported a high correlation
between the amount of cuticular fracturing in sweet cherries and the degree of fungal
infestation.
Borve et al. [102,113,114] also observed that a moderate amount of cuticular
cracking might occur in cherry fruit sometime before the harvest period. Beyer et
al. [115] observed cuticular cracks in cherry fruit preharvest but did not speculate