42 Produce Degradation: Reaction Pathways and their Prevention
In addition to flesh cracking, cuticular cracking occurs in apple fruit as well.
Wojcik [125] studied the epicuticular wax structure of seven apple varieties. They
found that the main difference between the varieties was the occurrence of cracks
in the wax layer of the cuticle. Interestingly, they found the sun-exposed side of the
apple with a blush typically had fewer cracks than the shaded, unblushed side. In
another study, Schirra et al. [126] used gibberellic acid sprays as a treatment to
prolong postharvest life. They noted that during normal fruit development the cuticle
starts out as a smooth, planar structure with very little crystalline wax structure. As
the fruit develops further, the cuticle begins to form epicuticular wax platelets that
are polygonal and arranged in a mosaic-type pattern. By the time the fruit has fully
matured, the platelets are lifted and fine cracks of the cuticle develop. The gibberellic
acid treatment delayed the formation of platelets and cracks and prolonged the fruit
storage period.
A common practice after harvest involves submerging fruits for washing and
conveying them to different places in the packinghouse. In some cases, fruit may
be dipped in a particular bath as a postharvest treatment. Exposing fruit to prolonged
dipping or washing treatments has been shown to induce cracking [124]. Byers et
al. [124] observed that dipping apples in water containing surfactant resulted in
higher water uptake and cracking compared to dipping them in water with no
surfactant. The primary sites of water uptake included the lenticels and puncture
wounds as well as the abrasions that penetrated the cuticle. In one study, Lurie et
al. [127] treated Golden Delicious apples (Malus domestica Borkh.) with a dip in
2% CaCl 2 solution. One sample was heated for 4 days at 38°C before dipping. The
heat treatment affected the wax surface of the apples. It appeared the heat-treated
apples had their cracks filled and had a smoother surface than the apples that did
not receive heat treatment. They concluded that the heat treatment softened the wax,
which then filled the cracks and crevices. Consequently, less Ca penetrated into the
heat-treated fruit compared to the control.
Roy et al. [128] used low-temperature scanning electron microscopy of frozen
hydrated apples to view the cuticular structure. They observed cuticular cracks in
older apple tissue. In a separate study, Roy et al. [129] studied infiltration of apples
with CaCl 2 solutions. They used a control and a heat treatment at 38°C on the fruit.
The results showed that the heat-treated fruit had less CaCl 2 infiltration. Also, the
SEM micrographs showed microcracks on the control fruit, but the epicuticular wax
had melted in the heat-treated sample. The results suggest that cracks on the fruit
surface may be an important pathway for the penetration of CaCl 2 solutions. In a
more in-depth study, they also showed that epicuticular wax cracks occurred in fruit
stored postharvest and that the cracks became deeper and wider for fruits stored for
longer periods [130]. In addition, the apples that were stored longer had a greater
uptake of CaCl 2 solutions.
Schirra et al. [126] used a postharvest heat treatment to prolong postharvest life.
Scanning electron micrographs revealed that heat treatment at 37°C actually melted
the epicuticular wax so that it filled the cracks and wounds in the fruit. In addition,
Veraverbeke et al. [89,131] used confocal laser scanning microscopy to perform a
nondestructive analysis of apple cuticle during storage. They noted the development
of cuticular cracks in fruit placed in controlled atmosphere storage. They suggested