Role of Cuticles in Produce Quality and Preservation 41
on their importance or the conditions responsible for their occurrence. Sekse
[101,116] described field conditions that could induce cuticular cracking, which
include poor irrigation practices and preharvest rains. Cultivar and rootstock selec-
tion have also been linked to the development of cuticular cracks in sweet cherries
[117–119]. Regardless of the cause, the fruit may appear to recover over time from
the stress that induced the cuticular cracks in the first place. Consequently, fruit with
preharvest cuticular cracks may be virtually indistinguishable from uninjured fruit
at harvest time, giving the impression that it has no residual injury effects. However,
the evidence shows that the presence of preharvest cuticular cracks predisposes fruit
to flesh cracking, desiccation, and pathogen infection after harvest [52,100]. Borve
et al. [102,113,114] observed that fruit with cuticular cracks had higher infection
rates when inoculated with fungal conidial suspensions than fruit without cuticular
cracks.
The pectin layer, which attaches the cuticle to the epidermal cell wall, appears
to have a profound effect on cracking of cherry fruit [100]. Calcium ions and some
other multivalent ions are believed to crosslink acidic pectin molecules [51,120]. In
addition, calcium ions are known to delay cracking in sweet cherries [100,121,122].
Glenn et al. [100] used aerated baths of different CaCl 2 solutions to study the effect
of calcium on cracking of cherry fruit. They found that fruit treated in solutions
containing 2.5 mM CaCl 2 had a significantly lower cracking index than fruit treated
in a water control. Fruits were also treated with a calcium chelating solution to
chelate any free calcium ions. The solution containing calcium chelator (2.5 mM)
increased the cracking index severalfold. Also, fruit treated in 2.5 mM solutions of
both CaCl 2 and the calcium chelator had a cracking index similar to that of fruit
treated in a water control. The study also showed that soluble pectin was leached
from fruit during cracking studies. The fruit treated with the CaCl 2 solution had
drastic reductions of leached soluble pectin. On the other hand, fruit treated in a
calcium chelator solution had much higher levels of leached soluble pectin. Micros-
copy data indicated that swelling occurred in the region of pectin solubilization.
Moreover, cuticular cracks were also associated with the swollen regions of the
cuticle. The results indicate that in addition to swelling from turgid epidermal cells,
the pectin-rich area of the cuticle directly accounts for much of the localized swelling
and cuticular cracking. Calcium has been shown to be effective in reducing the
swelling and localized stress responsible for cracking [51,100,123]. This suggests
that calcium ions prevent some of the swelling and the dissolution of the pectin-rich
region. In fact, field application of calcium compounds has been effective in reducing
cracking of sweet cherry varieties [122].
2.8.5.2 Apple
Flesh cracking in apple fruit can incur substantial losses [124]. Cracking in the field
appeared to be related to water uptake in the apple tree. Byers et al. [124] observed
that flesh cracking in Stayman apples only occurs during relatively long rainy periods
and after the fruit has grown to at least 5 cm in diameter. They found that applying
stress to the tree, such as scoring the trunk, could reduce the amount of cracking.
Applying gibberellic acid during the season also reduced cracking [124].