32 Produce Degradation: Reaction Pathways and their Prevention
just below the fungal appressoria has been demonstrated in barley [81]. Fauth et al.
[79] also showed that cutin monomers made from alkaline treatments were effective
in eliciting H 2 O 2 in cucumber hypocotyl segments. In addition, Fauth et al. [79]
found that cutin monomers enhance the activity of other H 2 O 2 elicitors.
2.6 ROLE OF CUTICLE ON MECHANICAL PROPERTIES
RELATED TO QUALITYThe mechanical properties of produce affect their postharvest storage life. Compres-
sion and impact stress can damage tissue, which causes cell contents to leak inter-
nally and results in discoloration and bruising. The compression and impact resis-
tance of most produce is largely determined by the strength of the epidermal,
subepidermal, and parenchyma cells that compose much of the edible tissue. Many
vegetable crops can be handled, stacked several layers deep in boxes or bins, and
shipped without incurring significant compression or impact damage. However,
packing, storing, and shipping fruits that become soft and fleshy as they ripen
becomes more difficult. Consequently, much of the produce that softens during
ripening is harvested prematurely when it can still withstand some degree of impact
and compressive stress. This is especially common for climacteric fruit such as
tomatoes, peaches, and pears that can ripen postharvest.
The primary protection that the cuticle provides for produce other than com-
pression and impact resistance may be resistance to damage from abrasion. Abrasion
resistance reduces the chances that the produce will develop open wounds that
provide sites for pathogen infestation. For produce that is soft and fleshy, the cuticle
may play an even greater role in providing mechanical protection. In addition to
providing abrasion resistance, the cuticle also provides support and resistance against
puncture wounds.
There have been only a few limited studies on the mechanical properties of plant
cuticles. Glenn and Johnston [82] studied the mechanical properties of wheat bran,
which includes a thin cuticle, several layers of periderm, and an aluerone layer. The
mechanical properties of wheat bran are important in obtaining good flour yield in
milling operations. Typically, the grain is conditioned to a desirable moisture level
to soften the endosperm and facilitate milling. A second application of moisture is
added approximately 30 minutes prior to milling to hydrate the bran tissue and make
it flexible and impact-resistant so that it remains relatively intact after the endosperm
tissue is stripped away. Glenn and Johnston [82] studied the tensile properties of
bran tissue as a function of moisture content. They showed that the elongation to
break markedly increased at higher bran moisture content. Consequently, the bran
tissue is more compatible with milling operations after moisture treatment.
Thompson [83] conducted a study on the rheological properties of tomato fruit
skin. The study focused on understanding the cell wall properties as well as the
biophysical and biochemical processes involved in fruit growth. However, the author
did not examine the relationship between mechanical properties and postharvest
quality or preservation. In another study on tomato fruit, Petracek and Bukovac [80]
measured the mechanical properties of enzymatically isolated cuticle. Based on