34 Produce Degradation: Reaction Pathways and their Prevention
in eggplants, apples, and mature green tomatoes. Of the three types of produce, the
eggplant had the highest gloss readings. It also had the smoothest surface and least
amount of epicuticular wax structures protruding from the surface of the cuticle.
Tipton and White [89] noted that glossy leaves of the Mexican Redbud had very
little epicuticular wax, whereas dull-leaf varieties had considerable amounts. Com-
modities such as plums have a heavy bloom of epicuticular wax on their surfaces
that give the fruit a dull appearance. However, the aesthetic quality of the fruit can
be improved dramatically by lightly buffing or polishing the fruit surface to remove
the bloom and create a smooth surface [31].
Apart from the epicuticular wax layer, the properties of the epidermal cell layer
also affect the amount of gloss on the surface of produce. An uneven epidermal cell
layer has a tendency to scatter light and reduce the degree of sheen, even if the
cuticle is relatively constant in thickness and has few, if any, epicuticular wax
structures on its surface [87].
2.8 ROLE OF CUTICLE ON PHYSIOLOGICAL DISORDERS
2.8.1 CHILLING INJURY
Postharvest physiological disorders that affect the quality of produce are not uncom-
mon. The many types and causes of a physiological disorder can be very specific
to a particular fruit or vegetable. One common disorder is chilling injury, which
reduces the storage life and quality of many kinds of produce [7]. Chilling injury
is thought to stem from the inability of cell membranes to function properly at lower
storage temperatures due to a phase transition in the membrane lipids [7]. A higher
proportion of unsaturated fatty acids are thought to distinguish chilling resistance
from susceptible produce. In one study, Aggarwal [7] enzymatically isolated cuticles
from mature fruits and used DSC to examine the effect of storage temperature on
phase transitions of the lipid component. The cuticular membrane underwent an
endothermic transition attributed to melting of the waxes. Cuticles isolated from
fruit stored in refrigeration exhibited a shift in the melting enthalpy of the waxes.
Aggarwal [7] speculated that the cuticular changes could be involved in chilling
injury of refrigerated fruit. However, further work is needed to support the claim.
Chilling injury in mandarins appears as small depressions or pits in the fruit
surface that may cover more than 50% of the fruit’s surface. In regions where pitting
occurs, the crystalline wax structures normally covering the fruit surface are absent
[90]. Other telltale signs of chilling injury include a collapse of the epidermal cells
and other cells adjacent to the pit. A question arises as to whether the cuticular
irregularities were the causal factor that preceded the chilling injury or whether the
injury to the epidermal cell layer occurred first and arrested further cuticular devel-
opment. Vercher et al. [90] agreed with the latter explanation. They believed the low
temperature stress disrupted normal epidermal cell function. The localized injury to
the epidermal and subepidermal cells ultimately caused the cells to collapse and
form a pit in the fruit surface. This injury impaired the normal development of the
crystalline wax structure and increased water permeability in the injured region.