Role of Cuticles in Produce Quality and Preservation 33
tensile tests, they determined that the cuticle consisted of a viscoelastic polymer
network. The hydrated cuticles started to swell and became weaker and fractured
more readily, but they also became more elastic, suggesting that water actually
plasticized the cuticle. The moisture in tomato fruit cuticle perhaps had a plasticizing
effect through the disruption of hydrogen-bonded crosslinks between chains in the
polymer matrix [2]. However, it may also be due to hydration of the carbohydrate
component of the cuticle, especially the pectic materials that can embrittle when
they become dry. The authors also used solvents to remove the wax fraction of the
cuticle to expose the cutin matrix. The cutin structure had greater elasticity but
fractured more easily compared to intact cuticles [84]. Petracek and Bukovac [84]
suggested that the wax acted like filler in the intact cuticle.
Round et al. [9] used atomic force microscopy (AFM) as a nonintrusive method
to probe the cuticle surface of tomato fruit and characterize its nanomechanical
properties. The authors showed that the surface elastic modulus decreased dramat-
ically with increasing water content. This result was corroborated by solid-state
NMR, which showed enhanced local mobility of acyl chain segments with higher
water content. In another study, Luque et al. [85] examined the hydration phenom-
enon in isolated tomato cuticle. They studied water sorption of cuticle equilibrated
at a range of relative humidity. The authors used water sorption isotherm and DSC
data to conclude that water does not plasticize the tomato fruit cuticle, in contrast
to the claims of Petracek and Bukovac [84]. They noted that the water sorption
properties reflect the hydrophobic nature of the cuticle.
Ozgen et al. [86] noted that the stage of ripeness of cranberries influences their
cuticular properties. In contrast to climacteric fruit, cranberries harvested when ripe
had longer storage life than those harvested at an immature stage. The reason was
that ripe fruit had a lower respiration rate and a thicker cuticle with a greater puncture
resistance than unripe fruit.
2.7 ROLE OF CUTICLE IN AESTHETIC QUALITY
The structure of the epicuticular wax layer is important in determining the finish
properties of produce. Produce such as apples may be coated with a film of carnauba
wax or shellac to provide a glossy finish that appeals to consumers [87]. However,
the films may actually worsen fruit quality. For instance, moisture from condensation
or from transpiration may become entrapped in the film coating, causing the film to
turn white or hazy and become unattractive. In addition, the natural wax coating
from the epicuticular wax layer can either enhance or detract from the aesthetic
quality [31,32]. Veraverbeke [88] mentioned that the cuticle could adversely affect
quality by producing an unacceptable bloom or a sticky film. The ideal finish on the
produce may vary for different commodities. For example, most consumers prefer
a glossy finish on apple fruit, but generally prefer a dull finish on green beans [32].
The surface characteristics of a particular type of produce are considered such an
important trait that they are often considered when developing new cultivars.
The amount of crystalline wax structure on the epicuticular wax layer has a large
effect on the gloss properties of commodities. Ward [32] studied the amount of gloss