26 Produce Degradation: Reaction Pathways and their Prevention
structure. The authors suggested that the cuticular cracks could have developed in
response to the low oxygen level in the controlled environment. The cracks could
have improved gas exchange and facilitated gas diffusion to oxygen-depleted tissue
within the fruit. Once removed from storage, the fruit became less oxygen deprived
but became exposed to higher temperatures and lower relative humidity, conditions
that favor moisture loss. The fruit adapted by filling in the cuticular cracks to form
a smooth wax layer that provided a better water vapor barrier. These results illustrate
that at least in some produce, the cuticle is a dynamic structure that can adapt to
the prevailing environmental conditions.
As mentioned previously, the cuticle provides a barrier against the penetration
of chemicals [8]. Although chemicals applied to the produce may have beneficial
effects, they may also have unintended effects. For instance, Roy et al. [34] studied
the effects of calcium treatments on apple fruit. Fruit with higher calcium content
were firmer and had lower respiration rates [34,35]. The authors found that the
calcium content in the flesh depended on the surfactant used in the dipping solution.
The most effective surfactants were those that altered the epicuticular wax [34].
However, it was not clear whether the surfactant or the calcium was directly respon-
sible for the observed effects. The surfactant may have simply facilitated calcium
penetration, which then caused the cuticular changes.
2.4 BARRIER PROPERTIES OF THE CUTICLE
2.4.1 BARRIER TO LIQUIDS AND GASES
The cuticle provides an effective barrier between the produce surface and the envi-
ronment. Cuticle penetration is important for improving nutrient uptake as well as
uptake of herbicides, fungicides, and other foliarly applied materials [6,9,36]. Indi-
rectly, the cuticle affects produce quality by restricting the passage of beneficial
chemicals applied on the cuticle surface while at the same time protecting the
produce from chemical injury [37]. In addition, the barrier properties of the cuticle
directly affect produce quality by minimizing desiccation, influencing gas exchange,
and preventing the leaching of plant solutes [1,12]. Schotsmans [38] studied different
methods of determining efflux of biological gases and found gases to be somewhat
regulated by the fruit’s skin. Banks [39] showed that the cuticle of pepper was about
10 times more permeable to CO 2 than to O 2. Amarante and Banks [40,41] found
that carnauba-based wax coatings reduced the permeance of water vapor, O 2 , and
CO 2 in pears.
The barrier properties of the cuticle are characterized by cuticle thickness,
composition, and structure. The cuticle layer on produce can be very thin (< 0.1 μm),
such as those found on leafy vegetables, or can be quite thick (> 30 μm), such as
those found on mature apple fruit [37,42]. Early in the development of a particular
fruit or leafy vegetable, the tissue is covered with a thin cuticle containing waxes
that provide an effective vapor barrier. The wax component rather than the cutin com-
ponent provides the greatest moisture resistance [4]. The cutin matrix is thought to form
a three-dimensional, porous structure that contributes little to the hydrophobic properties
of the cuticle, even though the cutin polymer itself is rather hydrophobic [4]. These