24 Produce Degradation: Reaction Pathways and their Prevention
2.2.2.2 Waxes
In contrast to cutin, cuticular wax is soluble in solvent and is made of a heterogeneous
mixture of lipids that vary considerably among different plants. Cuticular waxes are
a complex mixture of alcohols, alkanes, aldehydes, ketones, and esters made from
long-chain fatty acids [4,18]. These C16:1 and C18:1 fatty acids are first produced in
cell plastids. The fatty acids then form complex mixtures of long-chain aliphatics
outside the plastids. The mixtures contain alkanes, alcohols, aldehydes, fatty acids,
and esters with varying amounts of triterpenoids and hydroxycinnamic acid deriva-
tives [18,20]. The epidermal cells then exude the waxes, which traverse their way
through the cutin matrix. Upon reaching the cuticle surface, the waxes deposit as a
thin, amorphous layer known as the epicuticular wax layer. In certain conditions,
wax crystals may also form on the epicuticular layer. The structure and shape of the
wax crystals depend on the wax composition [21], which is influenced by environ-
mental and developmental factors as well as foliar applied chemicals [4,22–24].
2.3 CUTICULAR STRUCTURE AND COMPOSITION
IN PRODUCEThe cuticle is only a very thin layer on the plant surface, but it represents a significant
mass because it covers a large cumulative surface area of plants. In fact, Riederer
[25] estimated that, on average, the total mass of cuticle produced per hectare ranged
from 180 to 1,500 kg. Also, the cuticle is not simply an inert, innocuous layer on
the tissue surface, but it changes considerably during the plant’s development cycle.
Moreover, it is affected by genetic variation, environmental factors, and chemicals.
2.3.1 CUTICULAR GROWTH AND DEVELOPMENT
The cuticle forms very early in the development of produce and plays a significant
role in proper morphological development. Sieber et al. [12] noted that when the
cuticle was removed or missing in developing tissue, the adjoining tissue often fused
together and created deformities. The development of the cuticle often parallels the
growth and development of the produce it protects. In leafy vegetables, the devel-
opment of the cuticle is similar to that described previously for leaves and will not
be discussed in any further detail [15]. The cuticle development in bulky, fleshy
fruits is also very similar to that of leaves in the initial stages, but the cuticle continues
to develop as the fruits grow and mature. A well-documented example of this is the
apple [Malus domestica Borkh.].
Early in its development, apple fruit is protected by a thin, waxy cuticle, com-
parable to the procuticle structure described by Bird and Gray [15]. Eight weeks
after full bloom, the cuticle grows to a thickness of approximately 6 μm. The surface
remains smooth and has no distinct epicuticular wax formations. By 12 weeks,
however, small epicuticular wax structures develop on the cuticle surface and sub-
sequently increase throughout the remainder of the growing season [26]. The thick-
ness of the cuticle increases from approximately 6 μm at 8 weeks to approximately
22 μm at 20 weeks after full bloom [26]. Even after harvest and cold storage at 4°C,