Role of Cuticles in Produce Quality and Preservation 21
proper. As the leaf tissue continues to develop, an epicuticular wax layer forms on
the cuticle surface. A secondary cell wall forms beneath the primary cell wall and
the primary cell wall subsequently becomes incorporated into the cuticle structure.
The final stage in the development of the cuticle occurs when two layers of cutin
form just outside of the secondary cell wall [4,15].
2.2.1 CUTICLE MODEL
A simplified model of the cuticle is shown in Figure 2.1. The cuticular membrane (CM)
is attached to the epidermal cell wall structure (EC) by pectinaceous materials (P). The
epidermal cells contain a cell membrane (ECM) and a cell wall (CW). The pecti-
naceous materials (P), found in the middle lamellar region, bind adjacent cell walls
together as well as the cuticle to the epidermal cell wall. The cuticle or cuticular
membrane (CM) consists of a lamellar structure composed primarily of cutin, but
it may also contain carbohydrate polymers (LR) [12,15]. The cuticle may be isolated
from the underlying tissue using pectinases. The cuticular components are produced
in the epidermal cell layer and transported to the cuticular structure by an unknown
mechanism [12]. Jeffree [4] reported that two cutin layers, the internal and external
cuticular layers, were deposited below the cuticle proper. Bally [17] used the term
cuticle proper to describe the inner and outer cutin layers.
The cutin component of the cuticle forms a porous, three-dimensional structure that
is embedded with waxes [10,15]. Waxes that infiltrate the cutin structure also form a
layer on the cuticle surface [4], called the epicuticular wax layer (EW). In some plants,
additional crystalline wax structures develop on the epicuticular wax layer [4]. As the
plant tissue matures, the adaxial region of the primary epidermal cell wall may become
incorporated into the cuticle structure and may be replaced by a more fibrous secondary
cell wall [4]. Occasionally, the cuticle and epidermal cell layers, with perhaps one or
more underlying cell layers, are collectively referred to as the skin.
2.2.2 CUTICLE COMPOSITION
2.2.2.1 Cutin
Cutin is the major component of the cuticle and is a biopolymer that is insoluble in
organic solvents [12]. Heredia [2] reported that cutin might constitute 40 to 80% of
the entire mass of the cuticle. Cutin forms a three-dimensional, porous polymer
structure that provides mechanical strength. There is, however, little information on
chemical cross-linking or other structural information about cutin [18,19] because
of the insolubility of cutin [10]. However, a portion of the cutin fraction may dissolve
in strong alkali, leaving an insoluble residue refered to as cutan [4]. Cutan is thought
to consist of cross-linked cutin and certain wax constituents. Researchers have used
chemical depolymerization with LiAlH 4 , hydrolysis with alcoholic KOH or HCl,
transesterification with methanolic BF 3 , and digestion with cutinases as tools to
investigate the structure of cutin. These degradative approaches have established that
cutin is a polyester derived primarily from esterified, hydroxylated, and epoxy
hydroxylated C16:1 and C18:1 fatty acids [10,18].