Environmental Engineering Reference
In-Depth Information
integrated holistic biorefinery. Extraction of valuable phytochemicals including
waxes, prior to biomass destruction during biochemical and thermal treatments,
can significantly increase the overall financial returns.
In order to maintain a stable wax market, there is an ever-growing demand for
synthetic waxes due to the decline in petroleum waxes. This demand has led to the
continual search for natural resources that are readily available and found in
sufficient amounts. This, together with the use of environmentally benign tech-
nology, will allow for sustainable development [9, 10].
When adhering to the strict chemical definition, the term 'waxes' refers to the
ester products formed from the esterification of long-chain fatty acids with
long-chain primary alcohols [11]. However, the term 'plant wax' is often used
collectively to describe the complex mixture of surface lipids covering the aerial
tissues of herbaceous plants [12].
Early morphological work, reviewed by Martin, Juniper and Baker, led to the
first description of the terminology defining plant wax in which the 'cuticular
membrane' was used to describe the entire waxy coating which lined the outer
surface of the plant epidermal cells [13, 14]. It comprises three distinct regions: the
exterior epicuticular wax; the cuticle proper; and the interior intracuticular wax
[12]. The cuticle proper is made up of a biopolyester consisting of hydroxy and
epoxy fatty acids, collectively known as cutin, which covers the epidermal cells
forming an electrodense layer [12, 15, 16]. The intracuticular wax, which is embed-
ded in the cutin, contains amorphous mixtures of lipids that link the cuticle to the
cell wall matrix [12, 16]. The epicuticular wax refers to the complex mixture of
surface lipids, comprising cyclic and aliphatic long-chain molecules, which cover
the cuticle proper forming a smooth film exterior or crystalloids [11, 12]. It con-
sists of a large variety of chemicals which may be subdivided on the basis of their
structure, functional group type and their homologue distribution [11].
Plant wax consists of both cyclic and aliphatic long-chain components. The
most common long-chain aliphatic compounds include hydrocarbons, primary
alcohols, aldehydes, fatty acids and wax esters. Long-chain aliphatic compounds
that are less common include ketones, β-diketones and secondary alcohols [11].
Cyclic compounds that are found in the plant cuticular wax include sterols, flavo-
noids and terpenoids. These are summarised in Tables 2.1 and 2.2 [12].
In the majority of plant waxes, the hydrocarbon fraction consists of a number
of
n
-alkanes varying in chain length from C
25
to C
35
. The predominant
n
-alkanes
are those which contain an odd number of carbon atoms. In most cases, the
predominant chain length (usually 90% or more of the total hydrocarbon fraction)
is C
29
or C
31
[17]. This is a generalisation however, and there are a number of
exceptions in relation to the number of carbon atoms and the predominant chain
length. There have been some studies, in which alkanes having chain lengths less
than C
25
have been reported. In addition, in the majority of algae the predominant
chain length is C
17.
There have also been reports in which alkanes of chain length
C
62
have been identified in cane grass wax [17].
