Agriculture Reference
In-Depth Information
dichloromethane, acetone, etc.; Cappelletto
et al
., 2001).
All structural proteins capable of associat-
ing with pectins and hemicelluloses via their
sugar residues contribute to the pool of pro-
teins found and measured in fibres. These
include proteins rich in glycine (GRP), proline
(PRP), hydroxyproline (HRGP) and arabinoga-
lactan (AGP). Plants respond to an insult by
synthesizing proteins that generally will be
localized within the injured tissues. That said,
the proportion of proteins found in hemp fibres
is relatively low and evolves according to the
maturity of the plant, attaining a maximum
concentration of 1-2% (Vignon
et al
., 1995;
Cronier, 2005).
Among the plant extracts identified in
hemp phloem fibres are found a number of
lipophilic compounds (extracted using acetone)
such as the fatty acids (C4-C30), alkanes
(C21-C33), free sterols or sterol esters (sito-
sterol) and waxes (fatty acids and esterified
long chain aliphatic acids) (Gutierrez and del
Rio, 2005). Finally, among these extracts,
traces of cannabinoids can be detected in
industrial hemp fibre (2% of the total extracts)
(Kortekaas
et al
., 1995).
The last important biochemical group of
note in fibrous plants are the minerals. These
include silica and heavy metals derived from
the soil during the plant's growth. Silica is
present in hemp stems at low concentrations
only (<1%) (Kamat
et al.
, 2002) and is there-
fore barely present in the extracted fibres. By
contrast, heavy metals are taken up by the
Cannabis plant and distributed throughout the
plant, with residues found in the cortical and
xylem fibres. Hurds grown on contaminated
ground can therefore have high concentrations
of cadmium, lead and nickel (up to 3% by
weight) (Linger
et al
., 2002).
3- and 3,6-
β
-D-
galactans
4- and 4,6-
β
-D-
galactans
Ara
GalA-Rha
backbone
GalA
GalA
3- et 3, 5-
α
-L-arabinans
Fig. 3.7.
Schematic structure of a type I
rhamnogalacturonan. The primary chain is made
up of repeating dimers of galacturonic acid and
rhamnose. The branching oligosaccharides of flax
are primarily made up of arabinose and galactose
(Ara, Gal) (from Ridley, 2001).
The tridimensional polymer can contain up to a
dozen different intermonomeric bonds but, in
the case of hemp, its structure remains poorly
defined (Fig. 3.9). Lignification progresses by a
process of encrustation with the deposition of a
hydrophobic material within the polysaccharide
matrix. The extent of the lignifications varies
considerably, however, according to the type of
fibre. Primary periphloem fibres are poorly lig-
nified compared to the secondary woody fibres
used to make hurds, where nine-tenths of the
lignin in hemp stems is found (Han and Rowell,
1997) (Table 3.1).
The lignin of primary fibres encloses
type G and type S units, and especially type H
material (Cronier
et al
., 2005). In this sense,
the lignins are related more closely to the
graminaceae than to leaved trees with an S/G
ratio of 0.8 (del Rio
et al
., 2004).
Storage products and
secondary metabolites
PROTEINS AND PLANT EXTRACTS
.
Plant cell walls
also contain constituents such as proteins, oli-
gosaccharides and turpenes in relatively small
concentrations, although local concentrations
may be much more significant. Hemp is no
exception to this rule: up to 15% of the dry
matter is made up of plant extracts, i.e. soluble
molecules that can be extracted from plant
material using a suitable solvent (water, ether,
The full value of hemp can be realized by
exploiting a number of co-products other than
just hemp fibre. These include seed-derived
edible products and pharmacological products.
Various substances can be extracted from other
plant organs, notably the leaves and flowers of
certain varieties of hemp.
