Agriculture Reference
In-Depth Information
licles associated with each trio group begin to emerge,
forming a follicle group. At birth (day 149), all P follicles
are fully formed and are actively producing fi ber whereas
only a small but quite variable proportion of the S follicles
are producing fi bers. Twelve weeks after birth, most of the
secondary follicles are producing fi bers. The ratio of S : P
follicles in mature Angora goats ranges from 6 - 10 : 1
(compare merino sheep at 15 - 25 : 1). Prenatal and early
postnatal nutrition affects the rate of maturation and the
ultimate number of active S follicles. In fact, 75-80% of
the ultimate number of S follicles can be formed during
the fi rst 8 weeks of life. This amount has a direct infl uence
on the lifetime production of mohair.
A variable amount of fi bers produced in P follicles are
medullated (hollow), to varying degrees. When medulla-
tion exceeds 60% of the fi ber diameter, the fi ber is termed
a “kemp,” a chalky white, objectionable (from a textile
viewpoint) fi ber that appears not to accept dyestuff.
Centuries of visual selection against kemp have resulted in
low levels in most commercial animals. When the degree
of medullation in a fi ber is less than 60% of the fi ber
diameter, this fi ber is termed a “ med ” or “ heterotype. ”
The P and S follicles of Angora goats produce fi bers
having overlapping diameter distribution (Table 15.2) and
similar length giving rise to a nonshedding, “single-coated”
fl eece that is quite distinct from cashmere and other goat
breeds that produce double coats. In contrast, the P folli-
cles of cashmere goats produce individual fi bers having
diameters in the range 30 - 250 μm whereas S follicles
produce fi bers in the range 5-30 μ m. The S : P ratios in
cashmere goats range from 5 - 7 : 1.
chalk, and dipping fl uids. Excluding some types of VM,
the vast majority of these nonfi brous fl eece constituents are
removed in normal aqueous detergent scouring.
CHEMICAL COMPOSITION
Like other mammalian fi bers, mohair and cashmere (after
washing) are composed almost entirely of a family of
complex proteins referred to collectively as keratin. This
term is also used to describe the similar sulfur-containing
proteins that make up feathers, horns, and nails. Three
main fractions are referred to as low- and high-sulfur and
high-tyrosine proteins that are not uniformly distributed
throughout the fi ber. Most of the sulfur (3-4% overall) is
present in the amino acid cystine. It has been estimated
that wool contains more than 170 individual proteins.
There is no doubt that mohair and cashmere are equally
complex. Of the 22 naturally occurring amino acids, 18
are found in animal fi bers. The amino acid composition
of kid mohair has been shown to be very similar to that
of Merino and Lincoln wool (Ward et al., 1955). The
polypeptide chains are joined together by disulfi de and
isopeptide covalent crosslinks, ionic bonds (between
−
COO
−
groups), and by noncovalent interac-
tions such as hydrogen bonds. The positions of the disul-
fi de bonds are rearranged during the setting of fabrics, one
of the fi nishing processes that uses water and heat. The
ionic groups give animal fi bers their amphoteric nature
(capability to absorb and desorb acids and alkalis) and
also control dyeing behavior.
NH
3
+
and
−
MORPHOLOGY AND
PHYSICAL PROPERTIES
NONFIBROUS CONSTITUENTS
OF THE FLEECE
These typically make up 10-30% of the weight of the raw
material and are comprised of moisture (10-17%), com-
pounds produced by the goat, other natural contaminants,
and residuals introduced by man. The former group
includes mohair or cashmere wax (sometimes termed
grease, up to approximately 8% for mohair and cashmere)
secreted by the sebaceous glands and water-soluble dried
sweat (suint) produced by the sudoriferous glands. Other
natural contaminants include dirt, sand, and vegetable
matter (VM) such as burrs, grass, seeds, sticks, and leaves.
The amount and type of VM can have a major infl uence
on the value of the raw material. Most VM is removed
during normal mechanical processing, but when amounts
are excessive, carbonization with sulfuric acid and heat
may be necessitated. The last group of contaminants would
include such things as residual insecticides, branding paint,
General
The physical structure of all animal fi bers is complex
(Figure 15.3 ). Crystalline, water - impenetrable microfi brils
(approximately 0.2
m wide) and water-accessible, rela-
tively amorphous regions (cell membrane complex [CMC])
coexist at the molecular level. At the cellular level, the bulk
of the fi ber (the cortex) is composed of cortical cells that
are cigar-shaped having dimensions of approximately 8
μ
×
100
m. Animal fi bers are described as a composite assem-
bly of cuticle and cortical cells that are surrounded and held
together by the CMC, the main components of which are
low-sulfur proteins and lipids. Although the CMC consti-
tutes only a few percent by weight of the fi ber, it is respon-
sible for the exceptional mechanical properties of the fi bers
and fabrics made from them (for example, excellent abra-
sion resistance). The CMC also represents the only con-
tinuous phase in the fi ber and is used for diffusion of
μ
