O o d h e a d p u b L i s h I n g L i m I t e d

9.2
Wool colour
9.2.1
Evolution and genetics of natural white
wool colour
Most ancient sheep were not white but had a highly pigmented brown fleece,
probably similar to the wild Mouflon sheep that still inhabit parts of Sardinia
and Corsica. Sheep having a grey fleece, consisting of a mixture of white
and coloured fibres, are also known from Iron Age textile remains. Pigmentation
in the wool, hair and skin of mammals is due to the presence of active
melanin-producing cells (melanocytes). Within melanocytes, melanin granules
are produced (melanosomes) and this melanin can be transferred into adjacent
keratin-producing cells (keratinocytes) via finger-like projections. Melanin-
containing keratinocytes within the follicle bulb of a wool or hair fibre are
incorporated into the growing fibre (Forrest et al., 1985).
Melanin pigments exist in two chemically distinct forms, eumelanin and
pheomelanin. Eumelanin is generally black or dark brown and derived primarily
from tyrosine (Tyr) residues. Pheomelanin is a yellow to reddish brown
pigment formed from Tyr and cystine (Cys). Both pigments are polymeric
and are derived via a series of redox reactions from a common precursor,
dopaquinone, which is formed by the action of the enzyme tyrosinase on Tyr
residues (Ozeki et al., 1996). Melanocytes can form both eumelanin and
pheomelanin, and production of eumelanin is controlled by 
α-melanocyte
stimulating hormone (
α-MSH) (Jackson, 1994). In mammals a defect in the
gene that codes for tyrosinase synthesis results in a total lack of pigmentation
(albinism). In sheep albinism is very rare, but has been observed in Icelandic
sheep (Adalsteinsson, 1977, 1978) and in a flock of Suffolk sheep raised in
South Australia (Rowett and Fleet, 1993).
The stimulus for the selective breeding of sheep with white (unpigmented)
wool only arose after the development of dyeing in the Middle East around
1000
BC
. Textiles from the first millennium 
BC
 in the Middle East indicate an
increase in the number of white sheep, since almost all the wool textiles
found from this period are of white dyed wool (Ryder, 1987). Numerous
wool textile remains also exist from Roman times, and those from
Mediterranean sites are almost exclusively dyed white wools.
Selective breeding of unpigmented sheep in the first millennium 
BC
 would
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Improving the whiteness and photostability of wool
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have required the presence of some white animals in the domestic flocks that
had developed earlier. Breeding could have been carried out either by selecting
white-spotted sheep having large areas of white wool, or by selection of all-
white mutants (Maijala, 1997). White-spotting in mammals occurs in areas
where skin or hair follicles lack melanocytes. The absence of pigmentation
in modern-day Merino sheep is reported to be due to the deficiency of
melaninocytes resulting from the action of a complex of white spotting
genes (Fleet et al., 1991). A second general mechanism for achieving whiteness
is through dilution of pigment, which is accomplished by selection for decreased
efficiency of melanin production by melanocytes (Sponenberg, 1997).
The loci Agouti (A) and Extension (E) control the production of pigments
in the skin and hair of mammals. In sheep, wool whiteness is a multigenic
trait dependent on alleles of white spotting genes and modifiers which result
in low levels of melanocytes in the skin, and genes such as Agouti which
produce an inhibitor of melanocyte activity (Sutton et al., 1998). The Agouti
locus is responsible for production of a protein that nullifies the effects of 
α-
MSH on melanocytes, thus preventing the production of eumelanin. The E
gene is expressed in melanocytes and controls the synthesis of 
α-MSH.
One question that arises from knowledge of the evolution and genetics of
sheep is whether further improvements in wool whiteness are possible by
selective breeding. For Merinos a number of studies suggest significant
variance in wool colour within sheep populations. Several studies have
measured greasy and clean (scoured) wool colour and calculated the phenotypic
and genetic correlations and heritability. Phenotypic and genetic correlations
predict the direction and rate of change in a given trait that would result from
different selection practices. Heritability is the proportion of phenotypic
variation in a population that is attributable to genetic variation among
individuals. For wool colour the phenotypic correlation between greasy wool
colour and clean wool colour is not strong (<0.3), showing that greasy wool
colour is not a good indicator of scoured colour. This is due to greasy wool
colour being affected by the presence of high quantities of wool grease and
dust which are removed after scouring. However, a study on South Australian
Collinsville Merinos, the largest family group within the Australian Merino
population, showed that the genetic correlation between greasy and scoured
yellowness was moderate to high (0.4–0.9), indicating that selection on the
basis of greasy yellowness should lead to genetic improvements in scoured
yellowness (James et al., 1990b). Heritabilities of 0.42 for greasy colour
score and 0.35–0.54 for objectively measured clean wool colour parameters
were measured (James et al., 1990a,b). A recent study on New South Wales
(NSW) Merinos has confirmed that greasy wool colour is moderately heritable
(0.31–0.35) (Brown, 2006) and this is supported by work in New Zealand on
Corriedale sheep (Benavides and Maher, 2003).
These results suggest that scoured wool colour would respond fairly rapidly
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Advances in wool technology
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to selection. Few Merino sheep breeders in Australia currently select their
rams with the assistance of objectively measured scoured colour data, although
approved international test methods for clean wool colour exist and
measurement facilities are readily available from wool testing houses (see
Section 9.2.3).
 Selection for wool colour therefore appears to be an area
where significant improvements in wool quality could be achieved.
9.2.2
Environmental and storage effects on
wool colour
The colour of wool is adversely affected by weathering while exposed to
sunlight on the sheep’s back. Colour measurements on clean wool from
different sites on Merino fleeces grown in Armidale, NSW show a deterioration
of ~3 brightness (Y) units and ~1 yellowness (Y–Z) unit for wool grown on
the back of the sheep compared with mid-flank samples (Holt et al., 1994).
In contrast, on rugged sheep the back wool is slightly lighter than flank wool
and less yellow. Maintenance of good colour attributes is clearly one benefit
of rugging and shedding sheep. Holt et al. showed that a single application
of a UV absorber to the backs of sheep offered protection against weathering
equivalent to 70% of that offered by rugging, and maintained the brightness
and yellowness of back wool to within 0.7 and 0.1 units respectively of the
mid-flank values (Holt et al., 1994).
Wool stored in its greasy state undergoes colour changes that vary with sheep
breed (Baxter, 2001). Merino fleece wools deteriorate in yellowness (Y–Z)
by ~0.01–0.1 units per month and brightness (Y) values also decrease slightly
(see 
Section 9.2.3
 for definitions). New Zealand crossbred wools deteriorate
more rapidly, with the yellowness of Romney wool deteriorating by up to
~1.5 units per month. Greasy crossbred wools may change colour in a matter
of days, depending on the bloodline/environment combination and the moisture
content at the time of shearing. Although most of the colour deterioration
that occurs during storage is removed during scouring, it is not clear whether
protracted storage times in the greasy state can affect the achievable whiteness
on wool after processing. Differences in wool colour within individual Romney
and Merino fleeces have been linked to changes in the pH of an aqueous
extract of greasy wool (Sumner et al., 2003). A significantly higher pH was
measured for belly wool than mid-side wool, and the lowest pH was found
on back samples. Presumably weathering and gravity cause migration of
soluble suint components from the back and mid-side regions of the fleece
towards the belly. Normal (bleach-free) scouring would not be able to remove
any yellowing caused by exposure to alkaline conditions.
Scoured wool colour is generally stable for years (unless the wool was
bleached during the scouring process), provided that the wool is protected
from exposure to sunlight.
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Improving the whiteness and photostability of wool
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9.2.3
Measuring wool colour
The colour of greasy and clean unprocessed wools is most often expressed
in terms of the tristimulus values X,  Y and Z. These values represent the
amount of the red/orange (X), yellow/green (Y) and blue/indigo/violet (Z)
components of the spectrum of white light reflected from the sample. Using
these parameters, wool yellowness is normally quoted as Y–Z and brightness
as Y. The colour of fibrous materials can be objectively measured using a
reflectance spectrophotometer fitted with a standard illuminant (D65) that is
equivalent to outdoor daylight. Nowadays a 10
° field of view is normally
used to analyse reflected light from the sample, and the measured tristimulus
values are quoted as D65/10
° data. A more detailed review of the issues
involved in wool colour measurement has been published by Wood (2002).
The colour of wool is an important characteristic when assessing its
suitability for a particular end use, and hence colour affects the selling price.
The initial colour is particularly important to wool dyers for the production
of bright shades, and wool for this use needs to be both bright and white.
Using the same dyeing conditions, yellow wools produce significantly duller
shades and the colours obtained can often be quite different from those that
are desired.
The International Wool Textile Organisation (IWTO) has approved
international test methods for clean wool colour (IWTO, 2003b) and for
wool sliver and top (IWTO, 2003a) that use a special cell fitted at either end
with glass windows and which is packed with wool to a constant density or
pressure. A cell is unnecessary for textile fabrics and their colour is normally
measured by placing samples directly onto the spectrophotometer port. The
colour of yarn can be similarly assessed by winding a suitable quantity
around a white card. Table 9.1 shows the standard range of brightness and
yellowness values for clean wools and sliver and the descriptors that are
used to define clean wool colour.
Table 9.1 Brightness (Y) and yellowness (Y–Z) ranges and descriptors for clean
unprocessed wools.
D65/10
° Brightness (Y)
D65/10
° Yellowness (Y–Z)
Very bright
>70
Very white
<9
Bright
68–70
White
9–10.5
Average
64–68
Slightly creamy
10.5–9.5
Slightly dingy
59–64
Creamy
9.5–14.5
Dingy
<59
Quite yellow
14.5–16
Heavily stained/yellow
>16
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