Advances in wool technology
242
been very limited. The current rapid progress in genetics and molecular
biology suggests that application of these techniques to the genes that control
wool colour could be very useful in the longer
term for producing whiter
wools with improved photostability. Selection of sheep for improved wool
traits by conventional breeding has been limited by the genome and through
each selected generation the genetic profile of the original strains is altered
at an unknown number of loci. In contrast, transgenesis and cloning
methodologies allow the valuable genetic background of pre-existing strains
to be preserved. Moreover, these methods can now
be used to alter single or
multiple loci and to add or remove genes. Transgenic studies on sheep have
already shown that the properties of the wool fibre can be significantly
changed and that such changes are heritable (Powell
et al., 1994; Bawden
et
al., 1998). However, in order to clearly define the experimental options for
sheep transgenesis and wool fibre modification, a better knowledge of the
molecular biology of the wool follicle and the molecules which control
initiation of wool follicle growth is required. Realistically,
one would expect
transgenic studies to focus initially on traits such as wool yield, growth rate
and fibre diameter, which potentially offer a higher return to the grower than
improved colour.
It has been suggested that creating wool flocks where albinism could be
used as the basis for pigmentation control would be a novel area of research
(Fleet, 2002). Transgenics may also offer the opportunity to create small air
cavities within fine wool fibres that are similar
to the medulla found in
coarser wool fibres, such as the Scottish Blackface. If this could be
accomplished without significantly affecting the strength of the wool fibre,
then the increased reflectance and reduced light penetration through the fibre
would produce a brighter, whiter visual effect.
The origin and identity of the visible yellow chromophores in wool remains
an area where further fundamental research is essential. This would not only
allow geneticists to identify the genes responsible
for their production, offering
the possibility of control, but might also allow chemists to design more
effective bleaching processes to remove them from the fibre during commercial
processing.
Although a great deal of work has been done to characterise wool cortical
proteins and define their structure and function, knowledge of the cuticle
proteins remains very rudimentary. Increased
knowledge of the biology and
biochemistry of cuticle proteins in wool is an important issue, as most of the
high-energy UVB radiation in sunlight which causes photoyellowing will be
absorbed within the cuticle cells. One of the key questions is whether
chromophores form preferentially near the fibre surface in the cuticle cells
during photoyellowing by sunlight, as suggested by Simpson for UVB exposure
(Simpson, 1999), or whether they are more evenly
distributed throughout the
cuticle and cortex.
© 2009 Woodhead Publishing Limited
Improving the whiteness and photostability of wool
243
Zinc oxide is a UV-absorbing white pigment commonly used in sunscreens
to protect against skin damage. Currently it is not clear whether the application
of inorganic nanoparticulates to wool, capable of penetrating into the fibre,
might offer similar protection against photoyellowing. This would be unlikely
if most yellowing occurs in the highly crosslinked regions of the exocuticle.
Schäfer has claimed that the white inorganic pigments
titanium dioxide and
barium sulphate retard the photoyellowing of FWA-treated wool, but they
also impact on its whiteness by reducing the effectiveness of the FWA (Schäfer,
1990). The materials used were not nanoparticles and presumably were simply
deposited on the fibre surface.
A great deal of research has already been carried out on wool photoyellowing.
The complexity of the chemistry involved suggests that an empirical approach
to improving wool’s photostability is most unlikely to be successful. Detailed
knowledge of the photoyellowing mechanism(s) is therefore essential. Such
knowledge could then be used to develop a photostable FWA system for use
on wool and other protein fibres. This is urgently required to allow wool to
compete on a level playing field with other textile
fibres and to produce
photostable brilliant whites and pastel shades.
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