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

14.2
Overview of the structure and
characteristics of keratin protein
As a protein, keratin is characterised by a higher cystine content than other
proteins, typically from 4 to 28 residues per 100 in keratin when compared
with 0 to 2 residues per 100 for other proteins. This sulphur-rich amino acid
is associated with crosslinking, and so the building of tough and strong
materials. This crosslinking is of clear importance in the physical role that
keratin plays in nature as fibres and horny tissue to protecting DNA on a
cellular level (Hearle, 2000; Gilbert et al., 2001). In addition, cystine has
some known biological function in providing oxidative protection to cells
and is associated with treatment of a range of disease states from chronic
bronchitis to HIV and cardiovascular disease (Parcell, 2002). An ordered
and fibrous character is also of significance when considering defining
characteristics of wool keratins due to the tendency of this characteristic to
drive keratins to organise and form structured physical materials (Hermann
et al., 2000).
Given the widespread occurrence of keratin in nature it is important to
consider the features of wool keratins as compared with other sources.
Biological characteristics and practical considerations are both of importance
to evaluate future applications of wool. Horns and hooves from slaughterhouse
waste have traditionally been used as a source of keratin for a variety of
consumer and industrial uses. The high degree of additional animal tissue,
the physical form and consumer perceptions of the form have restricted use
of these materials to harsh processing methods and industrial applications of
low value. The focus in this chapter is the consideration of advanced
applications of wool beyond textiles and fibres.
Avian keratin sources, in particular feather keratins, also have this practical
limitation of source and consumer perception. In addition, fundamental
differences in protein structure and cystine content limit their functionality
and use for the development of robust materials. Feather keratins have an
inherently lower cystine content than wool and other hard alpha keratin
sources. In addition, they have a protein structure that comprises proteins of
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Application of wool keratins
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much lower molecular weight and a lower degree of fibrous organisation.
The absence of an ordered fibrous structure is a substantial disadvantage
when reconstituting keratins into advanced materials and applications. However,
some applications of feather proteins as bioplastics have been the focus of
academic research (Schrooyen and Dijkstra, 1998).