Page 204 - Advances in Textile Biotechnology

Enzymatic treatment of wool and silk fi bres 185

in the proportion of their β-conformation and an increase in the degree of
crystallinity. The crystallinity of the whole sericin is about 15% with the
fractions I 3%, II 18.2%, III 32.5% and IV 37.6%. The hydrophilic nature
of the sericin protein accounts for the relative ease with which it can be
solublized during degumming and its susceptibility to proteolytic enzymes.
As proteins, both silk fibroin and sericin are susceptible to degradation

by proteolytic enzymes. Most proteolytic enzymes can hydrolyse the mainly

amorphous sericin without much difficulty but not the highly crystalline

fibroin. Proteolytic attack on silk fibroin is limited to its amorphous regions.
Proteolytic enzymes such as chymotrypsin have been reported to degrade
the amorphous regions of fibroins to obtain a highly crystalline fi broin frac-

tion (Konishi et al., 1967; Tsukada, 1986). Successful removal of sericin from

fibroin for silk degumming requires the selection of suitable proteases in
order to limit enzymatic degradation to sericin to avoid damage to the silk
fibre. Various proteolytic enzymes (alkaline, neutral, and acidic proteases)

have been investigated for silk degumming (Freddi et al., 2003; Gulrajani
et al., 1996). Among them, alkaline proteases performed better than neutral
and acidic proteases. The degradation of sericin in terms of weight loss up
to 24% has a linear relationship with the amount of alkaline protease used
during the enzymatic degumming. Therefore the enzymatic process can be
controlled through the enzyme dosage and treatment time. After enzyme
treatment, the enzyme is inactivated at 80 °C for a short time and this
enhances dissolution of the partially hydrolyzed sericin fractions from the

silk. Enzymes have been used to remove sericin from fibroin in silk fabrics.
For the crepe fabric, it was reported from morphological analysis that
sericin was still found present in the highly twisted weft yarns whereas
sericin was completely removed from the warp yarns of the fabric. Extended
enzyme treatment might cause damage to the warp yarn fi bres. A certain
level of mechanical agitation during the degumming process might
be needed to enhance enzyme penetration and to make sericin removal
complete.
Because of the small amount of wax existing on the raw silk and addi-
tional waxes applied to the yarns, Gulrajani et al. (2000b) incorporated
lipase with the protease in the degumming process. It was reported that the
combination of a lipase and a protease resulted in effective de-waxing and

degumming and improved wettability of silk fibres and handle.
Recently, low-frequency and high-power (20–100 KHz) ultrasound has
been used to enhance enzymatic textile processing. The combination of
enzyme and ultrasound treatments has two effects. One is the effect on the
activity of enzymes. Very little is known about how ultrasound causes the
inactivation or activation of enzymes. Another is that ultrasound induces
vibration passing through a liquid and the resultant impact waves cause
cavitation. Ultrasound was used to try to accelerate the degumming process

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