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Enzymatic versus chemical processing of cotton 133

In this chapter the processes of conventional chemical preparation,
including desizing, scouring and bleaching, are compared with those of fully
enzymatic processes that, to date, include desizing and scouring but not
bleaching. There have been research efforts regarding the latter, but no
effective industrial methods. The processes will be evaluated with regard to
effectiveness on cotton properties, their environmental and energy impacts,
and their costs and benefi ts.

6.2 Chemistry and structure of the cotton fi ber

Cotton is by far the most important natural textile fiber in use. Most accounts
put the worldwide usage of cotton at about forty percent of all fi ber con-
sumed for textile purposes (Fiber Organon, 2005). Cotton comes from
plants of the genus Gossypium, with only 4 of the 33 species being of major
commercial importance (Chaudry and Guitchounts, 2003). For most of the
world, G. hirsutum (American Upland) and G. barbadense (Egyptian, and
other extra-long staple varieties) are the major cotton fi bers. Two other
coarse short staple cottons, G. herbaceum and G. arboretum, are grown in
southeast Asia.
Chemically, the cotton fiber is typically about 95% cellulose, the polymer

of β-d-glucose in which the units are linked at the 1 and 4 carbons (Fig. 6.1).

The other roughly 5% of the raw cotton fiber is a chemical mixture of
proteins, amino acids, pectic substances, hemicelluloses, waxes (high molec-
ular weight alcohols and esters), and various organic and inorganic salts.
Many of these low-molecular-weight constituents can be removed by hot
water, but the rest require stronger treatments, and these treatments have
conventionally been dilute (0.5–4.0%) sodium hydroxide (NaOH) at or
near the boil in the scouring process. The proteins, pectins and hemicellu-

loses are connected in complex ways to the cellulose microfibrils in the
primary wall. Figure 6.2 gives a representation of how these constituents
might be connected (Agrawal et al., 2007). A more detailed description of
how these materials are joined to one another, and the implications for
needs in enzymatic scouring are given in Fig. 6.3 (Agrawal, 2005).

OH
OH
O HO O
HO O O
OH
OH
n
6.1 Cellulose structure.

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