A Comparative Study of the Proposed Mechanisms of Cotton Fiber Yellowing

A study was designed to test the relative importance of different proposed mechanisms of cotton fiber yellowing.  Among the mechanisms considered were natural degradation of cellulosic materials to yield chromogenic products, fungal production of secondary metabolites, surface wax component oxidation, Maillard-type reactions between surface sugars and amino acids, and oxidative reactions caused by metal ions.  To study these mechanisms, a cotton print fabric that had been treated to remove most surface contaminants (i.e., scoured, desized, and mercerized) was spotted with different concentrations and combinations of various chemicals and agents and was then stored at 40 °C and 60% RH for 8 weeks.  The color of the treatment areas was measured on the L*, a*, b* color scale before and after storage.  While modest yellowing was noted for many treatments (Δb* of +0.2 to +0.4), treatments with physiological levels of Fe⁺² (50 ppm) were significantly more discolored than all of the other mechanics considered (Δb* of +2.0 to +4.0).   Cu⁺² was also found to induce considerable yellowing when applied at 50 ppm (Δb* of +3.0 to +5.0), but at physiological levels of ~2 ppm, the effect was more muted (Δb* of +0.8 to +1.2). The results suggest that iron is likely a significant causative factor in the rapid yellowing that occurs in some stored cotton bales.