Sorghum kafirin film property modification with hydrolysable and condensed tannins

Films made from kafirin, the prolamin protein of sorghum, could be an environmental-friendly alternative to synthetic plastic packaging films. However. because protein-based films have inferior functional properties to synthetic plastic packaging, tannic acid (TA) and sorghum-condensed tannins (SCT) were added at up to 20% (w/w) as modifying agents during kafirin film casting. Both TA and SCT were bound to kafirin protein in the film. Freeze-fracture surfaces of tannin-modified kafirin films were observed to be different from the control. Modification with both tannins at increasing levels resulted in an increase in tensile stress and Young's modulus by two-fold and four-fold, respectively. but a three-fold decrease in %strain and a 12-15% decrease in water absorbed. Modification with TA and SCT did not change the apparent water vapour permeability. However, a significant quadratic decrease was observed for oxygen permeability. The glass transition temperature (T-g) of the films showed a quadratic increase with increase in TA and SCT level. These findings indicate that TA and SCT can modify the properties of kafirin films. This probably there are no decreasing free volume in the film and possibly also by decreasing chain mobility between kafirin polypeptides by cross-linking. (C) 2004 Elsevier Ltd. All rights reserved.