HPLC Determination of the Reactive Lysine Content of Cottonseed Protein Films To Monitor the Extent of Cross-Linking by Formaldehyde, Glutaraldehyde, and Glyoxal

A sensitive and reproductible HPLC method to determine the reactive lysine content of films made from cottonseed flour was developed. This method is applied to follow the cross-linking of cottonseed proteins by formaldehyde, glutaraldehyde, or glyoxal. The cross-linking of these proteins resulted in the decrease of reactive lysine content within the film. The maximum puncture force of the films is correlated with the modified reactive lysine content after cross-linking treatments. Of the three cross-linking agents, formaldehyde was the most effective to enhance the maximum puncture force of the films despite its moderate reaction with only 50% of reactive lysine in the films. By contrast, glutaraldehyde, which reacted with nearly 100% of lysine, led to less resistant films. The results were interpreted as the impact of the molecular structure of the cross-link bridges on the mobility between protein chains.