How to monitor the protein cross‐linking by formaldehyde, glutaraldehyde or glyoxal in cotton‐seed protein‐based films?

Protein cross-linking treatments by formaldehyde, glutaraldehyde and glyoxal are applied to enhance the maximum puncture force of films made with cottonseed flours. A sensitive and reproductible HPLC method to determine reactive lysine content of films was developped. The cross-linking of cottonseed 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 films. By contrast, glutaraldehyde which reacted with nearly 100% of lysine, led to less resistant films. The impact of protein cross-linking on maximum puncture force of films cannot be explained solely by the number of lysyl groups involved in the cross-linking reaction. It is also determined by the modification of intermolecular interactions between protein chains caused by a "protein intemal plasticization effect" due to the chemical modifications in protein structure after cross-linking treatments. (Resume d'auteur)