Glyoxal modification of gelatin leads to change in properties of solutions and resulting films

The influence of different pretreated film forming solutions (FFSs) on the physical, chemical and barrier properties of gelatin films was studied. Hydrolyzation with hydrochloric acid and acetylation with acetic acid anhydride at different pHs were performed. Subsequently, gelatin was crosslinked with glyoxal to result in biopolymer films with properties predetermined by the rheological behavior of the respective FFS. Gel time, storage modulus and tan delta of FFSs were obtained from small-strain oscillatory measurements and were correlated with swelling and solubility of the resulting films as well as with chemical analysis of lysine and arginine and the respective amino acid modifications N-epsilon-carboxymethyllysine (CML), glyoxal derived lysine dimer (GOLD) and 5-(2-imino-5-oxo-1-imidazolidinyl) norvaline (imidazolinone). In general, chemical modification reduced the storage modulus whether it was hydrolyzation, acetylation or modification with glyoxal. Treatment by hydrolyzation or acetylation led to differences in gel time and in the ratio between viscous and elastic portion of the different hydrogels, also affecting the relationship between monovalent amino acid modification and bivalent crosslinking after glyoxal addition. Thus, the structural character of FFS hydrogels and of the protein network of films both determined the physical and barrier properties of chemically modified gelatin films.