Investigation of the influence of pH on the properties and morphology of gelatin hydrogels

The behavior of gelatin hydrogels is influenced by the charges located on the amino acid side chains throughout the gelatin molecules. The presence and distribution of ionisable side chains influences the surface activity of gelatin and ultimately determines the material properties. Herein, we report the influence of pH on mechanical properties as studied by texture analysis supported by data from polarimetry, zeta potential, pH titrations and NMR experiments. When adjusted to more extreme pH values (pH 2 and 12), softer gelatin blocks were observed. However, at pH values close to the isoelectric point (pH 5–10), the material is firmer. This behavior is related to the helical content. At pH 2 and pH 12 the surface of the gelatin carries a net charge, positive and negative, respectively, that inhibits the formation of tight helices and lowers the physical crosslink network density. Chemical shift perturbations were observed for the acidic amino acids glutamic and aspartic acid, under acidic pH, where their peaks shifted to higher ppm. Intense amide signals were observed at acidic pH but diminished with increasing pH. This was due to an increase in the rate of chemical exchange between the solvent and peptide amide protons as the pH increases.