Effect of pH and protein-polysaccharide ratio on the intermolecular interactions between amaranth proteins and xanthan gum to produce electrostatic hydrogels

The development of new acid-induced mixed hydrogels based on amaranth proteins and xanthan gum is dis-cussed. The influence of pH and protein-polysaccharide ratio on intermolecular interactions and gel properties was investigated. From a protein solubility assay and FTIR analyses, it was shown that while proteins form electrostatic hydrogels with xanthan gum at pH < pI (4.0), room temperature, and without the addition of crosslinking agents, at pH > pI (5.5) a higher incompatibility between charged biopolymers led to the formation of cross-linked soft hydrogels, promoted by hydrophobic interactions and to a lesser extent by electrostatic forces. Raman spectroscopy results, on the other hand, suggested there were no significant changes in the sec-ondary structure of amaranth proteins. However, changes in their tertiary structure as a function of pH and ratio were evidenced. Regarding gel properties, the electrostatic hydrogels obtained at pH 4.0 showed more aggre-gated, denser, and stronger structures than gels prepared at pH 5.5. In addition, at both pH values, the increase of protein-polysaccharide ratio showed an adverse effect due to increased protein-protein associations. The overall results suggest that acidification of API:XG systems until pH 4.0 is a promising pathway for producing electro-static hydrogels for food applications.