The Hofmeister series: Anion effect on microbial transglutaminase cross-linked soybean protein isolate hydrogels

In this paper, the possibility of the kosmotropic anion (CO₃^2-, Citrate^3-, SO₄^2-, H₂PO₄^-, CH₃COO^- and Cl^-) to improve the gel properties of microbial transglutaminase (MTG)-crosslinked soybean isolate protein (SPI)-based hydrogels was explored using a soaking strategy. The results of this experiment demonstrated that the hardness of the hydrogel undergoes different degrees of enhancement after different salt treatments in the following order: H₂PO₄^- > CH₃COO^- > SO₄^2- > Citrate^3- > Cl^- > CO₃^2-. Rheological results showed that salt treatment led to enhancement of the energy storage modulus of the hydrogels. Further experiments indicated that the water-binding capacity of different salts depended on the salting out effect. Based on the Hofmeister effect, hardness-enhanced SPI-based hydrogels were successfully prepared. The Hofmeister effect offers a simple, an effective and a novel method for the preparation of functional SPI hydrogels.