Co-folding of soy protein isolates and shellac by structural interplays to induce hydrogels

The feasibility of co-folding of soy protein isolates (SPI) and shellac by pH-shifting method to induce a protein hydrogel was firstly investigated. SPI and shellac at different mass ratios (1.3, 1.5, 1.7, 2.1, and 2.5 w/w) were co-folded by adjusting pH back to 7, 8, and 9, after unfolding at pH 12. The pH-shifting treatments resulted in the conformational change of SPI and the formation of covalent bonds between SPI and shellac, as confirmed by sodium dodecyl sulfate−polyacrylamide gel electrophoresis (SDS-PAGE), Fourier-transform infrared (FTIR) spectroscopy, UV–Vis, fluorescence spectroscopy, and as well as the changes of thermogravimetry. The cold-set hydrogel was prepared from the pH-shifting process by adjust pH to 9 which showed the optimal hardness and rheological properties, accompanied by excellent water-holding capacity (WHC> 90%). Meanwhile, the microscopic observations revealed the formation of uniform microstructure in the gel network. The co-folded hydrogels were proved to have a lower degree of protein hydrolysis (DH) than the same pH-shifted SPI alone during in vitro simulated digestion. Thus, the simple pH manipulation of SPI and shellac may be applied to induce hydrogels, reducing the protein DH during in vitro digestion which provides the potential for delivery of sensitive nutrients.