Hydrogels formed with shellac and zein: pH-responsiveness, rheology, and gelation mechanism

Hydrogels responsive to pH have unique applications. Shellac is a pH-responsive molecule soluble above pH 7 but insoluble in acidic conditions. However, shellac-based hydrogels responding to pH have not been reported. In this study, 0.50-4.0 % w/v zein was incorporated in 4 % w/v shellac solutions to form hydrogels during pH-shifting from 12.0 to 6.2-6.4 enabled by gradual and uniform acidification due to hydrolysis of glucono-δ-lactone. These hydrogels maintained their volume when immersed in phosphate buffer at pH 7.0 but shrank at pH 6.0 and 6.5 and swelled or dissolved at pH 7.5 and 8.0. Increasing zein concentration led to greater changes in the hydrogel volume responding to pH changes. An increase in zein concentration facilitated gelation but excess zein weakened the network according to dynamic rheological tests and texture profile analyses. Molecularly, the alkaline pH led to multiple shellac molecules covalently bonded with zein according to gel electrophoresis. Analyses of hydrogel volume changes responding to NaCl, sodium dodecyl sulfate, and urea revealed the primary role of hydrophobic interaction and the secondary role of hydrogel bonds in forming hydrogels. These hydrogels with pH-responsiveness around pH 7.0 may find novel food and non-food applications.