pH-responsive hydrogels formed with shellac and zein acidified with glucono-δ-lactone

Hydrogels with a continuous water phase and responsive to environmental conditions such as pH have unique applications. Shellac, a wax used in food coating, is a pH-responsive molecule soluble above pH 7 but insoluble in acidic conditions. The hypothesis of this thesis is that addition of zein in alkaline shellac solutions enables the formation of pH-responsive hydrogels after gradual and uniform acidification to slightly acidic pH enabled by hydrolysis of glucono-δ-lactone. In the first objective, the formation of zein-shellac hydrogels and physical properties of hydrogels were characterized. With 0.50-4.0% w/v zein supplemented to 4.0% w/v shellac at pH 12.0, dissolving 1.2% w/v glucono-δ-lactone lowered pH to 6.2-6.4 gradually during 8-h incubation at 22 °C and formed self-standing hydrogels. These hydrogels maintained their volume when immersed in phosphate buffer at pH 7.0, shrank at pH 6.0, but swelled or dissolved at pH 8.0. Small amplitude oscillatory tests were used to characterize rheological properties of hydrogels. These hydrogels showed a limit of linear viscoelastic regime of >1% strain, independent of zein concentration. A higher content of zein shortened the gelation time, and the storage modulus at 1 Hz was the highest (ca. 8,000 Pa) at 3.0% zein. Heating at 4-50 °C reduced the moduli, signifying hydrogen bonds in networks. In texture profile analysis, hydrogels had the highest hardness, adhesiveness, gumminess, and chewiness at 2.0-3.0% zein, while springiness and resilience were less dependent on zein concentration. The second objective was to investigate possible gelation mechanisms. Protein electrophoresis showed the grafting of multiple shellac to zein. Shrinking of hydrogels in 100-600 mM NaCl solutions at pH 7.0 indicates the insignificance of electrostatic attraction in gelation. Dissolving of hydrogels in 0.2-0.8% sodium dodecyl sulfate solutions and swelling of hydrogels in 1.0-4.0 M urea solutions at pH 7.0 unveil the major role of hydrophobic attraction bringing free shellac and shellac-grafted zein molecules in proximity to form hydrogen bonds and thus hydrogels. The significance of covalent bonding, hydrophobic attraction, and hydrogen bonds in hydrogel formation was confirmed in Fourier-transform infrared spectroscopy. These hydrogels with pH-responsiveness around pH 7.0 may find novel food and non-food applications.