Preparation of starch–poly–glutamic acid graft copolymers by microwave irradiation and the characterization of their properties

•Poly-γ-glutamic acid was grafted onto waxy starch using microwave irradiation.•The reaction conditions were optimized to temperature and pH.•The starch–g–PGA gel absorbed more than 20 times of its own weight amount of water.•The starch–g–PGA gel exhibited viscoelastic solid behavior.

Graft copolymers of waxy maize starch and poly-γ-glutamic acid (PGA) were produced in an aqueous solution using microwave irradiation. The microwave reaction conditions were optimized with regard to temperature and pH. The temperature of 180 °C and pH7.0 were the best reaction conditions resulting in a PGA graft of 0.45% based on nitrogen analysis. The average graft content and graft efficiency for the starch–PGA graft copolymer prepared at 180 °C and pH7.0 were 4.20% and 2.73%, respectively. The starch–PGA graft copolymer produced at 180 °C and pH7.0 could absorb more than 20 times its own weight amount of water and form a gel. The preliminary rheology study revealed that the starch–PGA graft copolymer gel exhibited viscoelastic solid behavior while the control sample of waxy starch showed viscoelastic liquid behavior.