Rice protein (particle size of 0.15 mm, crude protein content of 813 g/kg) was purchased from Zhengzhou tianshun food additives Co., Ltd. (Henan, China). Alcalase 2.4 L with an activity of 23,400 U/mL was purchased from Novozymes Co., Ltd. (Tianjin, China). All reagents used in the experiment were of analytical grade.