Enzymatic hydrolysis of corn gluten: Optimization of process parameters, hydrolysis, solubilization and enzyme inactivation kinetics

Over the last decade, modified plant proteins as an alternative to proteins from animal sources are increasingly being used as food additives. Corn gluten which contains approximately 60 % proteins is a by-product from the production of starch from maize. Currently, its main use is as an animal feed. The major drawback of using corn gluten in foods is that it is very insoluble in water. However, its solubility can be increased by modification. In the present work, a comprehensive study on the hydrolysis of corn gluten was performed to improve its utilization in food products.In the present study, firstly, a general survey of enzymatic hydrolysis and solubilization of corn gluten was performed with five commercial enzyme preparations (Alcalase, Neutrase, Flavourzyme, Protamex and PTN) in order to choose appropriate enzyme; and as a result Alcalase was chosen as proper enzyme among them. Then, the calibration of pH-stat for corn gluten hydrolysis by Alcalase was performed and the pK values of the corn gluten hydrolysates were determined as 6.82, 6.60 and 6.37 for the temperatures of 40°C, 50°C and 60°C respectively. For optimization of the process parameters, hydrolysis reactions were performed at different substrate concentrations (10-60 g protein/L), enzyme concentrations (0.05-0.35 % (v/v)), temperatures (40-65°C) and pH values (6.5-8.5); and the degree of hydrolysis, solubility of corn gluten and residual enzyme activity were investigated depending on processing time. The optimum process conditions for hydrolysis and solubilization were obtained as 30 g/L substrate concentration, 0.25 % (v/v) enzyme concentration, 55°C and pH 8. Under these conditions, the values of degree of hydrolysis and solubilization were found as 28.4 % and 85.3 % respectively; and enzyme lost its activity by 74 % at the end of 120 min processing time. The effect of hydrolysis products and pretreatments applied to substrate; such as thermal treatment, microwave irradiation, sonic treatment and starch removal; on hydrolysis were also investigated. Hydrolysis experiments under ultrasonication were performed in order to investigate the effects of sonication parameters on hydrolysis and enzyme stability.Modelling studies were performed to determine the kinetics of hydrolysis, solubilization and enzyme inactivation. The Michaelis-Menten kinetics was examined with initial rate assays, and the kinetic parameters, Km and Vmax were estimated as 53.77 g/L and 5.94 mmol/L.min. The effects of temperature and pH on Michaelis-Menten parameters were also examined. The type of inhibition for Alcalase determined as uncompetitive; and the inhibition constant, Ki, was estimated as 44.68 % (hydrolysate /substrate mixture; v/v).Key Words: Corn gluten, Alcalase, enzymatic hydrolysis, process parameters, optimisation, kinetic