Due to the increasing global demand for Gamma-aminobutyric acid (GABA) in the food and pharmaceutical industries, the expression of the recombinant Glutamate decarboxylase (GAD) and its industrial production is currently requested. Culture conditions were optimized to increase the expression level of the recombinant enzyme in different pH, temperature, incubation time, aeration levels, inoculation concentrations, concentrations of IPTG, and several carbon sources using the RSM based on a central composite design. According to the results of the quadratic regression equation, recombinant Escherichia coli BL21 (DE3) had the highest GAD expression at pH=7.2, aeration at about 120 rpm, inoculation concentrations of about 3% v/v, 2.25 mM IPTG, 37 °C, and 6 h of incubation time in the presence of 0.2% glucose. Using pure glucose as a carbon source on an industrial scale is not cost-effective for producing recombinant proteins. Therefore, using low-cost carbon sources such as corn syrup and molasses with concentrations of 1.5 and 5.65% (w/v) is an efficient method for the industrial production of recombinant GAD. The concentration of purified recombinant GAD in carbon sources of 0.2% glucose, 1.5 corn syrup and 5.65% molasses was 2.155, 2.07 and 1.96 mg/mL, respectively. In this way, the global need for GABA can be met by the industrial production of GAD.

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