BACKGROUND: Gamma(gamma)-aminobutyric acid (GABA) has been used extensively in pharmaceuticals and functional foods and is also a building block for bioplastics. GABA is produced from glutamate through decarboxylation catalyzed by glutamate decarboxylase (GAD). The reaction medium should be kept acidic because a pH rise resulting from the reaction inactivates the enzyme catalyst, which is active only at acidic pH. The use of conventional buffers and acids inevitably accompanies salts, which cause serious problems in separation and purification of GABA. In this work, we have applied heterogeneous solid acids for the first time. RESULTS: The GAD-catalyzed reaction was conducted in 0.2 mol L-1 sodium acetate buffer (pH 4.6) with 1 mol L-1 monosodium glutamate at 37 degrees C. When commercial cation-exchange resins as solid acids were simply added to the reaction medium, the conversion improved from 13% to 67% without salt formation. Even when water was used as the reaction medium, acidic ion-exchange resins enhanced the reaction conversion significantly. CONCLUSION: In a salt-free manner, acidic resins suppress the pH rise during the reaction so that they can enhance the reaction conversion. In addition, they can be recovered and reused easily after the reaction. Heterogeneous solid acids make the GABA production process more economical and eco-friendly. (C) 2013 Society of Chemical Industry