In the present study, we describe changes in the primary and secondary structural patterns of glucoamylase during starch hydrolysis under microwave irradiation using SDS-PAGE and circular dichroism (CD) spectroscopy. Our SDS-PAGE results show that the primary structure of glucoamylase did not change after microwave irradiation. According to the CD spectra, the positive peak height (lambda = 193 nm) of the microwave-irradiated samples decreased by 36.4-68.2% compared to those without irradiation, whereas the double negative peak height (lambda = 206 nm, lambda = 220 nm) increased by 10.8-31.4%. In addition, the positive peak (lambda = 193 nm) shifted by 0.2-3 nm. After treatment of glucoamylase with microwave irradiation, the alpha-helical content of glucoamylase decreased sharply, whereas the beta-sheet, beta-turn and random coil content increased gradually. The conformational changes of glucoamylase after microwave irradiation provide theoretical support for the mechanism whereby microwave irradiation accelerates starch hydrolysis catalyzed by glucoamylase. (C) 2012 Elsevier Ltd. All rights reserved.