In this study, ESR was used to characterize the differences between irradiated lactose and non-irradiated lactose. A multi-component ESR spectrum was observed from most of the non-irradiated commercial lactose powder as well as the gamma-irradiated lactose powder. However, the signal pattern of non-irradiated lactose powder was different from that of irradiated lactose powder. Moreover, the multi-component ESR spectrum intensity of radicals generated by milling was weaker than that by gamma-irradiation. Among the several possible causes of generating radicals such as irradiation, lipid oxidation, protein oxidation, influence of ash or mineral and mechanical radical by grinding, the multi-component ESR spectrum from the non-irradiated lactose powder was shown to have resulted from the generation of mechanical radicals during milling in the manufacturing processes. Crown Copyright (C) 2010 Published by Elsevier Ltd. All rights reserved.