Effect of microwave roasting on roast characteristics and bioactive compounds in coffee beans

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Coffee roasting influences the nutritional value and bioactive compounds of coffee. However, the effects of microwave roasting as an alternative to conventional methods remain insufficiently understood. In this study, a microwave roasting system was developed to investigate the effects of microwave power, stirring speed, and heating time on the mass loss rate, roasting uniformity, roasting degree, caffeine and 3-Caffeoylquinic acid (3-CQA) content of coffee beans. Meanwhile, volatile compounds were comparatively analyzed between microwave and hot-air roasting at the same roasting degree. The mass loss rate increased with higher microwave power and decreased with longer heating time. Microwave power, roasting time, and stirring speed jointly influenced coffee bean roasting uniformity, with optimal performance achieved at 100 W and 140 r/min for 8–10 min and 100 W and 180 r/min for 12 min roasting. Roasting degree of coffee beans was primarily determined by microwave power and heating time, and was deepened as microwave power increased and heating time prolonged. Caffeine remained stable (1.06–1.43%), whereas 3-CQA decreased from 4.55% to 0.80%. In addition, noticeable differences in volatile compound profiles were observed between roasting conditions, indicating that roasting method and intensity jointly influence aroma formation. Overall, microwave roasting showed distinct process characteristics in terms of heating behavior and compositional changes of coffee beans, providing a potential alternative approach for coffee roasting.