Enhanced inactivation of Aspergillus niger in low water activity raisins by microwave processing: evidence of non-thermal effects

This study aimed to investigate the inactivation efficiency of microwave treatment on Aspergillus niger inoculated in low water activity raisins, with a particular focus on potential non-thermal effects. To differentiate thermal and non-thermal contributions, corresponding water bath treatment was designed to match the time-temperature profiles of microwave processing. Different microwave processing with varying exposure times (2, 3 and 4 min) each followed by different holding times (0, 1 and 2 min) were developed. Compared to water bath treatment, microwave-treated samples exhibited significantly higher microbial inactivation values in Aspergillus niger (P < 0.05) under the same thermal processing level, which proved the presence of microwave non-thermal effects on mold inactivation in low water activity raisins. Furthermore, the non-thermal effects exhibit a nonlinear increase with prolonged microwave exposure durations. This phenomenon indicated that elevated temperatures intensified the non-thermal effects. The combined thermal and non-thermal effects of microwave processing significantly enhanced the inactivation of Aspergillus niger inoculated in low a_w raisins, offering a promising approach for improving the safety and quality of low water activity foods.