Evaluation of Aspergillus oryzae as a Surrogate of Aspergillus flavus and Radiofrequency Dielectric Heating to Control Aspergillus Mold in Inshell Hazelnuts

Abstract

This study evaluated the thermal inactivation properties of Aspergillus flavus (A. flavus) and Aspergillus oryzae (A. oryzae) in hazelnut kernels and shells to assess the suitability of using A. oryzae as a surrogate and explored the potential of radiofrequency (RF) dielectric heating to control Aspergillus mold. Both molds exhibited similar heat sensitivities at 45, 55, and 65°C, achieving complete inactivation at 65°C without holding time, though resistance persisted at 45°C even with prolonged heating. Notably, A. flavus was 5°C more heat-resistant and more sensitive to holding time compared with A. oryzae. The critical inactivation temperature was 55°C for A. oryzae and 60°C for A. flavus. At 50°C, A. oryzae displayed comparable thermal resistance in hazelnut shell and kernel powders, but its behavior diverged at 55°C. While A. oryzae reduction was observed in shells during inoculation, no reduction occurred in kernels. RF heating achieved a 0.48-log reduction in A. oryzae in inoculated in-shell hazelnuts when the kernel temperature reached 70°C, increasing to a 0.81-log reduction when the shell temperature reached 65°C. The incomplete inactivation on the shell surface might be attributed to the low moisture content and poor thermal conductivity of the shells. Lipid oxidation of RF-heated kernels was evaluated by fatty acid content and K values, and no significant differences were observed from the unheated samples. These findings highlight the potential of using A. oryzae as a surrogate of A. flavus and RF heating to inactivate Aspergillus to ensure hazelnut safety and prevent lipid deterioration.