Microwave-Induced Thermal Inactivation of Fusarium solani Strain Isolated from Cocoa Pods: Experimental Determination of the Temperature Threshold for Fungal Lethality

Cocoa (Theobroma cacao L.) postharvest quality can be compromised by fungal colonization across the farm-to-chocolate continuum, motivating non-chemical control options that can be engineered as unit operations. This study evaluates microwave (MW) irradiation (2.45 GHz) as a physical disinfestation process against Fusarium solani (F. solani). Experiments were conducted under a completely randomized design (CRD) by tuning MW power (140–210 W) and exposure time to reach predefined surface-temperature bands verified by infrared (IR) thermography. Post-treatment viability was assessed by incubation (27 °C, 7–10 days) and classified as Reduced Growth Rate (RGR; sublethal regrowth slower than control) or Death (D; no regrowth comparable to autoclaved negative controls). Outcomes were temperature-governed: RGR predominated in sublethal bands (60–80 °C), whereas consistent D occurred once surface temperature exceeded 80 °C. Thermodynamic modeling established a median lethal temperature (LT50) of 81.0 °C, and a 99% eradication threshold (LT99) of 87.2 °C. Maximizing the heating rate (Hr) to 0.381 °C/s at 210 W ensured rapid and profound cellular suppression. Scanning electron microscopy (SEM) corroborated severe ultrastructural damage (collapsed macroconidia and disrupted hyphae) under lethal conditions, and turbidimetric monitoring over 120 h showed sustained growth suppression in treated samples. These findings define a practical lethal threshold and power–time windows to inform MW scale-up as a postharvest unit operation while minimizing thermal residence time.