Contribution of ultrasound and conventional hot water to the inactivation of Rhizopus stolonifer in sweet potato

Effects of ultrasound (US, 100, 200, 300, 400, and 500 W) and hot water (HW, 25, 40, and 55 °C) treatment combination on Rhizopus stolonifer in sweet potato tuberous roots (TRs) were investigated. US (300, 400, and 500 W) simultaneous HW (55 °C) for 10 min significantly inhibited colony diameters (from 90 mm to 80.08, 65.60, and 6.00 mm) and spores germination (p < 0.05). US+HW treatment significantly increased the leakage of nucleic acids and proteins (p < 0.05). Scanning and transmission electron microscopy analyses showed that US+HW led to significant damage on the appearance and ultrastructure of R. stolonifer. These damages resulted in crumpled hyphae, thickened cell walls, disappeared membranous organelles, massive vacuolation of the cytoplasm. US500+HW55 treatment could lead to cell membrane permeability increase and mitochondrial membrane potential decrease of R. stolonifer. In addition, US500+HW55 controlled the R. stolonifer development in sweet potato TRs during 20 days of storage at 28 °C, suggesting that US500+HW55 effectively inhibited the infection of R. stolonifer. Therefore, US500+HW55 treatment could be an efficient alternative method for storing and preserving sweet potato TRs.