Response surface methodology centred optimization of mono-frequency ultrasound reduction of bacteria in fresh-cut Chinese cabbage and its effect on quality

Mono-frequency ultrasound washing (MFUW) was investigated for the removal of inoculated bacteria in freshcut Chinese cabbage. Based on single factor experiments, ranges of the main conditions affecting reduction levels were confirmed as ultrasound frequency (20-60 kHz), ultrasound power (120-200 W/L), and washing time (5-15 min). Response surface method centred on a 3-factor Box-Behnken Design was applied to optimize MFUW conditions on the log reduction of inoculated Escherichia colt ATCC 8739 and Listeria innocua CICC 10417. The models generated were significant (P < 0.05) and exhibited adequate fitness with the experimental data as designated by high coefficients of determination (R-2 >= 0.97) and non-significant (P > 0.05) lack of fit values. Optimum decontamination conditions for both responses were as follows: ultrasound frequency of 40 kHz, ultrasound power density of 125.45 W/L and a washing time of 15 min. Under these conditions, log reduction predictions were 5.6 and 4/CFU/g for E. coli and L innocua respectively. The reduction counts of both bacteria obtained through the optimized MFUW were higher than that of the conventional water washing (CWW) method. Moreover, optimized MFUW extended Chinese cabbage storability up to 7 days devoid of physical quality deterioration.