Pilot-Scale Continuous Ultrasonic Cleaning Equipment Reduces Listeria monocytogenes Levels on Conveyor Belts

Abstract

Ultrasonic cleaning of a conveyor belt was studied by building a pilot-scale conveyor with an ultrasonic cleaning bath. A piece of the stainless steel conveyor belt was contaminated with meat-based soil and Listeria monocytogenes strains (V1, V3, and B9) and incubated for 72 h to allow bacteria to attach to the conveyor belt surfaces. The effect of ultrasound with a potassium hydroxide–based cleaning detergent was determined by using the cleaning bath at 45 and 50°C for 30 s with and without ultrasound. The detachment of L. monocytogenes from the conveyor belt caused by the ultrasonic treatment was significantly greater at 45°C (independent samples t test, P < 0.001) and at 50°C (independent samples t test, P = 0.04) than without ultrasound. Ultrasonic cleaning efficiency was tested with different cleaning durations (10, 15, 20, and 30 s) and temperatures (30, 45, and 50°C). The differences in the log reduction between cleaning treatments were analyzed by analysis of variance with Tamhane's T2 posthoc test using SPSS (Chicago, IL). The lengthening of the treatment time from 10 to 30 s did not significantly increase the detachment of L. monocytogenes (ANOVA 0.633). At 30°C and at the longest time tested (30 s), the treatment reduced L. monocytogenes counts by only 2.68 log units. However, an increase in temperature from 30 to 50°C improved the effect of the ultrasonic treatment significantly (P < 0.01). Ultrasonic cleaning for 10 s at 50°C reduced L. monocytogenes counts by more than 5 log units. These results indicate that ultrasonic cleaning of a conveyor belt is effective even with short treatment times.