Combined effects of ultrasound and surfactants to reduce Bacillus cereus spores on lettuce and carrots.

This study was performed to compare the effectiveness of ultrasound treatment singly and in combination with surfactants as an alternative method to conventional sanitizers containing chlorine for reducing numbers of Bacillus cereus spores on fresh produce. A cocktail of three strains of B. cereus (10876, ATCC 13061, and W-1) spores was inoculated onto iceberg lettuce and then treated with ultrasound for 0, 5, 10, 20 and 60 min. Five minutes was found to be an adequate ultrasound (40 kHz, 30 W/L) treatment time which also caused no damage to lettuce leaf surfaces as observed through a field-emission scanning electron microscope (FE-SEM). Iceberg lettuce and carrots were inoculated with a cocktail of three strains of B. cereus spores and treated with combinations of ultrasound and various concentrations (0.03 to 0.3%) of surfactant (Tween 20, 40, 60, 80 and Span 20, 80, 85) solutions for 5 min. The efficacy of the combination of ultrasound and surfactant increased depending on the hydrophile-lipophile balance (HLB). The most effective treatment for reducing levels of B. cereus spores was the combination of ultrasound and 0.1% Tween 20, yielding reductions of 2.49 and 2.22 log CFU/g on lettuce and carrots, respectively, without causing deterioration of quality. These reductions were 1 log greater than those obtained by immersion in 200 ppm chlorine for 5 min. Further research for elimination of B. cereus spores involving study of spore adhesion and removal mechanisms from food surfaces is needed, as well as devising an industrial-scale ultrasound system for the food industry.