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.

Synergistic effect of steam and lactic acid against Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes biofilms on polyvinyl chloride and stainless steel.

This study was designed to investigate the individual and combined effects of steam and lactic acid (LA) on the inactivation of biofilms formed by Escherichia coli O157:H7, Salmonella Typhimurium, and Listeria monocytogenes on polyvinyl chloride (PVC) and stainless steel. Six day old biofilms were developed on PVC and stainless steel coupons by using a mixture of three strains each of three foodborne pathogens at 25°C. After biofilm development, PVC and stainless steel coupons were treated with LA alone (immersed in 0.5% or 2% for 5s, 15s, and 30s), steam alone (on both sides for 5, 10, and 20s), and the combination of steam and LA. The numbers of biofilm cells of the three foodborne pathogens were significantly (p<0.05) reduced as the amount of LA and duration of steam exposure increased. There was a synergistic effect of steam and LA on the viability of biofilm cells of the three pathogens. For all biofilm cells of the three foodborne pathogens, reduction levels of individual treatments ranged from 0.11 to 2.12 log CFU/coupon. The combination treatment of steam and LA achieved an additional 0.2 to 2.11 log reduction compared to the sum of individual treatments. After a combined treatment of immersion in 2% LA for 15s or 30s followed by exposure to steam for 20s, biofilm cells of the three pathogens were reduced to below the detection limit (1.48 log). From the results of this study, bacterial populations of biofilms on PVC coupons did not receive the same thermal effect as on stainless steel coupons. Effectiveness of steam and LA may be attributed to the difference between Gram-negative and Gram-positive characteristics of the bacteria studied. The results of this study suggest that the combination of steam and LA has potential as a biofilm control intervention for food processing facilities.