Ultrasound Inactivation of Bacteria and Yeast in Aqueous Solutions and Skim Milks

Abstract

Ultrasonic treatment is considered a potential method to inactivate microorganisms in food. The aims of this study were to determine the effects of both low-frequency (20 kHz) and high-frequency (850 kHz) ultrasound treatment on bacteria and yeast, and to develop a fundamental understanding of the mechanisms involving ultrasonic cavitation. Enterobacter aerogenes, Bacillus subtilis, Staphylococcus epidermidis, S. epidermidis SK, S. pseudintermedius and a yeast, Aureobasidium pullulans, were sonicated at 20 kHz (0.04–0.85 W/ml, 5–60 min) or 850 kHz (0.04–0.25 W/ml, 2.5– 60 min) by batch sonication equipment under controlled temperatures (< 30°C). The bacteria and yeast were suspended in aqueous solutions and skim milks. The relationship between the effects of inactivation and physico-chemical properties of bacteria at different growth phases was investigated by low-frequency ultrasound. Lethal damage of E. aerogenes and B. subtilis was caused by ultrasonication, while Staphylococcus spp. were not affected markedly. This was mainly due to the protection of the capsule layer (extracellular polysaccharides) outside this gram-positive cell against mechanical damage induced by ultrasound cavitation. A theoretical model based on shear forces produced by cavitation was established. This model was validated on further low-frequency sonication experiments performed on E. aerogenes and A. pullulans. E. aerogenes, B. subtilis and S. epidermidis were also treated by high-frequency ultrasound. It was found that all the bacteria were inactivated noticeably due to the generation of hydroxyl radicals and hydrogen peroxide during sonication. A post-ultrasonication effect was identified where bacteria were inactivated even after sonication was completed. E. aerogenes was also sonicated in skim milk with varying protein concentrations, and the effects of ultrasonication on the physic-chemical properties of milk were considered. The bacteria in milk were markedly inactivated in the low-frequency system, while they were not affected by the high-frequency one. It was also found that ultrasonication, under the conditions used in this thesis, did not notably influence the milk proteins or the structure and size of the casein micelles. However, it partially denatured whey proteins and reduced the size of the residual fat globules. Further, the proteins at the surface of the casein micelles might be cleaved.