Electron microscopic analysis of dairy microbes inactivated by ultrasound

Ultrasonication is a non-thermal method of food preservation that has the advantage of inactivating microbes in food without causing the common side-effects associated with conventional heat treatments, such as nutrient and flavour loss. The aim of this study was to evaluate the use of ultrasound as an alternative to heat pasteurisation and to assess cell damage using transmission electron microscopy (TEM). Three spoilage microbes, previously isolated from pasteurised milk, were used as "test" microbes. Saline solution (SSS) and UHT milk were used as suspension media and were inoculated with exponential growth phase "test" microbes at a microbial concentration of 1 x 10(4) cfu ml(-1). The samples were subjected to power ultrasound (20 kHz, 750 W), at 100%/124 microm wave amplitude for different time intervals. Both Escherichia coli and Saccharomyces cerevisiae were reduced by >99% (for both suspension media) after ultrasonication and Lactobacillus acidophilus was reduced by 72% and 84% in SSS and milk, respectively. Transmission electron microscope micrographs showed that ultrasonication inflicts extensive microbicidal/microbistatic external and internal damage on all three "test" microbes. In E. coli, sonication-induced emulsification caused the formation of unique minute lipopolysaccharide vesicles from the fragmenting cell envelope.