Impact of ultrasound on dairy spoilage microbes and milk components

Abstract

Numerous reports in the literature suggest pasteurisation failures in the dairy industry as a possible cause for an end product with a poor quality. Ultrasonication offers the dairy industry a non-thermal alternative to pasteurisation. The aim of this study was to evaluate the use of ultrasonication as an alternative to heat pasteurisation. Ultrasound was found to eliminate spoilage and potential pathogens to zero or to levels acceptable by South African and British milk legislation, even when initial inoculum loads of 5× higher than permitted were present before treatment. Viable cell counts of E. coli were reduced by 100% after 10.0 min of ultrasonication. The data obtained also showed that viable counts of Pseudomonas fluorescens were reduced by 100% after 6.0 min and Listeria monocytogenes was reduced by 99% after 10.0 min. An infra-red based apparatus was used to analyse raw and pasteurised milk after an ultrasonic treatment. Ultrasonication did not lead to decreases in the protein or lactose content of both raw and pasteurised milk. Kjeldahl nitrogen determinations confirmed that ultrasonication had no detrimental effect on the total protein or casein content of pasteurised milk. This study indicated that ultrasonication lead to an increase in the fat concentration. This was explained by the larger surface area of the fat globules after ultrasonication, which led to an increase in light scattering as observed by the MilkoScan. Alkaline phosphatase and lactoperoxidase activities were also investigated as potential indicators of an effective ultrasonic treatment. Ultrasonication was, however, found to be ineffective in deactivating both enzymes used regularly by the dairy industry as indicators of effective thermal processes.