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.

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-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. Résumé -Impact des ultrasons sur les microbes d'altération du lait et sur ses composants.De nombreuses publications suggèrent que dans l'industrie laitière, la mauvaise qualité du produit fini peut provenir de défaillances de la pasteurisation. Le but de cette étude a été d'évaluer l'utilisation de l'ultrasonication comme alternative au traitement thermique. Les résultats ont montré que les ultrasons éliminaient totalement, ou à des niveaux acceptables par la législation sud-africaine et anglaise, la flore d'altération et les pathogènes potentiels, et ce même quand les charges d'inoculum initiales avant traitement étaient 5 fois supérieures à celles autorisées. La réduction du nombre de cellules viables après ultrasonication était de 100 % après 10.0 min pour E. coli, de 100 % après 6.0 min pour Pseudomonas fluorescens et de 99 % après 10.0 min pour Listeria monocytogenes. L'analyse par infrarouge du lait cru et du lait pasteurisé après traitement aux ultrasons ne montrait aucune diminution de la teneur en protéines et en lactose dans ces deux laits. La quantification de l'azote par Kjeldahl confirmait l'absence d'effet préjudiciable de l'ultrasonication sur les teneurs en protéines totales ou en caséines du lait pasteurisé. Cette étude montre que l'ultrasonication conduit à ...

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