Impact of low-frequency high-power ultrasound on spoilage and potentially pathogenic dairy microbes

[摘要] Thermal pasteurisation failures in the dairy industry have often been found to causeend-products of poor quality and short shelf-life. Therefore, alternative methods toeliminate microbial contaminants in raw milk are being studied. Ultrasonication is onesuch non-thermal technology that could offer the dairy industry an alternative totraditional pasteurisation.The main objective of this dissertation was to evaluate the use of high-power lowfrequencyultrasound (20 kHz, 750 W, 124 μm) applied in batch mode to eliminate aselection of spoilage and potentially pathogenic microbes, commonly associated withmilk. These included Gram-positive and negative microbes, comprising of rods andcocci, an endospore-former, and a yeast (Escherichia coli, Bacillus cereus,Chryseobacterium meningosepticum, Lactobacillus acidophilus, Lactococcus lactis,Listeria monocytogenes, Micrococcus luteus, Pseudomonas fluorescens andSaccharomyces cerevisiae).Three strains of E. coli (1 x 106 cfu.ml-1) tested, viz. ATCC 11775, a wild strainfrom raw milk, and an O157:H7 strain from milk were sensitive to ultrasonication.Complete elimination of viable cells occurred within 10 min. Viable counts ofP. fluorescens were reduced by 100% within 6 min of ultrasonication andL. monocytogenes was reduced by 99.0% within 10 min. Lactococcus lactis wasreduced by 97.0% and M. luteus, B. cereus and C. meningosepticum by 88.0%, 87.0%and 85.0% respectively. Lactobacillus acidophilus showed the most resistance toultrasound with only 78.0% of viable cells being eliminated. Under similar conditions,S. cerevisiae was reduced by 99.7%. Microbial cell morphology, size and Gram statusdid not necessarily influence the efficacy of ultrasonication. Sterile saline solution andUHT milk were used as the suspension media, and the reputed protective effect of milkfat was not observed under the parameters used in this study. A higher wave amplitude(100%; 124 μm) was found to be more efficient in eliminating microbes than a lowerwave amplitude (50%; 62 μm). Pulsed-ultrasonication did not enhance the efficiency ofultrasonication indicating that standing waves were absent.Limited success was achieved by ultrasonication itself, and the long batchtreatment time (10 min or more) was found to be unrealistic for industrialimplementation. Hence the simultaneous application of ultrasound and heat (thermoultrasonication)was examined. Thermo-ultrasonication proved to be more effective than either an ultrasonic orheat treatment with all viable M. luteus cells being eliminated within 4 min (100%amplitude at 72°C). Similarly, to eliminate E. coli and Lb. acidophilus from milk, only 2min and 4 min thermo-ultrasonication was required, respectively. Bacillus cereusendospores remained resistant and after a 10 min thermo-ultrasonic treatment only78.04% were eliminated.During this investigation both extensive surface (SEM) and internal (TEM) celldamage caused by ultrasonication were observed in E. coli, Lb. acidophilus andS. cerevisiae. Hence ultrasonication physically/mechanically damages these microbialcells causing cell death/injury.Microbial proteins and DNA released from cells into the environment after anultrasonic treatment was measured and an increase in released microbial proteins andDNA was found to be indicative of a decrease in the number of viable cells, providingthat the initial cell concentration was high enough. It was, however, not possible tocorrelate the concentration of released microbial proteins and DNA with the exactnumber of viable cells eliminated, rendering it an ineffective quality indicator for theindustry.Ultrasonication had no statistically significant influence on the protein, fat andlactose content of both raw and pasteurised milk. The somatic cell count of raw andpasteurised milk was found to decrease after ultrasonication. Unlike with heating,activity of alkaline phosphatase and lactoperoxidase were not reduced byultrasonication. Hence neither enzyme can be used to indicate a successful ultrasonictreatment of milk.This study has demonstrated that ultrasonication offers a viable alternative topasteurisation as it is effective in eliminating microbes, and does not alter native milkcomponents. However, to attain a more effective killing, thermo-ultrasonication isrecommended for the treatment of milk to be used for the production of different dairyproducts.

[发布机构] Stellenbosch University