Impact of processing temperatures on survival of microbial contaminants from pasteurised milk

[摘要] ENGLISH ABSTRACT: Milk has been identified as having the potential of being a carrier of humanpathogens, and it is thus essential to eliminate or reduce the likelihood of milk bornecontamination. This problem of milk contamination is generally solved by the processof pasteurisation which is achieved by heating the raw material for a sufficientperiod of time to destroy any pathogenic and spoilage bacteria which may be presentat a temperature of below 100°C. Presently, there are two basic methods ofpasteurisation in use in the dairy industry, the LTLT and the HTST methods, wherethe applied heat treatment is considered sufficient to ensure public safety andadequate keeping quality. In addition to these, there is another method, the potpasteurisation, to be found in Southern Africa that was designed to eliminatepotential pathogenic and spoilage bacteria present in raw milk. As far as it is knownno thermal studies have been done on the pot pasteurisation method. Theobjectives of this study were to determine the impact of different milk pasteurisationtemperature and time combinations on the survival of selected microbes. Theaccuracy of the pot pasteurisation method and how it differs from the otherpasteurisation methods was also determined using the same selected microbes.The six selected microbes were thermally inactivated by using the LTLT,HTST and the pot pasteurisation methods at low and high inoculum levels of 104and 106 cfu.ml-1. The thermal death curves were constructed for each selectedspecies. The selected microbes included the strains Bacillus cereus (S4),Chryseobacterium meningosepticum (S5), Pseudomonas putida (S6), Acinetobacterbaumannii (C3), Escherichia coli (58) and Candida lipolytica (G1). Survivors wereenumerated after heating for 0, 5, 10, 15, 20, 25, 30, 35 and 40 min for both theLTLT and HTST pasteurisation methods and after heating for 0, 10, 20 and 30 minfor the pot pasteurisation method.The results from this study showed that with the exception of the B. cereusstrain, the other selected microbes at both high and low concentration levels did notsurvive the LTLT or the HTST pasteurisation methods. It was found that for all theorganisms used in this study, there was a rapid initial death rate just before therequired pasteurisation temperatures of 63°, 72° and 90°C were reached, during thecome-up period. In contrast, the results from the pot pasteuriser showed that theB. cereus (S4), Chr. meningosepticum (S5), P. putida (S6), A. baumannii (C3) andE. coli (58) strains survived the pasteurisation conditions applied.From these results it was thus concluded that the pot pasteuriser under theconditions evaluated in this study, did not pasteurise effectively. Therefore, it isrecommended that the manufacturer improves the heating quality of the potpasteuriser. As it was found that only the B. cereus (S4) strain survived all thedifferent pasteurisation methods, future research needs to be done to determine atwhich temperature this heat resistant bacterial strain will be destroyed. This is veryimportant because there is a need to destroy all the spoilage microorganisms thatcan lead to the deterioration of food products.

[发布机构] Stellenbosch University