Microbial contaminants of milk processed by high-temperature short-time pasteurization

Veterinarni Medicina 44, 1999, 301-307

The aim of the study was to determine whether the high temperature, short-time (next HTST) pasteurization method can be applied in the production of market milk and to assess the effects of this method on milk quality. Samples of raw and pasteurized milk were collected repeatedly from four pasteurization facilities in three dairies using the HTST method of heat processing for the production of curds and cheeses. One part of the samples was processed immediately after their delivery to the laboratory and the other was examined after 5 days of storage at +6 degrees C. The examinations included the determination of total count, psychrophilic and coliform bacteria, mould, yeast, Bacillus cereus and Staphylococcus aureus counts and the detection of Salmonella sp., Listeria monocytogenes, Escherichia coli, and Escherichia coli serotype O157 : H7. Standard (ISO) methods were used. The rate of bacterial contamination of raw milk intended for HTST pasteurization, expressed in terms of total and coliform bacteria counts, is shown in Tab. IA. It is evident that total bacteria counts considerably exceeded the limits laid down in the EC Regulations No. 46/92 and in recommendations of the Czech Standard CSN 57 0529. Rather surprising were also the unacceptably high counts of coliform bacteria. The rate of contamination of raw milk by psychrophilic bacteria, moulds and yeasts is shown in Tab. IB. The counts of psychrophilic bacteria exceeded the recommendations of Czech Standard (CSN 57 0529) several times. The increased counts of mesophilic aerobic, facultatively anaerobic and psychrophilic bacteria in raw milk tested immediately after withdrawal indicate an unsatisfactory sanitation regime and varying hygienic conditions upon milk collection transport and storage in dairies. No limits for mould and yeast counts have been laid down and therefore the respective data are only illustrative. Tab. IC shows the contamination of raw milk intended for pasteurization by the major causative agents of foodborne infections. Of great importance is particularly the contamination by Escherichia coli including the serotype O157 : H7. Salmonellae were undetectable in all the samples under examination The contamination by the other pathogenic agents cannot be regarded as dangerous. Data on bacterial contamination of milk after the mild pasteurization is shown in Tabs. IIA-IIC. Unacceptable psychrophilic and coliform bacteria counts were found in 14 and 20% of the samples respectively. As can be seen in Tab. IIC, the unacceptable samples included also those contaminated by Escherichia coli incl. the serotype O157 : H7. Owing to the bacterial contamination, the HTST pasteurization is not suitable for the production of market milk. Tab. IIA also shows marked differences among the dairies and individual samplings indicating the possibility to reduce the contamination rate by observation of technological rules during the transport, take-over and handling of milk prior to heat processing. Considering the high rate of bacterial contamination of raw milk immediately before pasteurization, the HTST method of pasteurization cannot be recommended for the production of market milk in the Czech Republic. A high rate of contamination of raw milk intended for the HTST method of pasteurization was found, although, in general, the quality of milk purchased from farmers in the Czech Republic is very good. The HTST method of pasteurization would increase the hazard of foodborne infections and intoxications, particularly those for which Listeria monocytogenes and strains of serotype O157 : H7 Escherichia coli are responsible. Shelf life of HTST pasteurized milk would be very short and its sensoric properties unacceptable. Therefore, the HTST method of pasteurization is not currently used in the production of market milk in the Czech Republic

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