HEAT RESISTANCE OF Bacillus spp. SPORES ISOLATED FROM COW’S MILK AND FARM ENVIRONMENT

Jan‰tová B. , J . Luká‰ová: Heat Resistance of Bacillus spp. Spores Isolated from Cow’s Milk and Farm Environment. Acta Vet. Brno 2001, 70: 179-184. The aim of this study was to compare heat resistance of the spores of mesophile Bacillus spp. from raw milk (bulk tank milk and milk from individual cows) and farm environment (faeces, silage, litter, swabs from skin and udder) as related to their survival in heattreated dairy products. Fifty-eight strains of 9 species of Bacillus were tested for termoresistance within the temperature range of 95 135 °C and various exposure times. The highest termoresistance was found in B. licheniformis spores, which survived the temperature of 135 °C. Except for the spores B. coagulans and B. pumilus all remaining spores were able to germinate after heat treatment of up to 120 °C. The greatest reduction of the spore numbers was found in B. cereus. The results indicate that some Bacillus spores may survive in milk even after heat treatment. Bacillus spp., raw milk, faeces, silage, litter, swabs from skin and udder, spores, termoresistance Aerobic and facultative anaerobic spore-forming bacteria of the genus Bacillus present a serious problem in milk industry. Because of the heat resistance of spores and ability of vegetative cells to produce extra-cellular enzymes they may cause milk and milk product deterioration. Bacillus spp. are quite common in the agricultural environment and may contaminate milk from various sources both during the production, storage and processing. Raw milk is most frequently contaminated under conditions of inadequate udder hygiene, from soil, feed, dust and faeces (Chris t iansson et al. 1999). Species such as B. licheniformis, B. subtilis and B. cereus (Criel ly et al.1994; Páãová et al. 1996) are most commonly isolated from raw milk. B. licheniformis together with B. subtilis and B. pumilus belong to mesophiles, whereas B. cereus is rather a psychrophile able to grow in milk and milk products at cold-storage temperatures and even cause alimentary diseases (Chris t iansson 1992). It is known that the spores of bacilli survive pasteurisation and have suitable conditions for germination created in the pasteurised milk. B. cereus is a predominating micro-organism influencing the maintenance of pasteurised milk. It causes sweet curdling of milk and changes to milk odour and taste due to the production of the enzymes proteinase, lipase and phospholipase (Meer et al. 1991). Sterilisation and UHT milk treatment are the most effective means of spore destruction showing 99.99% success (Cox 1974). Heat resistance is an important character of Bacillus spp. spores. Appropriate temperature selection and duration of heating are crucial for the required microbial quality of the milk product, as with a linear temperature increase the time needed to achieve nearly complete abiosis becomes shorter exponentially. When determining the heat resistance and thus the sufficient duration of thermic treatment, it is useful to define the D value (decimal reduction time), i.e., a temperature value resulting in a decrease of the number of live microorganisms by one order (Kyzl ink 1980 ). ACTA VET. BRNO 2001, 70: 179–184 Address for correspondence: MVDr.Bohumíra Jan‰tová University of Veterinary and Pharmaceutical Sciences Brno Palackého1-3, 612 42 Brno, Czech Republic Phone: +420 5 4156 2607 E-mail: [email protected] http://www.vfu.cz/acta-vet/actavet.htm