Origin and seasonal variation of bacterial contamination of milk
Degree GrantorUniversity of Canterbury
Degree NameMaster of Science
Type, origin and seasonal variation of psychrotrophic bacteria contaminating milk from a Christchurch milk-processing factory was investigated. Bacteria were monitored in bottled milk over ten days at 7°C. Bacillus licheniformis, B. subtilis, Pseudomonas fluorescens and Ps. putida were identified, with populations exceeding 5x105 CFU/ml after five days. Origin of the bacterial contamination was determined by isolating from milk at various points on the processing line including newly pasteurised, storage tank and pre-filler, as well as filler and bottled milk. Environmental isolations were made from chlorinated water, recycled glass bottles and factory swabs. Ps. fluorescens and Ps. putida were present in the milk immediately before entering bottles and Ps. fluorescens was also isolated from environmental swabs. Additionally, Bacillus spp were isolated and included B. circulans and B. cereus both of which were found in newly pasteurised milk. Seasonal variation in psychrotrophic bacterial populations were established by comparing bacteria isolated from milk at various sites throughout the milk process line in the summer with those found in winter. The most significant seasonal difference was seen in raw milk which contained high levels of Pseudomonas spp in the winter and smaller populations of Gram-positive cocci in the summer. For bottled milk, at the consumer expiry date, increased levels of Pseudomonas spp were seen in early summer, mid-autumn and late autumn, but decreased levels were seen in mid-spring.