Effect of prolonged storage on microbiological quality of raw milk

The changing of milk microbiota composition in collection tank due to two-day collection was investigated. Each out of 10 independent cycles of every two-day collection was sampled 4 times and microbiologically examined. PCA medium supplemented with skim milk powder was used to determine the total counts of mesophilic aerobic bacteria, proteolytic bacteria, psychrotrophic and proteolytic psychrotrophic bacteria, respectively. MRS and M17 media were used to determine the counts of lactobacilli and cocci (lactococci and enterococci), respectively. As expected the counts of microbial groups increased but particularly the ratio between the groups of microorganisms was changed. The most evident shift in composition of milk microbiota was detected for psychrotrophic, particularly proteolytic psychrotrophic bacteria that are of particular concern to the dairy industry. Proteolytic psychrotrophs are notorious for their proteolytic activities which adversely influence milk proteins. Moreover, secreted proteolytic enzymes cannot be destroyed by heat treatment processes that are normally used in milk processing. The counts of microbial groups sharply increased on the second day of two-day collection. From selected bacterial consortia, the presence of the individual group of microbes was also PCR examined. Results indicate that the microbiological quality of milk in a two-day collection system is remarkably lower in comparison with daily collection system