Characterization of microorganisms associated with pasteurized milk

Growth of psychrotrophic bacteria in refrigerated milk can be detrimental to the shelf life of the product. Normally, the presence of these bacteria are a result of post-pasteurization contamination. In some cases however, the microorganisms which affect shelf life are those that survive pasteurization and grow during refrigerated storage. The objectives of this study were to determine the influence of producer handling of raw milk on the shelf life of the pasteurized product and characterize the microorganisms which survived pasteurization.

Raw milk from producers rated as having good or poor sanitation was transported to the University of Tennessee on ice. The samples were transferred aseptically into sterile bottles and pasteurized for 30 min at 62.8°C. The samples were stored at 7.2°C and sampled at the raw, pasteurized and stored stages. Pasteurized samples from poor quality raw milk producers had generally higher standard plate counts than samples from good producers, but no significant difference was detected. No correlations were detected between the shelf life of the pasteurized milk and characteristics of the raw milk from good and poor producers.

Bacterial isolates were selected from raw, pasteurized and stored sample plates from one random good and poor producer. All isolates were in two groups: gram positive sporeforming rods and gram negative non-sporeforming rods. These isolates were analyzed for their heat resistance and growth under refrigerated storage. Their heat and growth characteristics were compared to two common milk spoilage microorganisms, Pseudomonas fluorescens and Pseudomonas fragi. Some gram negative isolates were found to survive certain heat treatments better than the Pseudomonas strains. Most gram positive sporeformers survived heating treatments of 80°C to 90°C for 30 min, but they were not unusually heat resistant.

In growth testing, selected isolates were inoculated into sterile reconstituted skim milk and stored at 7.2°C for 15 days. Gram positive sporeforming isolates did not grow well and in some cases actually decreased in number over the 15 day incubation. In contrast, the gram negative isolates increased at the same rate as the P. fluorescens and P. fragi.

It was therefore concluded that it was possible for microorganisms to survive pasteurization and grow at a relatively rapid rate in the absence of post-pasteurization contamination.