Date of Award:


Document Type:


Degree Name:

Master of Science (MS)


Nutrition, Dietetics, and Food Sciences

Committee Chair(s)

Donald J. McMahon


Donald J. McMahon


Almut H. Vollmer


Craig J. Oberg


This project was funded by the Western Dairy Center to understand how long a milk pasteurizer can be operated before increases in bacterial numbers are observed in the pasteurized milk. While pasteurization kills pathogenic bacteria there are some non-pathogenic bacteria that can survive and have the ability to become attached to the surfaces in the cooling sections of the pasteurizer. Some bacteria can also produce spores that survive pasteurization even if the bacterial cells are killed. Temperatures in the cooling section remain in a range suitable for growth of these heat-tolerant bacteria and can allow germination of bacterial spores. While this is not a health issue, it can affect the quality of the milk and other dairy foods if spoilage bacterial numbers become high.

We constructed a laboratory-scale heat exchanger for pasteurizing milk and monitored the number and type of bacteria contained in the milk. The system was operated for 18 hours with a continuous flow of milk being heated (to 72°C (161°F) for 15 seconds) followed by cooling. Sample of milk were collected every hour and then analysed for the number of bacteria and the number of bacterial spores.

Bacteria that would have survived pasteurization (thermophilic bacteria) of the milk stayed at the baseline level for the first 7 hours of processing. There was a 10 to 20 fold higher level of bacteria in the milk after 8 hours processing, followed by another 10 fold increase after 14 hours of processing. Operating a pasteurizer for extended times will lead to increased bacterial load in pasteurized milk which can cause quality problems.