The Effect of Different Preservation Techniques on Souring Milk

The problem of this experiment was to investigate how different ways of preserving milk would affect the pH of milk; whether to slow down the process of souring or not affect it at all.  The objective was to determine which preservation method: refrigeration (or untreated), pasteurization, or Ultra-High Temperature pasteurization (UHT) would slow the process of souring the most, thus allowing milk to have an extended time of usage.  The three different test groups of milk were left out to sour over a period of three days.  The potential of hydrogen (pH in other words) was tested before the milk was souring (to measure the extent of the souring) and when it was souring for all test groups to ensure the pH lowered at the same time.  One group (untreated) went directly into a refrigerator, one group (pasteurized) was heated to the average pasteurizing temperature, 145oF, for 20 minutes, and the final group (UHT pasteurized) was heated to 280oF for three seconds.  The heated samples were then immediately placed into a refrigerator and the pH of all test groups were tested after two days in the refrigerator.  Multiple two-Sample t-tests analyzed the pH of the milk groups and concluded the results.  The analysis showed that the UHT pasteurized milk had a significant difference in the souring time of milk (the pH dropped at a slower rate than in the two other groups).  Thus, heating the milk to 280oF when milk is close to the published expiration and souring date can prolong the expiration and allow the milk to have a further time of usage.

Research Done By:

Lance Henkel
Centerline High School

Christopher Mataczynski
Cousino High School