Posted 12-4-02

Question: I am doing an experiment to see whether whole or skim milk goes sour quickest. I thought that whole milk would because of the high fat content. But my results showed that skim milk did. What could the reason be? I read that lactose turning into lactic acid is the reason for milk going sour. Lactose is sugar and I see that whole milk contains more sugar than skim milk. Could this be the reason? Close parental supervision is strongly encouraged for an experiment of this kind due to the possible growth of bacteria in spoiling milk

Reply 1 (from Peggy Tomasula, research leader of the USDA-ARS Dairy Processing and Products Research Unit, Wyndmoor, Penn.). "In responding, I'm going to assume that all milk samples were treated by the student in the same manner; were subjected to the same refrigeration temperatures; reached room temperature at the same time; were not stirred or handled in any way, etc. ; milk was poured into extremely clean glasses or beakers, and that the milk samples used were from the same processing plant. I [agree] with the student's observation. The skim milk soured a bit faster than the whole milk did because it contains slightly more lactose, providing slightly more food for the bacteria. Whole milk contains slightly less lactose. The student can look at the lactose content on the side of the container and see that skim milk has slightly more lactose than whole milk. I would also [suggest] that the student repeat the experiment."

Reply 2 (from Harold Farrell, USDA-ARS Dairy Processing and Products Research Unit). Please note: Sci4Kids emailed Dr. Farrell about this question, and in his reply, he asks for additional information that other students probably should consider before doing a sour milk experiment of their own.

He writes: "We need some information from the student before answering the "sour milk" question. We need... 1) a Definition: by "sour" do you mean curdled or smelling bad? 2) Temperature: Was the temperature for holding the milk refrigerator- or room- temperature? 3) Handling: did you pour the milk, or was the container covered? 4) Light: in the dark or out in the light?

If pasteurized milks from a store, did they have the same sell-by dates? Finally, a word of caution! If, by souring, you mean that the milk was curdled, and you can see curds and whey (a yellowish fluid), then spoilage has occurred. If you see gas bubbles, or holes in the curd caused by gas bubbles, then you must be careful because there are bacteria growing in the milk. Depending on how you handled the milk, some [of these bacteria] could be harmful.

We know that our hands are the chief source of germs in our environment because we are always touching things that have been handled by other people. As you can see, this type of experiment sounds simple, but milk is a good place for all bacteria to grow--some good and some bad. Always be careful with any food that is left out at room temperature for a long time and never eat it."

-- Peggy Tomasula, Harold Farrell, Dairy Processing and Products Research Unit, Wyndmoor, Pennsylvania.

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