milkQ&Apage

Posted 5-4-00

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I am doing my science fair project on milk spoiling. I know from what I have read that milk spoils because it has bacteria in it. But I have noticed that the milk goes through different stages as the spoiling goes on. First, it gets sort of solid like yoghurt, and then it separates into a solid and liquid, and it really smells bad! When I let it get really separated.. you can even take the solid part out and it is like plastic! I tried the experiment with whole milk, two percent and skim milk. The skim milk took the longest to spoil. What is going on here? Thanks.Before answering your question about milk, we first want to tell you something about it. Milk is a very good food for both people and bacteria.

From the time the milk leaves the cow till you drink it, it can become contaminated with many different kinds of bacteria--some good, but some bad. At the dairy, the milk is heated (pasteurized) to kill bacteria, and sealed. This kills most of the bacteria, but not all.

Photo of a carton of milk. When you open the milk at home, new bacteria from the air, from your hands, and from the glass you use can get into the milk. Normally this is OK, because you drink the milk right away. However, in your experiments, as you keep the milk, you are encouraging all kinds of bacteria to grow. You must be careful not to touch, drink or handle the samples. Depending on how you treat these samples, they may contain harmful bacteria. A good rule of food safety is that if it smells bad, do not eat or touch it. Please be careful; wash your hands and be careful with the spoiled milk, when you throw it out.

Now about what happens to the milk. As the bacteria grow in the milk, they eat some of the protein and some of the milk sugar. As they eat the sugar, the bacteria produce acid, and the acid causes the milk to clot.

A similar thing happens when you cut your finger; the blood clots. After you keep the milk clot for a while, the clot shrinks Photo of whole and sliced cheese. and a yellow fluid (whey) is released. You can make this happen more quickly by squeezing a little lemon juice (acid) into a small amount of milk. The curds are the white caseins, or milk proteins, shown here, and they are sticky (people once used them as glue). Remember the old nursery rhyme, "Miss Muffet sat on a tuffet and ate curds and whey..."?

However, in your case we do not know if good or harmful bacteria caused the milk to clot. The bacteria are also partially eating the fat and protein. This releases the bad smell. And since we don't know which kind of bacteria are at work, PLEASE be careful. You also made a very good observation that the milks containing fats clotted first. This is because the fat droplets are suspended in the milk by a process called homogenization, which is done at the dairy. These small droplets of fat attract the caseins (the sticky proteins) and make the clot form faster. There are no fat droplets in the skim milk so there are fewer attachment points and the skim milk clots more slowly.

-- Harold Farrell, supervisory research chemist and colleagues, Dairy Products Laboratory, Wyndmoor, Pennsylvania.

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