No Crying Over Spoiled Milk

Question: What does milk look like when it spoils?

Experiment Overview

In this experiment, you will be comparing several types of milk to see how they change as they spoil over time. White milk comes in several varieties, including skim, 2%, and whole. You can also buy chocolate milk and make milk from a dry mix. You will prepare several jars, each with its own kind of milk, and then you will observe them each day for at least one week to see how they change. It will be important that you label the jars properly so that you can easily observe and record the changes you see.

Science Concept

Milk is a complex combination of water, nutrients such as calcium, and … bacteria. The bacteria that live in milk are not usually harmful at all. In fact, you can typically find these bacteria living in cheese, yogurt, and other dairy products. However, it's important that these bacteria be kept at a certain level. If there are too many of them, the milk can spoil and then it becomes foul-tasting and possibly harmful. Refrigeration helps keep bacteria populations from growing. Also, boiling milk kills many of the bacteria it contains, which helps keep the bacteria population down. But if you simply leave milk out at room temperature, the bacteria will multiply rapidly and soon the milk will spoil.



  • Several small sealable containers, such as baby food jars, with lids

  • ½ cup of the following types of milk (or others if you have access to them):

    • -Skim milk

    • -2% milk

    • -Whole milk

    • -Chocolate (or other flavor) milk

    • -Buttermilk

    • -Milk made from a dry mix

    • -White milk that has been boiled for at least 5 minutes

    • Small strips of paper (to be used as labels)

    • Tape

    • Marking pen

    • Adult helper


1. Prepare each of the jars with its own type of milk.

2. Use refrigerated milk in all cases except for the one made from a mix and the one you boil.

3. Ask your adult helper to boil the white milk you chose for at least five minutes.

4. Write a label on a strip of paper for each type of milk you are testing and attach it to the proper jar with tape.

5. Each day for one week, observe each jar. Make a note of any changes you see in the milk, and any signs that the milk is beginning to spoil.

6. Optional: If you wish, each time you make an observation, you may open the jars briefly to see if the milk inside has started to smell. Be careful, though. This step may produce some very strong and unpleasant odors, and is best done outdoors. Be sure to seal the jars firmly after this step.

Questions for the Scientist

1. What sort of changes did you notice in the milk?

2. Which type of milk changed the most?

3. Did any of the types of milk start to form into solid pieces? Were you able to identify anything these types of milk had in common?

4. Which of the types of milk smelled the strongest as they began to spoil?

5. Did the boiled milk experience the same kinds of changes that its refrigerated counterpart experienced?

6. How long do you think milk should be allowed to sit out on the counter before being placed back into the refrigerator?

7. Why do you think people store food in the refrigerator?

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