When we talk about food and milk spoiling or "going bad," we typically mean that the food has become contaminated with bacteria or some other microorganism. To delay bacterial contamination of milk, milk is generally heated up to a high temperature to kill potentially harmful bacteria. Some types of milk, known as ultra-high temperature milk, is heated to at least 280 degrees Fahrenheit and can be stored without refrigeration until it is opened. Produce can also be protected from spoilage by frequent washing or UV radiation.
To properly set up an experiment testing what affects spoilage of food and milk, children will need to clearly identify three different aspects of the experiment: the variable, which is the thing that will be changed to test its effect on spoilage; the control conditions, which will be used as a point of comparison; and the hypothesis, which is the child's prediction. For example, for an experiment testing the effects of temperature on how fast milk spoils, the variable would be temperature, the control group could be milk that is stored in a refrigerator and the hypothesis could be that higher temperatures will speed spoilage.
The simplest experiment to do with milk spoilage is to test the effects of storing it at different temperatures. Although milk is typically heated to keep it from spoiling, bacteria will grow more quickly at warm temperatures. Store the milk at room temperature, under a heat lamp, in a refrigerator and in a freezer as the different storage conditions. Bacteria growth can be measured by placing small amounts of the milk on plastic dishes with agar or some other growth medium to see how much bacteria each milk sample has. Pasteurized vs. unpasteurized milk can also be tested.
Similar experiments can be done testing the effects of temperature on the spoilage of fruit. Use many different types of fruit, such as blueberries, apples, bananas and strawberries and have your children store them at different temperatures. Have the children store the fruit in different packaging, such as wax paper, saran wrap, aluminum foil and a paper bag and see how long it takes the fruit to begin to rot.