The two main groups of bacteria in milk are Lactic acids and Coliforms. Lactic acid is the natural bacteria present in milk and dairy products. Coliforms are the main reason for milk spoilage. Pasteurization kills almost all of the bacteria, but some of the bacteria that cause milk to spoil still remain. If these bacteria are given an environment that promotes growth, they will rapidly multiply.
When milk spoils it changes in consistency, appearance, and smell. Spoiled milk also undergoes a chemical change. As the milk spoils, the bacteria produce acid. It is the acid that causes the milk to clot. You can compare the acidity of the test milks by using indicator strips.
Easy to Moderate.
$3 (not including lamp).
20 minutes setup; about 10 minutes daily for 4 to 5 days.
When conducting experiments with microorganisms, treat them all as if they could cause disease. Do not touch the milk and, if you do, wash your hands thoroughly. Do not taste or ingest any of the milk. Be careful when working at the stove.
Examine your results and note the acidity level of the milk(s) that spoiled at the fastest rate. How did the control milk compare to the boiled milk? Compare the appearance of the milk at the warm environment to the cool environment. How did the spoiled milk's appearance change daily? When acid causes milk to curdle it forms solids called curds, and a liquid, called whey. Which of the test milks formed curds and whey? In an analysis of this experiment summarize what conclusions you can draw about the environment(s) that promote bacterial spoilage.
After you keep the milk clot for a while, the clot shrinks 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, and they are sticky (people once used them as glue). If you touch them, remember to wash your hands immediately.
Problem: After several days, the milk at room temperature appeared to have the same amount of spoilage as the milk in the refrigerator.
Possible cause: The room may be at a cool temperature and the bacteria could need longer to grow. Continue the experiment for several more days.
In this experiment you can change the variables in several ways. You can change the fat content of the milk by comparing skim milk, whole milk, 2 percent milk, and other types. You can add a substance to the milk, such as sugar or chocolate, that may alter the speed of bacteria growth. Another way to change the experiment is to vary how much light the milk is exposed to by leaving the same type of milk out in a bright and dark area. You could also alter the food substance by using different beverages or solid foods instead of milk.