Souring of the Milk
Fresh milk right from the cow has a pH value that is almost neutral. Gradually, however, milk becomes sour, even when it is stored in a refrigerator. In this investigation we will see if we can tell how many days old the milk is simply by measuring its pH. We will also investigate the microorganisms at work in its change in acidity.
Natural milk, ‘straight from the cow," has a pH close to neutral. In this project, we will first measure the acidity of fresh, just purchased milk, and then check how the pH value of the milk changes every day over one week.
Together we may collect enough data to estimate the age of milk by knowing its acidity.
While doing this project, we can learn about microorganisms that make lactic acid. These are responsible for the sour taste and acid pH-values of the sour milk. The microorganisms need energy, and they get most of it from milk sugar, "lactose." When milk bacteria use lactose as an energy source, they change it into lactic acid, which increases the milk's acidity. The acidity of the milk, therefore, is an indicator of its freshness.
(Extension: pH of lactose-free milk. Some people are "lactose-intolerant" and need medicine to digest whole milk, or they can buy milk with the lactose already removed. Would you expect such "lactose-free milk" to be more or less easily made acid?)