Why does milk sour?

Milk sours due to the fermentation of lactose, the natural sugar found in milk, by lactic acid bacteria (LAB). These bacteria are commonly present in raw milk and can also enter milk during the milking process or through improper storage.

When LAB come into contact with lactose, they convert it into lactic acid. This process produces a sour taste and a thicker consistency in milk. It also lowers the pH of milk, making it more acidic.

Factors that contribute to the souring of milk include:

1. Temperature: LAB grow and multiply rapidly at warm temperatures. Milk left at room temperature for several hours is more likely to sour compared to milk that is refrigerated.

2. Storage Time: The longer milk is stored, the higher the chances of LAB growth and fermentation. Milk should be consumed or refrigerated as soon as possible after opening.

3. Initial Bacterial Load: The initial number of LAB present in milk can affect the rate at which it sours. Milk from cows with mastitis or other infections may have higher levels of bacteria and sour more quickly.

4. Packaging: Milk packaged in containers that allow air to enter is more prone to contamination with LAB compared to airtight packaging.

5. Pasteurization: Pasteurization is a heat treatment process that kills most bacteria, including LAB, in milk. Pasteurized milk has a longer shelf life and is less likely to sour quickly.

To prevent milk from souring, refrigeration is crucial. Keeping milk below 4°C (39°F) slows down the growth of LAB and extends the milk's shelf life. Additionally, proper hygiene practices during milking, storage, and handling can help reduce the risk of LAB contamination.