What is causes milk to sour?

The primary cause of milk turning sour is the presence of bacteria, particularly lactic acid bacteria (LAB), which are commonly found in raw milk or improperly handled pasteurized milk. These bacteria consume the lactose (milk sugar) and convert it into lactic acid through a process called fermentation. As the lactic acid accumulates, it causes the pH level of the milk to drop, resulting in a sour taste and eventually spoilage.

Key factors contributing to milk souring include:

1. Temperature: Warm temperatures provide an ideal environment for the growth and multiplication of bacteria. Leaving milk at room temperature for extended periods allows bacteria to thrive, leading to faster souring.

2. Contamination: Milk can become contaminated with bacteria during the milking process, from unclean milking equipment, or through contact with surfaces or containers that carry bacteria. Inadequate sanitization practices during milking, storage, and handling can contribute to milk souring.

3. Storage Conditions: Improper storage conditions can accelerate the growth of bacteria in milk. Leaving milk unrefrigerated or at temperatures above 40 degrees Fahrenheit (4 degrees Celsius) promotes bacterial growth and spoilage.

4. Pasteurization: Pasteurization, when done correctly, kills most bacteria in milk, including those responsible for souring. However, if milk is not pasteurized properly or if it is recontaminated after pasteurization, bacteria can still multiply and cause souring.

5. Exposure to Air: Contact with air can introduce spoilage-causing bacteria into milk. Exposing milk to the open air or leaving it unsealed after opening allows bacteria to enter and begin the process of fermentation.

6. Light Exposure: Exposure to light, particularly sunlight, can contribute to the degradation of milk components and increase its susceptibility to bacterial growth.

7. Packaging and Container Materials: Certain packaging materials, such as plastic containers or poorly sealed containers, may not provide an adequate barrier against bacteria or oxygen, allowing milk to spoil more quickly.

To prevent milk from souring, it is crucial to maintain proper hygiene practices during milking and handling, promptly refrigerate milk after purchase, and consume or refrigerate milk before the expiration date.