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Why Did My Milk Turn Sour?

Published in Milk Spoilage 2 mins read

Your milk turned sour because naturally occurring bacteria, primarily Lactobacillus, converted the lactose (milk sugar) into lactic acid. This process lowers the milk's pH, resulting in the characteristic sour taste and smell.

Understanding Milk Spoilage

Milk, even when refrigerated, isn't sterile. It contains various microorganisms. When these bacteria, like Lactobacillus, multiply, they metabolize the lactose, producing lactic acid as a byproduct. The increased lactic acid concentration is what causes the sour taste. The speed of this process depends on several factors:

Factors Affecting Milk Souring:

  • Temperature: Warmer temperatures accelerate bacterial growth, leading to faster souring. Keeping milk refrigerated at or below 40°F (4°C) is crucial to slow down this process. As noted in various sources, temperature significantly impacts souring; the higher the temperature, the faster the milk sours.
  • Container cleanliness: Contamination from unclean containers can introduce additional bacteria, speeding up spoilage.
  • Storage duration: Even refrigerated milk will eventually spoil, as bacteria will continue to multiply over time. The "best by" or "expiration" date is an indicator, but the milk might still be safe for consumption for a short time after. However, if the milk smells or tastes off, discard it. One source mentions that milk doesn't necessarily "go bad" at midnight on the expiration date, highlighting that it's a guideline, not a hard cutoff.
  • Exposure to saliva: Avoid drinking directly from the carton; saliva introduces additional enzymes and bacteria that can accelerate souring, as mentioned in a Reddit post.

Some sources mention that even sealed milk can spoil due to the presence of non-sterile bacteria already present in the milk at the time of packaging. The speed at which this happens will vary based on the factors mentioned above. Soy milk, and other types of milk, also sour following the same principles.

Preventing Milk from Souring

  • Refrigerate promptly: Place milk in the refrigerator immediately after purchase.
  • Maintain proper temperature: Ensure your refrigerator is maintaining the correct temperature (40°F or 4°C or lower).
  • Cleanliness is key: Wash your hands before handling milk and use clean containers.
  • Avoid cross-contamination: Don't let other foods drip or contaminate the milk.
  • Check expiration dates: While not foolproof, using milk before the expiration date reduces the risk of spoilage.