Why is My Milk Sour?
Your milk is sour because bacteria have multiplied, fermenting the lactose (milk sugar) into lactic acid. This process is natural and happens more quickly at warmer temperatures and once the milk's airtight seal is broken.
What Causes Sour Milk?
Souring is primarily caused by the growth of lactic acid bacteria. These bacteria are naturally present in milk, even pasteurized milk. These bacteria are typically harmless, and while they can create a sour taste and smell, they usually aren't harmful to consume unless significant spoilage has occurred resulting in further bacterial growth and the production of other substances. Raw milk, however, will sour more quickly and may harbor more harmful bacteria (reference: Spoiled milk usually refers to pasteurized milk that has gone bad, while sour milk may refer to raw milk that has begun to ferment). The speed of souring depends on several factors:
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Temperature: Warmer temperatures accelerate bacterial growth. Storing milk at or below 40°F (4°C) significantly slows down the process. (reference: Putrid flavours are the result of bacterial contamination, storage temperature above 40°F, and age. Spoilage of the milk is by bacterial action)
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Exposure to Air: Once the carton or container is opened, exposure to air introduces more bacteria. It is important to ensure appropriate storage and resealing immediately after use.
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Contamination: Improper handling or storage can introduce unwanted bacteria, accelerating the souring process.
Is Sour Milk Harmful?
If the milk only smells or tastes slightly sour, it might still be safe to consume. However, if there is a strong unpleasant smell, a significantly thick consistency, or noticeable mold, it's best to discard it. (reference: If the milk does not have any off-putting smell or taste, you can still use it. If milk has gone a little sour, it is still safe for consumption)
Other Factors Affecting Milk Taste
Sometimes, milk may taste "off" even if it's not sour. This can be caused by several factors:
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Lipase: This enzyme naturally occurs in milk, both human and cow's milk. It continues to work even when frozen, leading to a change in taste and smell. This may cause it to taste and smell soapy or sour, but this is usually not harmful. (references: One theory is that lipases, which are enzymes that naturally occur in human milk, continue working even when milk is frozen; Some moms notice that after defrosting, their milk smells unpleasant – soapy or even sour. This is normal! Breast milk contains lipase, an enzyme that is)
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Diet: In the case of breast milk, maternal diet can affect the taste and smell. Some additives can cause a sour taste or smell. (reference: Sometimes your milk can smell and taste sour. This can happen very quickly if you have certain additives in your diet — like polyunsaturated and)
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Storage: Improper storage, even if the milk is not technically spoiled, can impact flavor.
Preventing Sour Milk
- Store milk at the coldest part of your refrigerator.
- After opening, seal the container tightly.
- Use milk within its best-by date.
- Avoid repeatedly pouring milk in and out of the container.
