How Does Milk Sour?
Milk sours due to the action of bacteria already present in the milk, even after pasteurization. These bacteria ferment the lactose (milk sugar), converting it into lactic acid. This process is a chemical change, resulting in a sour taste and altered texture.
The souring of milk is primarily a result of bacterial fermentation. Specifically:
- Lactose Fermentation: Bacteria, such as Lactobacillus, consume the lactose in milk as an energy source. [Source: When milk turns sour, the bacteria are converting the lactose sugars in milk into energy, with lactic acid as a byproduct.]
- Lactic Acid Production: As a byproduct of this fermentation process, lactic acid is produced. The increase in lactic acid is what causes the milk to become sour. [Source: When milk turns sour, the bacteria are converting the lactose sugars in milk into energy, with lactic acid as a byproduct.]
- Acidity Increase & Protein Change: The increased acidity alters the milk's pH, causing proteins to denature and coagulate. This leads to the separation of whey (liquid) from the curdled solids. [Source: The acidity of the milk increases, which in its turn causes the protein to fold and liquid to separate into whey and a viscous mass.]
- Sour Taste & Odor: The resulting lactic acid imparts the characteristic sour taste and often a slightly unpleasant odor to the soured milk. [Source: You know your milk is spoiled if it has an unpleasant odor or taste or changes...]
Factors Affecting Souring Rate
Several factors influence how quickly milk sours:
- Initial Bacterial Count: The higher the initial number of bacteria present, the faster the souring process.
- Temperature: Warmer temperatures accelerate bacterial growth and thus souring. Refrigeration slows down this process. [Source: It's true, eventually unopened milk will spoil. That is because the milk you buy in a grocery store is not sterile. It contains a certain number of harmless...]
- Type of Milk: Different types of milk (whole milk, skim milk, etc.) may sour at slightly different rates due to variations in composition.
Sour Milk vs. Cultured Dairy Products
It's important to differentiate between naturally soured milk (spoiled milk) and cultured dairy products like yogurt or buttermilk. Cultured products involve controlled fermentation using specific bacteria strains to create desirable flavors and textures. In contrast, spoiled milk is the result of uncontrolled bacterial growth. [Source: Sour or Soured milk is milk that either has acid (vinegar or lemon juice) added to it or has gone sour and is now fermented milk.]
Using Soured Milk
While spoiled milk should be discarded, slightly soured milk can sometimes be used in baking. The acidity can add depth of flavor and even act as a substitute for buttermilk in some recipes. [Source: Sour milk also adds complexity to a stovetop porridge with a blend of steel-cut and quick oats, and it works especially well in recipes that...]
