Is Milk an Acid or Base?

Milk is slightly acidic and becomes more acidic over time.
Milk is slightly acidic and becomes more acidic over time. Fresh milk has a pH around 6.8, but the value drops to 6.4 as Lactobacillus changes lactose into lactic acid.

Is milk an acid or a base or is it neutral? Many factors affect the pH value of milk, but it generally ranges between 6.4 and 6.9. This makes it slightly acidic. However, these value are close to a neutral pH of 7, so many sources say milk is neutral.

On average, the pH of milk is about 6.5. This is slightly acidic, but close to neutral.

Milk Becomes More Acidic

Fresh milk has the highest pH, but milk becomes more acidic over time. Milk contains the sugar lactose. Harmless Lactobacillus bacteria ferment lactose to use as an energy source, converting it into lactic acid. As the bacteria produce more lactic acid, milk becomes more acidic and tastes sour. Refrigeration and pasteurization slow the process, but even sterilized milk eventually sours after it has been exposed to air.

Milk pH According to Species

While all mammals produce slightly acidic milk, the pH value depends on the species. Sheep milk is quite a bit more acidic than cow milk. Human milk is nearly neutral, but with a pH slightly lower than that of plasma.

Milk pH by species (note: Ahmed measured a lower value for cow milk than cited by other sources.)

Plant “milk” (e.g., soy milk, oat milk, almond milk) is quite different from true mammalian milk. However, plant-based milk is slightly acidic, too.

Factors That Affect Milk pH

Milk contains natural buffers, so it doesn’t readily change its pH, but several factors affect the level of acidity.

  • Species
  • Storage conditions: Warmer temperatures encourage Lactobacillus growth, causing pH to drop more quickly. Longer times between milking and use typically result in more acidic milk. Milk closer to its “use by” date is more acidic than a fresher product.
  • Processing: Milk pH varies slightly depending on whether it’s whole, skim, or evaporated. Evaporated milk is more acidic than regular whole or skim milk. Preservation methods also affect pH. For example ultrahigh temperature (UHT) processing kills more bacteria than pasteurization, so it takes longer for UHT-processed milk to turn more acidic.
  • Lactation stage: Colostrum (the first milk a female produces) is more acidic than regular milk. Human colostrum is more acidic than cow’s milk.
  • Animal health: The female’s health affects milk characteristics. Mastitis tends to raise milk pH. In some cases, cow’s milk may even be alkaline.