Milk is generally considered slightly acidic or very close to neutral. Its precise acidity level is a significant factor in how it is processed and how long it stays fresh. Understanding whether milk is basic or acidic requires looking at the chemical scale used to measure acidity and the dynamic composition of the liquid itself.
Understanding the pH Scale
The pH scale measures the concentration of hydrogen ions in a water-based substance to define its acidity or alkalinity. This scale ranges from 0 to 14, with 7.0 representing a neutral substance, such as pure water.
A pH reading below 7.0 is considered acidic, with lower numbers indicating stronger acids. Conversely, a reading above 7.0 is considered basic or alkaline, with higher numbers indicating stronger bases. The scale is logarithmic, meaning each whole number change represents a tenfold difference in acidity or alkalinity.
The pH of Fresh Milk and Its Components
Fresh cow’s milk typically has a pH that falls within the narrow range of 6.5 to 6.7, making it slightly acidic. This slight acidity is inherent to the milk’s natural composition, coming primarily from two main organic components: casein and lactose.
Casein, the primary protein, contributes to this pH because its amino acid groups can release hydrogen ions. Other components, such as whey proteins and dissolved minerals like phosphates and citrates, also influence the overall acidity. These proteins and minerals function as buffering agents, meaning they resist changes to the pH when small amounts of acid or base are introduced. This helps maintain the milk’s stability during processing and storage.
Why Milk Becomes Acidic Over Time
The familiar souring of milk is a clear example of its pH dropping. This change results from a biological process called lactic acid fermentation. Milk naturally contains various microorganisms, including lactic acid bacteria, such as Lactobacillus species.
These bacteria use lactose, the natural sugar in milk, as their food source. As they consume the lactose, they metabolize it and produce lactic acid as a byproduct. The accumulation of lactic acid significantly lowers the milk’s pH, causing its characteristic sour taste and leading to the curdling of milk proteins. This decrease in pH indicates that the milk is no longer fresh. It is also the basis for preserving milk in fermented products like yogurt and cheese.