The pH scale measures a substance’s acidity or alkalinity, ranging from 0 to 14. A pH of 7 is neutral; below 7 is acidic, and above 7 is alkaline. Milk, like many other food products, has a specific pH level, indicating its chemical balance.
The pH of Fresh Milk
Fresh cow’s milk typically has a pH ranging from 6.4 to 6.8, making it slightly acidic but close to neutral. This pH results from milk’s complex composition, including lactose, proteins, and minerals. Casein proteins, whey proteins, and dissolved minerals like phosphates and citrates are primary contributors to this acidity. Casein, which makes up about 80% of milk’s protein, acts as a buffering system. These proteins, along with minerals like calcium phosphate and citrates, help maintain milk’s pH within its normal range by resisting significant changes.
Factors Influencing Milk pH
The most common reason for a change in milk’s pH is microbial activity. Lactic acid bacteria (LAB), naturally present in milk, ferment lactose into lactic acid. This process increases acidity and causes the pH to decrease, leading to the sour taste of spoiled milk.
Processing methods also influence milk pH. Pasteurization, which heats milk to destroy harmful microorganisms, slows the pH drop by reducing bacterial load. Ultra-high temperature (UHT) treatment can cause a slight pH decrease over time due to chemical reactions. Beyond microbial activity and processing, factors like the cow’s diet, breed, lactation stage, and health conditions such as mastitis can also cause minor pH variations.
Why Milk pH Matters
Monitoring milk’s pH level is a standard practice in the dairy industry, serving as a reliable indicator of its quality and freshness. A significant drop in pH can signal spoilage, often before noticeable changes in taste or smell. This allows producers to ensure high-quality milk for consumption or further processing.
The pH level is also important for food safety, as certain pH ranges can either inhibit or promote the growth of harmful bacteria. For example, the acidity created by lactic acid bacteria helps preserve milk by inhibiting spoilage microorganisms.
pH plays a role in various dairy processing applications. In cheese making, controlled acidification, driven by bacteria converting lactose to lactic acid, is essential for curd formation, texture development, and flavor. Milk’s pH typically drops from around 6.7 to a range between 4.6 and 5.3, depending on the specific cheese variety.
Similarly, in yogurt production, lactic acid bacteria ferment lactose, reducing the pH to between 4.0 and 4.6. This causes milk proteins to coagulate and create the characteristic gel-like structure and tangy flavor. Precisely monitoring pH during fermentation helps determine the optimal time to halt the process, ensuring consistent product quality.