pH measures how acidic or alkaline a substance is on a scale from 0 to 14. A pH of 7 is neutral; values below 7 are acidic, and above 7 are alkaline. Fresh cow’s milk typically has a pH between 6.4 and 6.8, making it slightly acidic but close to neutral.
The Typical pH of Milk
The stability of milk’s pH is attributed to its natural buffering capacity, allowing it to resist significant changes in acidity or alkalinity.
Milk contains several components that act as buffers. Casein proteins are major contributors to this system. Other buffering agents include whey proteins, phosphates, citrates, and dissolved carbon dioxide. These compounds absorb or release hydrogen ions, preventing rapid pH fluctuations.
Factors That Change Milk pH
Milk’s pH can shift due to several internal and external influences. A common cause is the action of lactic acid bacteria (LAB) during spoilage. These bacteria ferment lactose, the natural sugar in milk, converting it into lactic acid, which causes a decrease in pH and leads to the milk souring and curdling.
Mastitis, an infection of a cow’s udder, can also alter milk pH. Milk from cows with mastitis tends to have a higher, more alkaline pH, sometimes reaching 7.5 in severe cases. This pH increase occurs because inflammation in the udder changes the permeability of mammary gland cell walls, allowing components from the cow’s blood serum, such as sodium, potassium, citrates, and bicarbonates, to leak into the milk. These blood components are slightly alkaline, increasing the milk’s overall pH.
Processing methods, such as heat treatments, can also subtly impact milk’s pH. Ultra-high temperature (UHT) processing, for example, may cause a slight decrease in pH. Additionally, deliberate or accidental adulteration of milk with certain substances can significantly alter its pH. The addition of neutralizers like sodium bicarbonate, often used to mask spoilage or extend shelf life, can increase milk’s pH. Conversely, adding water to milk can cause its pH to shift closer to neutral.
Why Milk pH Matters
Monitoring milk pH helps assess its quality and freshness. A decline in pH below 6.7 signals bacterial degradation and the onset of spoilage, indicating the milk is no longer safe for consumption. This makes pH a reliable indicator for consumers and producers alike.
The pH of milk is also fundamental in the manufacturing of various dairy products. In cheesemaking, a controlled reduction in pH is necessary for the coagulation of milk proteins and the expulsion of whey, which influences the final texture and flavor of the cheese. Different cheese varieties require specific pH ranges for proper development. Similarly, in yogurt production, lactic acid bacteria lower the milk’s pH, contributing to its characteristic tangy taste and thick consistency.
Monitoring pH is a standard practice in the dairy industry’s quality control measures. It helps in detecting issues such as early spoilage, mastitis, or potential contamination, ensuring that products meet safety standards. Extreme deviations in pH can also affect the stability of certain nutrients in milk, impacting its overall nutritional integrity.