The pH scale measures how acidic or alkaline a substance is. This scale ranges from 0 to 14, with a pH of 7 indicating neutrality. Values below 7 signify acidity, while values above 7 denote alkalinity. Each whole pH unit represents a tenfold change in the substance’s acidity or alkalinity. Understanding pH is important for assessing the quality, freshness, and stability of various food products.
The pH of Fresh Milk
Fresh cow’s milk has a pH ranging from 6.4 to 6.8, indicating it is slightly acidic but close to neutral. This natural acidity comes from components such as casein and whey proteins, along with minerals like phosphates and citrates.
Variations in milk’s pH can occur. These are influenced by factors like the animal’s diet, breed, and stage of lactation. Processing methods and storage conditions also affect milk’s pH. Colostrum, the first milk produced, is more acidic, while milk from cows with an infection like mastitis is more alkaline.
How Milk’s pH Changes and Why It Matters
Milk’s pH level is not static; it changes due to bacterial activity and is also intentionally altered for the creation of dairy products. When milk spoils, it is primarily due to the action of lactic acid bacteria. These bacteria ferment lactose, the natural sugar in milk, converting it into lactic acid. This process leads to a decrease in the milk’s pH, making it increasingly acidic.
A drop in pH below 5 causes milk proteins, particularly casein, to coagulate. This coagulation results in the milk souring and forming curds. The increased acidity is a primary indicator of spoilage. Both temperature and storage duration significantly impact the rate at which these bacteria grow and produce acid, accelerating spoilage in warmer conditions.
Controlled changes in pH are important for producing various dairy items. In yogurt production, specific lactic acid bacteria are introduced to milk. These bacteria convert lactose into lactic acid, lowering the pH to an ideal range of 4.0 to 4.6. This controlled acidification gives yogurt its characteristic tangy flavor, helps achieve its creamy texture, and causes the milk proteins to coagulate, forming the gel-like structure.
Cheesemaking relies on controlled pH reduction to achieve desired textures and flavors. Lowering the pH, either through acid addition or bacterial fermentation, is important for the coagulation of milk proteins, which forms the cheese curd. The pH level during cheesemaking influences the curd’s firmness and elasticity, and also affects the activity of enzymes like rennet, which functions best around a pH of 6.0. The final pH of cheese can vary between 4.4 and 5.3, depending on the specific cheese variety, directly impacting its taste and texture development during aging.