The pH scale measures acidity or alkalinity from 0 (most acidic) to 14 (most alkaline), with 7.0 being neutral. Fresh cow’s milk naturally exists in a slightly acidic or near-neutral state, typically falling between pH 6.5 and 6.7. Monitoring this narrow pH range is a simple yet powerful method for assessing milk quality, stability, and suitability for processing.
Understanding Milk’s Baseline pH
The baseline pH of fresh milk is maintained by its buffering capacity, which is the ability to resist significant changes in acidity or alkalinity. This stability is largely due to components dissolved within the liquid. Casein proteins, which make up about 80% of the total protein, are the primary contributors to this buffering system. They accept or donate hydrogen ions, effectively neutralizing small amounts of added acid or base to maintain the milk’s initial pH.
Inorganic phosphates and citrates also function as internal buffers, absorbing excess ions. The natural pH of milk can exhibit slight variations based on the cow’s health or stage of lactation. For instance, milk from a cow suffering from mastitis tends to have a higher, more alkaline pH, sometimes exceeding 7.0, due to an influx of blood components. Conversely, milk produced immediately after calving, which contains colostrum, often presents a slightly lower pH.
pH as the Indicator of Milk Spoilage
The most common form of milk spoilage is linked to a sharp decline in pH caused by microbial activity. Lactic acid bacteria (LAB), naturally present in milk, metabolize lactose (the main milk sugar) and convert it into lactic acid. The accumulation of this acid increases the concentration of hydrogen ions, driving the pH value downward.
As the pH drops below 6.0, the milk develops a distinct sour taste, indicating spoilage is underway. The most dramatic physical change occurs when the pH approaches the isoelectric point of casein, which is around 4.6. At this acidity level, the casein micelles lose their electrical charge and coagulate, causing the milk to curdle into visible clumps.
Dairy processors strictly monitor the pH of raw milk because a reading below 6.5 suggests bacterial growth and indicates poor quality or improper handling. This monitoring is a quality control measure, as rapid acidification ruins the texture and flavor and signals that the milk may no longer be safe for consumption. A high-quality, fresh sample should consistently remain within the 6.6 to 6.8 range before processing.
The Role of pH in Commercial Milk Processing
The intentional manipulation of pH is fundamental to creating many fermented dairy products, transforming liquid milk into various textures and flavors. In cheesemaking, a controlled drop in pH enables curd formation. A starter culture containing lactic acid bacteria is added to slowly lower the pH, facilitating the coagulation of casein proteins.
Monitoring the pH is important at every step; for instance, the milk is often cut into curds when the pH is between 6.0 and 6.2. This level promotes the proper expulsion of whey, known as syneresis. The final texture of the cheese depends on the ultimate pH achieved, with many hard cheeses reaching a final acidity between 5.1 and 5.3.
Maintaining pH stability is equally important during high-heat treatments like pasteurization or Ultra-High Temperature (UHT) processing. If milk is too acidic before heating, the proteins can prematurely denature and clump together, leading to sedimentation and a grainy texture. Processors rely on the milk’s natural buffering capacity and precise monitoring to prevent heat-induced instability during thermal treatment.
pH Differences Between Dairy and Plant-Based Milks
The slightly acidic nature of dairy milk (pH 6.5–6.7) differs from many popular plant-based alternatives, which often range from neutral to slightly alkaline. Almond, oat, and soy milks typically have a different protein composition and are often formulated with a higher baseline pH to enhance shelf stability. This difference in baseline acidity can lead to issues when plant milks are mixed with hot, acidic beverages like coffee.
Coffee has a pH that ranges between 4.0 and 5.5, which is acidic enough to challenge milk protein stability. When plant-based milk, which often has a lower protein content and different structure than dairy, is exposed to the heat and acidity of coffee, its proteins denature and coagulate. For example, soy milk can curdle in coffee at pH 4.7, while dairy milk is stable until the acidity reaches about pH 4.1. This immediate separation is a visible consequence of the differing sensitivities of plant and dairy proteins to a low-pH, high-temperature environment.