What is the pH of milk?

Milk is a very complex mixture that contains water, proteins, minerals, sugars or carbohydrates, fats, and even cells of different types. These components can be found either dissolved in the aqueous phase, dispersed as a colloid of very small insoluble particles, or emulsified as small droplets of fats or oils dispersed in the aqueous phase.

The pH of cow’s milk has a slightly acid value ranging from 6.65 to 6.71, for a fresh milk sample just milked, at 25°C obtained from a cow that has been lactating for several days, according to a report from 2016 from the Institute for Agricultural and Fisheries Research and Training of the Junta de Andalucía, Spain. This slight acidity is mainly due to the high content of proteins such as casein, as well as a buffer system formed mainly by phosphate ions.

Maintaining a constant pH is a necessity for the stability of a system as complex as milk, since a change, even of just a couple of pH units, can upset the entire physicochemical balance to the point where it separates. the emulsion, the proteins precipitate and the milk is curdled.

Factors that determine the pH of milk

Despite its complex composition, the level of acidity or pH of milk varies very little, as we have just seen, and, under normal conditions, it is generally within a narrow range of slightly acidic pH. However, the pH of milk can be affected by a number of factors, some of which make it more acidic (lowering its pH) while other factors make it more basic or alkaline.

For this reason, pH can be an important indicator that things are not going well, either for a batch of milk itself, or for the cow that produced it. Let’s see how the pH of milk varies under different conditions.

Variation of milk pH with temperature

The pH of any aqueous solution always varies with temperature. However, this is doubly true for milk, whose pH decreases as the temperature increases. In fact, it has been determined that, on average, the pH of milk drops by 0.01 units for every 1°C increase in temperature. Although this may seem small, it is actually a very big change.

This happens because the solubility of calcium phosphate decreases with increasing temperature, so it precipitates, reducing the available amount of the base in the milk buffer system.

Variation of milk pH with processing

Pasteurization and other types of thermal processing lower the pH for the same reason explained above. Also, when pasteurized at very high temperatures (as in the case of Extended Life, or UHT) milk, sugars like lactose break down into organic acids like lactic acid, further lowering the pH .

Variation of the pH of the milk with the phase of the lactation period

Another factor that affects the pH of milk is the phase of the lactation period. Colostrum, that is, the first milk that is produced during the first days postpartum, has much more protein than normal milk, which makes it considerably more acidic, with a pH close to 6.0. As time passes, the milk gradually becomes more alkaline and by the end of the lactation period, the milk has a pH greater than 7.4.

Variation of the pH of the milk with the state of health or the age of the cow

Cows with mastitis generally produce milk with a higher pH than healthy cows. In these cases, pH values ​​between 6.9 and 7.5 can be obtained.

microbiological contamination

When milk is not pasteurized or left exposed to air for a long time without refrigeration, it can be colonized by different types of bacteria and other microorganisms that turn it acidic and cut it (producing sour milk). This occurs because many bacteria ferment the sugar present in milk (lactose) to lactic acid or other organic acids that lower the pH.

The pH of milk from other mammalian species

Finally, cow’s milk is not the only milk out there. Obviously all mammals produce milk for their young, and each species produces it with a particular composition and a characteristic pH.

Among milks for human consumption, the most common, along with their respective pH, are:

As can be seen, what would be a problematically high pH for cow’s milk is a perfectly normal value for human milk.

References

Lopez, AL; Belly, D. (2016). Milk, composition and characteristics. – Ministry of Agriculture, Fisheries and Rural Development, Institute for Agricultural and Fisheries Research and Training, 2016.

Negri, L.M. (2005). The pH and acidity of milk. Manual of Technical References for the achievement of quality milk . 2nd ed., 2005, INTA.