🧪 Lactic acid

Lactic acid (E270, 2-hydroxypropanoic acid) is the organic acid that makes fermented milk sour. Lactic acid bacteria produce it when they ferment lactose, the milk sugar. On food labels it carries the code E270 and counts as an acidity regulator.

Here is what happens in milk. Bacteria feed on lactose and give off acid, and the pH falls. The protein casein, which is 75–80% of milk protein, stays dispersed because its particles carry a charge and push each other away. Acid cancels that charge, and near pH 4.2 the particles stick together. That is how milk thickens. A curd made by bacteria is soft and breaks up easily. The acid from a starter culture also helps expel whey and holds back harmful bacteria.

There is not much of it in the finished food. FAO gives the acidity of a finished yoghurt as usually 0.8–0.9% lactic acid. The Indian dahi should reach 0.9–1.0%. Dairy people measure acidity just this way: they titrate milk with alkali and write the result as percent lactic acid. See acidity for more on that number.

Lactose and lactic acid are easy to mix up. Lactose is already in fresh milk; the acid appears only when bacteria work on that sugar. Kefir, as FAO describes it, is made by lactic fermentation, and yoghurt by fermenting lactose with a pure culture of lactic acid bacteria. To taste the result, try kefir, yoghurt or labneh.

On a label, E270 sits among the acidity regulators. The JECFA committee of FAO and WHO evaluated it in 2001: the acceptable daily intake is not limited, and there is no safety concern at current intakes when used as a flavouring. In FAO's additive rules most food groups allow it at the level of good manufacturing practice, with numeric limits for some groups, such as 6000 mg/kg in fresh minced meat.

For acids in wine, see malolactic fermentation and tartaric acid. For souring milk at home, see acidification. The composition of milk as a whole is in the section on milk composition, health and substitutes.