🔄 Fermentation

Milk fermentation is the conversion of milk into a fermented milk product by microorganisms. Lactic acid bacteria feed on milk sugar — lactose — and turn it into lactic acid. The Codex Alimentarius describes the result this way: the pH of the milk drops, and it either coagulates or stays liquid. When the acidity reaches about pH 4.6, the protein casein gathers into a network and the milk thickens.

Each product has its own microbes and temperature. Yogurt is cultured with the heat-loving Lactobacillus bulgaricus and Streptococcus thermophilus: the milk is heated to 90–95 °C, cooled to 40–45 °C and held for 4–5 hours at 42–43 °C until the pH falls to 4.2–4.5. Kefir is made with kefir grains, in which yeasts live alongside bacteria, at around 23 °C; the yeasts produce a little carbon dioxide. Koumiss, made from mare's milk, has to contain at least 0.5 % alcohol under the Codex.

The Codex also sets minimum acidity: 0.3 % lactic acid for fermented milk, 0.6 % for yogurt and kefir, 0.7 % for koumiss. At least 10⁷ live starter microbes per gram have to remain until the end of the shelf life.

Fermentation or spoilage? Both begin when bacteria turn lactose into acid, but spoiled milk is soured by whatever microbes happen to be present. In fermented milk a chosen starter does the work at a set temperature, and the Codex Alimentarius fixes minimum acidity (0.6 % lactic acid for yogurt and kefir) and at least 10⁷ live starter microbes per gram until the end of the shelf life. It also differs from pickling: here the acid comes from milk sugar, not from salt or vinegar. Kefir and koumiss involve yeasts as well, so they are not only a bacterial process.

On a pack, look for the product name that says what was done: yogurt, kefir, fermented milk, buttermilk or skyr, with “live cultures” or “live yogurt cultures” in the ingredients. Check whether the product was heat-treated after fermentation, because then live microbes are not guaranteed. You will find them in the chilled dairy aisle of supermarkets, in dairies, and at market stalls selling traditional fermented milk.

Fermentation is one of the oldest ways of preserving milk. FAO writes that in hot parts of Africa milk sours within 12–24 hours without refrigeration, so natural fermentation is the main way of processing it there. The acid holds back harmful microbes and the lactose content falls; in the EU it is permitted to state that live yogurt cultures improve lactose digestion in people who have difficulty digesting it. This is how kefir, yogurt, clabbered milk, ryazhenka, ayran, matsoni, skyr, dahi, amasi and many other products are made.