🧀 Soured milk

Kysliak, or soured milk, is the simplest fermented milk product: milk that has thickened through the action of lactic acid bacteria. Zinovia Klynovetska recorded it in 1913 as “samokysh”, and ethnographer Lidiia Artiukh explains in her notes to the reprint that samokysh is the same thing as kysliak. Kysliak was drunk with thick porridge, and varenyky were washed down with cold fresh or soured milk.

Modern requirements for fermented milks are set by the Codex Alimentarius: the starter bacteria acidify the milk, and the finished product needs to contain at least 2.7% milk protein, less than 10% fat and at least 0.3% lactic acid. There have to be at least 10 million live starter bacteria per 1 g right up to the end of the shelf life, and draining off the whey is not allowed. The Ukrainian terminology standard DSTU 2212:2003 requires at least one million live cells per 1 g at the end of the shelf life.

Kysliak differs from kefir in that the kefir starter also contains yeasts, and from yogurt in its set of bacteria: under the Codex, yogurt is made specifically with thermophilic streptococcus and Lactobacillus bulgaricus. Fermented milk products were made famous by Ilya Mechnikov, born near Kharkiv and awarded the Nobel Prize in 1908: he proposed a diet including milk soured by bacteria that produce a lot of lactic acid.

Soured milk or kefir? Soured milk is the plainest fermented milk: lactic acid bacteria thicken it, with no yeasts, which are part of the kefir starter. From yogurt it differs in its bacteria, since the Codex makes yogurt with thermophilic streptococcus and Lactobacillus bulgaricus. It also differs from fresh milk that went off by itself: that souring is uncontrolled, whereas here a starter acidifies milk that has been boiled or pasteurised. Under the Codex the finished product has at least 2.7% milk protein and 0.3% lactic acid, and the whey is not drained off.

Look for the name “soured milk” or “sour milk”, or the Ukrainian kysliak, on the pack of a fermented milk product. There is no protected name, so read the label: milk and starter cultures should be listed, and a note about live cultures suggests it was not heated after souring. It sits in the chilled dairy aisle of supermarkets, at dairy stalls and in shops selling Eastern European food. At home keep it in the fridge.

Why it matters to you. Don't leave raw milk to sour on its own: EFSA warns that raw milk can carry Campylobacter, Salmonella, pathogenic E. coli and other pathogens, and it is boiling, not souring, that reliably kills them. It is safer to boil the milk or take pasteurised milk and sour it with a spoonful of ready kysliak or a starter culture. Keep kysliak in the fridge.