Kiselo mlyako (кисело мляко, literally “sour milk”) is Bulgarian yogurt: milk set into a firm, spoonable curd with a clean, pronounced sourness, eaten plain, with honey (not for babies under 1), or thinned into the cold cucumber soup tarator. It’s made with the two classic yogurt bacteria, Lactobacillus delbrueckii subsp. bulgaricus and Streptococcus thermophilus, from cow’s, sheep’s, goat’s or buffalo milk, at home, by artisans or industrially (Teneva-Angelova et al., 2018).
The Bacterium Named After Bulgaria
In 1905 the Bulgarian physician and microbiologist Stamen Grigorov showed that the culture of Bulgarian yogurt consisted of two lactic acid bacteria, a rod and a coccus; the rod was named Lactobacillus bulgaricus in 1907 (Teneva-Angelova et al., 2018). Around the same time Ilya Metchnikoff at the Pasteur Institute – who shared the 1908 Nobel Prize in Physiology or Medicine for work on immunity (Nobel Prize) – argued that the lactobacilli in Bulgarian yogurt restricted putrefactive bacteria in the gut and linked it to the many centenarians he believed lived in Bulgaria. That idea helped launch the modern interest in fermented milk, but it was a hypothesis, not evidence that yogurt extends life.
The same two species are now the standard yogurt culture far beyond Bulgaria: in the US, a product can only be called “yogurt” if it’s cultured with L. bulgaricus and S. thermophilus (21 CFR 131.200). What makes kiselo mlyako distinctive is the particular strains, a warm set (around 42–45°C) and the fact that it isn’t strained, so it keeps its whey and a looser, more sour character than labneh or Greek yogurt.
Starter, Temperature and Time
- Starter: a plain Bulgarian-style yogurt with live L. bulgaricus and S. thermophilus on the label, a few spoonfuls of your last batch, or a freeze-dried Bulgarian yogurt starter.
- Milk: pasteurized whole milk, heated to about 85°C first for a firmer set. Raw milk can carry harmful bacteria, and fermentation doesn’t reliably make it safe (CDC).
- Heat: these are thermophilic (heat-loving) bacteria. Inoculate at 42–45°C and keep the milk at that temperature until it sets, usually 4–8 hours. An oven with only the light on is usually too cool; use a yogurt maker, an insulated cooler with a jar of hot water, or a well-wrapped pot.
The same method makes the Georgian fermented milk matsoni; room-temperature milks such as Moroccan raib use different, mesophilic bacteria. For a general walkthrough, see how to make yogurt at home. For milk that is baked for hours before it is soured, see Russian ryazhenka.
Lactose and Live Cultures
The live cultures in yogurt help digest its lactose – a health claim accepted by the European Food Safety Authority (EFSA, 2010) – so many people who don’t tolerate milk do fine with kiselo mlyako. Store-bought yogurts that are heat-treated after fermentation no longer have live cultures; check the label. Bulgarians also thin it with water and salt into a drink much like Turkish ayran.