Becky asked: “What’s the difference between yogurt and kefir?”
Shortly after our prehistoric forebears first domesticated livestock, they noticed that the animals’ wonderful milk changed if it wasn’t drunk right away. It developed odd flavors and textures, and sometimes it made them sick. But other times, it became tart, and more digestible, and stayed drinkable for weeks. Using the same container for the next batch—the pores of the ceramic or leather bag retaining populations of invisible microbes—helped ensure that it would be good in the same way. Gradually, patterns were established, and a diverse array of fermented milk products sprang up around the world.
The most popular of these exist thanks to lactic acid bacteria, including such famous species as Streptococcus thermophilus and Lactobacillus delbrueckii. When these bacteria get into warm milk, they thrive and reproduce, eating the lactose sugar that’s naturally prevalent in milk, and converting it to lactic acid. The buildup of acid has a few simultaneous effects: It makes the milk taste tart and pleasant. It coagulates some of the protein in the milk, causing it to thicken. And most other microorganisms can’t survive in such an acidic bath, so it prevents them from getting in and spoiling the milk.