What is yoghurt, and how is it different from other cultured dairy?
Milk fermented by specific bacteria that convert lactose into lactic acid, which lowers the pH until the milk proteins coagulate — producing thickness from chemistry rather than from anything added.
What defines yoghurt specifically. Two bacteria working together: Streptococcus thermophilus and Lactobacillus delbrueckii subsp. bulgaricus. They are symbiotic — each produces compounds the other needs, so together they ferment faster than either alone. In many jurisdictions a product cannot legally be called yoghurt without them.
Why it thickens. As acidity rises, casein proteins lose their charge and stop repelling each other, clumping into a gel that traps water. No thickener is required — though many commercial yoghurts add milk powder, starch or gelatin to get there faster or firmer.
The variations:
Greek or strained yoghurt — ordinary yoghurt with the whey drained off, concentrating protein and fat and removing some lactose. "Greek-style" frequently means thickened rather than strained, which is a different and cheaper product.
Skyr, an Icelandic strained product, traditionally made from skimmed milk and very high in protein.
Set versus stirred, depending on whether it fermented in the pot or was fermented then stirred and poured.
Live or bio, meaning the cultures were not killed afterwards — heat-treated yoghurt contains none, which is worth checking if that is the point.
The other cultured dairy, and how they differ:
Kefir, fermented with a complex community of bacteria and yeasts in grains, giving a thinner, fizzier, slightly alcoholic drink with far more microbial diversity than yoghurt.
Buttermilk, traditionally the liquid left after churning butter; cultured buttermilk is milk fermented with different bacteria.
Soured cream and crème fraîche, cultured cream — crème fraîche has higher fat and does not split when boiled, which is why it is used in sauces where soured cream curdles.
Quark and cottage cheese, which cross into fresh cheese.
On lactose: fermentation consumes some, and the live bacteria help digest more, which is why many lactose-intolerant people tolerate yoghurt.