Two bacteria, one transformation
Warm milk, add a starter culture of two bacteria — Lactobacillus bulgaricus and Streptococcus thermophilus — and leave them to work. As they ferment the milk's natural sugars, they produce lactic acid. When the acidity reaches a certain point, the milk proteins — mainly casein — tighten and trap liquid, turning something thin and white into something thick, tangy and cohesive.
That much is true of every yoghurt. What diverges is everything afterwards: whether the gel is left undisturbed or broken up, whether whey is removed, and whether the living cultures survive to reach the spoon.
Set versus stirred
A set yoghurt is incubated directly in the pot it's sold in. The culture does its work there, the proteins gel in place, and nothing disturbs the structure. The result holds a clean spoon mark and has a slightly firmer, more fragile texture — tilt the pot and the surface stays put rather than flowing.
Stirred yoghurt takes a different route. The milk is cultured in a large vat, the resulting gel is broken up mechanically, then poured into pots. That breaking step gives stirred yoghurt its smooth, flowing consistency — the familiar spooning yoghurt that collapses gently under its own weight and dominates most supermarket shelves.
Neither is superior. They are different products built by different processes, suited to different uses.
What Greek yoghurt actually means
Genuine Greek yoghurt goes a step further: after culturing, the whey is strained out. Whey is the watery liquid component of milk, and removing it concentrates everything left behind — protein, fat, solids. The result is that characteristic thick, almost spreadable body, achieved without added cream or thickeners, simply by having less water in the pot.
The label matters here. Greek yoghurt should be made by straining. Greek-style yoghurt often isn't — the thick texture is achieved instead with added milk powder, modified starch, or other stabilisers. This isn't automatically a worse product, but it is a different one, and the ingredients list will tell you which you're holding. A genuine strained yoghurt will typically list only milk and live cultures. Anything else in that list, and straining almost certainly hasn't happened.
Straining also produces a significant quantity of leftover whey — itself a useful ingredient in breadmaking, drinks, or dried into powder. Nothing is wasted in a well-run dairy.
Live cultures and what kills them
The bacteria that make yoghurt aren't just a means to an end — in a live yoghurt, they're still present and active in the finished pot. This is what labels mean by live or active cultures. Those cultures continue to develop slowly in the fridge, which is why a pot of live yoghurt grows gradually tangier with age.
Heat-treated yoghurt is pasteurised again after fermentation, which kills the bacterial cultures and extends shelf life considerably — heat-treated yoghurt can sit at room temperature until opened. The living cultures are gone, and with them that slow, evolving flavour. The product is stable and consistent, but it is no longer alive in the same sense.
Reading the label carefully tells you which you have. Live yoghurt will say so. Heat-treated yoghurt often won't advertise the fact prominently, so look for the cultures claim — or its absence.
Yoghurt's wilder cousin
If yoghurt represents a controlled, two-bacterium fermentation, kefir sits at the other end of the spectrum. Made with kefir grains — complex communities of bacteria and yeasts — it produces a thinner, sharper, slightly effervescent drink with a much broader microbial range. We've covered the craft and character of kefir separately; the two products share a common principle but lead very different lives.
The same glass of milk, handled differently, becomes a firm set yoghurt, a thick strained Greek, a pourable stirred pot, or a fizzing fermented drink. The bacteria do the heavy lifting; the maker decides everything else.
