Straining pulls a good share of the liquid whey out of cultured milk, and that whey is carrying most of the protein responsible for curdling under heat. What's left behind is heavier in casein and fat, lighter in the protein that clumps and turns grainy in a hot pan. That's the whole reason Greek yogurt tolerates a simmering sauce that would break regular yogurt in seconds. It can still split if you push it too hard.
What straining actually removes
Cultured milk starts as a loose gel. Lactic-acid bacteria feed on the milk sugar and drop the pH low enough that casein proteins link into a soft mesh, which is what turns milk into yogurt in the first place. Straining that yogurt through cloth or a fine filter lets the liquid caught in that mesh drain away — water, dissolved lactose, minerals, and whey proteins, mostly one called beta-lactoglobulin. A few hours of draining removes roughly half the liquid. Push it further, the way labneh does, and you end up with something closer to spreadable cheese. Greek yogurt sits partway along that line: strained enough to hold its shape on a spoon, not strained all the way to a cheese.
Why losing the whey protein matters in a hot pan
Beta-lactoglobulin is the protein that does the damage. It unfolds when it gets hot, then grabs onto its neighbors and clumps — the same reaction that turns a raw egg white opaque, and the one that curdles milk when wine hits it in a hot pan. Casein, what Greek yogurt is mostly made of after straining, doesn't behave that way at normal cooking temperatures. It stays folded and holds its structure instead of seizing. Strain out most of the whey and you strain out most of the protein that was going to curdle. What's left resists heat by default, not because anyone stabilized it on purpose.
Why it can still split
Resistance isn't immunity. Drop cold Greek yogurt straight into a pot at a hard simmer and the temperature shock hits the casein hard enough to seize it anyway, whey protein or not. Whisking in a spoonful of the hot liquid first — tempering — brings the yogurt's temperature up gradually, so the proteins denature slowly instead of all at once. Low heat and constant stirring do the rest. Skip either step and even well-strained yogurt separates into curds floating in a thin, watery liquid.
Why it behaves differently served cold
None of this matters for a bowl of tzatziki or a dip whisked with garlic and lemon. Cold, the advantage is just water: less of it makes for a thicker sauce that clings to meat instead of running off, and a higher proportion of fat gives it a rounder mouthfeel instead of a thin, sour one. The heat-stability story only shows up once a pan gets involved.
What it does in a batter
In baking, Greek yogurt is doing a different job. Its lower water content means it adds less thin liquid to a batter than regular yogurt or buttermilk would for the same volume, so recipes built around it often call for slightly more liquid elsewhere to balance that out. Its acidity, inherited from the same fermentation that made it yogurt in the first place, still reacts with baking soda the way buttermilk's does, releasing carbon dioxide that helps a batter rise. Straining changes the ratio of solids to liquid. It doesn't touch the acid doing the leavening.