Yogurt added carelessly to a hot pan turns grainy and separates into curds and watery liquid. The behaviour is entirely predictable once you know what yogurt is structurally.
What yogurt is before it meets heat
Yogurt is milk whose protein has been partly destabilised by bacterial acid, forming a loose gel that traps water. The gel is stable at fridge temperature but is not built to withstand heat.
Casein proteins in that gel are already close to the acidity at which they lose their mutual repulsion. They sit near the edge of clumping rather than comfortably away from it.
Heat supplies the energy that pushes them over. Once they aggregate, they squeeze out the trapped water and the smooth texture is gone.
Why the split is not reversible
Protein aggregation of this kind is a one-way change. Stirring afterwards breaks the curds into smaller pieces but cannot restore the original network.
The dish is still edible, and several regional preparations accept a slightly grainy finish. But a sauce meant to be glossy will not become glossy again.
Prevention is therefore the only real option, and every traditional technique addresses the same underlying problem.
How starch and fat prevent it
Whisking a spoon of gram flour or rice flour into the yogurt before it enters the pan is the most common fix. Starch granules physically interfere with proteins finding each other.
Higher-fat yogurt survives better for a related reason, since fat globules dilute the protein network and slow aggregation. Thin, watery yogurt splits most readily.
Cream or ground nuts perform a similar function in richer gravies, which is one reason those styles tolerate more aggressive heat.
Why temperature and order matter
Adding cold yogurt straight into a boiling pan creates the worst case, because a small quantity of yogurt meets a large mass of very hot liquid instantly.
Cooks avoid this by lowering the heat, then tempering the yogurt with a few spoons of hot gravy before returning the mixture to the pan. The temperature gap narrows in stages.
Stirring continuously during that period keeps any local hot spots from forming, since a pan bottom is always hotter than the liquid above it.
Why salt goes in afterwards
Salt added at the same moment as the yogurt increases the risk noticeably. Salt ions screen the charges that keep casein particles apart, so aggregation begins more easily.
Holding the salt until the yogurt has been incorporated and the sauce has settled removes that additional pressure at the point where the mixture is most vulnerable.
The same logic applies to acidic ingredients such as tamarind or lemon, which are almost always the last things to enter a yogurt-based gravy.