Yogurt curdles when the protein strands congeal, or tighten up, when they are exposed to heat. Fat coats the protein strands, preventing them from tangling up with each other to create curds. If you add a nonfat or low-fat yogurt to a simmering sauce, it will curdle more easily than the full-fat version because it does not have as much fat to protect the proteins from the heat of the sauce.

If you do not have full-fat yogurt, or you just want to be doubly certain your yogurt will not curdle when you cook it, add 1 teaspoon of flour or cornstarch per 1 cup of yogurt. Stir the flour or cornstarch into the yogurt before you add it to the sauce to prevent lumps. The starch in the flour or cornstarch prevents the proteins in the yogurt from tangling together.

High heat is the root cause of curdled yogurt in cooked sauces. Lowering the heat to a bare simmer will cook the yogurt more gently, and lessen the risk of curdling.

If you have time, let the yogurt come to room temperature before adding it to your sauce. If you must start with cold yogurt, temper it before adding it to your sauce. Transfer the yogurt to a heat-proof bowl. Ladle a spoonful of the hot sauce into the bowl and whisk to combine it with the yogurt. Continue to add the simmering sauce to the yogurt, one spoonful at a time, until the yogurt is warm to the touch. Whisk the tempered yogurt into the sauce.