Hello! This is a very common issue when cooking curries with dairy, but with a few adjustments to your technique and heat management, you can easily achieve a smooth, rich sauce.
Yogurt curdles because its primary proteins, called caseins, rapidly denature, tighten, and separate from the whey when exposed to sudden high heat and acidity. Yogurt begins to destabilize and coagulate around 140°F to 150°F (60°C to 65°C), and adding cold yogurt directly into a simmering or boiling sauce (212°F / 100°C) will cause it to split almost instantly.
Here is the exact method and temperature guidelines to prevent splitting:
1. Use full-fat yogurt: Always choose whole milk or full-fat Greek yogurt. The fat globules act as a protective barrier around the protein molecules, making them significantly more resistant to heat than low-fat or non-fat varieties.
2. Bring to room temperature and whisk: Take the yogurt out of the refrigerator 20 to 30 minutes before use. Whisk it thoroughly in a small bowl until it is completely smooth and loose.
3. Add a starch stabilizer: Whisk 1 teaspoon of cornstarch, all-purpose flour, or chickpea flour (besan) per cup of yogurt directly into the whisked yogurt. The starch granules swell and physically block the proteins from clumping together.
4. Temper the dairy: Take your curry off the heat and let it cool below 140°F (60°C), where steam is barely visible. Scoop a ladleful of the warm curry sauce and slowly whisk it into your bowl of yogurt. This gradually raises the yogurt's temperature and equalizes the acidity without shocking the proteins.
5. Stir gently on low heat: Pour the tempered yogurt mixture back into the main pot while stirring continuously. Keep the heat very low and avoid bringing the sauce back to a rapid boil.
Since you are batch cooking for the freezer, keep in mind that dairy emulsions can also separate slightly during the freezing and thawing process. Stabilizing with cornstarch helps tremendously, or alternatively, you can freeze the spiced base without yogurt and stir in fresh tempered yogurt when reheating the portion to serve.
