What causes yogurt in sauces to split? How to prevent it?

Question

A lot of my favorite curry recipes have a yogurt based sauce in them, but on a pretty regular basis when I make them, the yogurt ends up splitting into basically curds and whey. What causes that, and how can I prevent it?

As an illustrative example, last night I made a simple tofu curry as follows:

1. warm evoo in a pan
2. add chopped onion, saute briefly
3. add curry seasonings, allow to warm/season the oil
4. add cubed tofu, toss to coat
5. cook for a while, tossing periodically to lightly brown the cubes
6. turn heat off
7. add plain greek yogurt
8. stir to combine

In the few minutes it took to finish the rest of the meal and start plating, the yogurt had separated so I had a clumpy, lumpy, yogurt soup instead of a smooth creamy sauce.

Answer 1

You are trying to add the yoghurt at too high a temperature. Let the dish cool to around 75 deg C before adding the yoghurt, and make adding the yoghurt the last thing you do before serving.

Answer 2

Yogurt is a mesh of denatured milk protein that traps the whey.

When yogurt is over-heated those proteins tighten and squeeze out the extra whey. When the protein matrix is cut it will also leak whey.

To combat this add a little starch. A little cornstarch mixed into the yogurt will prevent the yogurt proteins from over-coagulating. All heated yogurt sauces that I have seen include starch for this reason.

I should also add that this problem occurs mostly with lowfat yogurt. Extra milk fat in yogurt will interfere with the protein's ability to coagulate the same way the starch does.

Answer 3

In addition to reducing the temperature of the curry, you can also:

• Temper the yoghurt - combine a small amount of the warm sauce to the yoghurt before adding it to the curry. This helps when adding cream, milk, or eggs to a sauce.
• Whisk the yoghurt - use a fork or whisk and vigorously mix the yoghurt. As the fats and proteins are emulsified in the liquid, this ensures an even starting distribution.
• Add an emulsifier - add a teaspoon of mustard powder or lethicin to the curry before combining the yoghurt.

Answer 4

One option is to substitute cream for yoghurt.

That's what's done in a lot of Indian cooking. When yoghurt is used, it's usually added at the end, and not at high heat, just as James points out.