Cooking with Yogurt

When yogurt is used to create a sauce for a curry, sometimes we find that it curdles. I've had many questions about this, so I thought I'd write up a post about it. First we will talk science, then about what you can do to prevent curdling, and then a strategy if your yogurt sauce curdles and you need to bring it back to life. Hope this will be beneficial to you.

Science of Milk and Yogurt

Remember the nursery rhyme we used to sing: "Little Miss Muffet, she sat on her tuffet, eating her curds eating and whey...." While there are many different types of proteins in milk, we can divide the proteins into two classes: Curds and Whey. They are different in how they react to acids. Some of the curd proteins, called caseins (read more here), coagulate (clump up in solids) in the presence of acids. Whey proteins, on the other hand, do not coagulate in presence of acid. Special microbes often convert the milk's sugar into acid, which then creates curds and whey, resulting in fermentation. Lactic acid bacteria is usually responsible for a range if fermented milk products: Yogurt, Buttermilk, Creme fraiche, Sour cream, Ropy milks, Koumiss, and Kefir. There are two types of lactic acid bacteria: Lactococcus and Lactobacillus. The proportion of these bacteria that is encouraged in the fermentation process often results in the flavor, texture, and the type of the fermented product produced. Yogurt is popular in India, and westwards through Middle East. The milk needs to be at 106-114F/ 41-45C over 3 - 5 hours, or, around 86F/ 30C for 6 - 12 hours for the prefered lactic acid bacteria (mainly lactobacillus, delbrueckii, streptococcus, salivarius, each active at different times during the process) to develop. The acidity develops to about 1 - 4 %, and a tart gel is formed, which is smooth and semisolid, thick in consistency. The fermentation process leads to formation of acetaldehyde, which gives the impressions of a refreshing taste, similar to green apples. So, milk is first heated, sometimes for a prolonged time to concentrate the proteins (to get a firmer final product), and then cooled to the appropriate temperature, and held at that temperature, to help the bacteria to ferment the milk, and set the milk into yogurt.

So when yogurt is added to heated foods (or heated by itself) or to higher acidic conditions, it curdles. The coagulation is caused because the protein network in the gel shrinks further and squeezes out the whey and produces the distinct white protein curds floating in the thinned whey. Sometimes additional salt added with the yogurt sauce, or stirring vigorously also causes curdling. 

How to prevent yogurt in sauces to curdle?

Higher the fat content in yogurt, the fat prevents further coagulation and the separation of curds and whey. Also note, yogurt added to sauce cannot really develop new thickness to the the sauce, because the caseins are already coagulated, only the existing thickness of the yogurt is available. Here are a few strategies to prevent curdling:

• Make the sauce on low heat. As low as possible.
• Use high fat yogurt. Never use low fat or fat free yogurt.
• When whipping yogurt to add as a sauce, first drain out as much whey as you can, and use the curds only. You can add water to help with the whipping. Make sure the yogurt is smooth before adding to the sauce.
• Do not stir the sauce in the cooking process too vigorously.
• Make sure the sauce has low acid content before the yogurt is added. (No vinegar or a lot of tomatoes, etc.)
• Some people say we can add some flour (almond, wheat, chickpea, etc) to the yogurt, stir it well/ whip, and then add to the sauce. Claim is that the flour particles act as a thickener. (I have not done this myself, I add yogurt to my sauces over a very low flame and it does not curdle.)

Some companies promise that their yogurt will not curdle!

The sauce has curdled, now what? How can I fix it?

I suggest that once the sauce has curdled, do the following:

• Take some fat and starch and make a roux. Prepare the roux separately, making sure that the roux is very smooth and has a particle quality smaller than the coagulated solids in the "broken" sauce.
• Add this on very low flame, not to a boiling sauce. Mix it in gradually.

This will change the taste of the sauce and make it a bit mellowed from the tangy tartness of a typical yogurt sauce, unfortunately that sacrifice will have to be made to "fix" a "broken" situation.