I got an email with this question recently:
Does cooking with yogurt make the yogurt no longer tolerable to those of us with LI?
Yogurt, as people with lactose intolerance get told all the time, is not only supposedly healthy but well-tolerated because the live cultures in the yogurt work in your intestines to make lactase and digest any lactose in the yogurt.
I also warn people to check the yogurt carton for the words "live and active cultures." Some yogurts want to have long shelf lives, so they use a quasi-pasteurization technique called heat-treating, in which the yogurt is raised to a temperature near boiling. This reduces spoiling, but also kills off the yogurt that helps us digest the lactose.
The question took this the next logical step. In cooking or baking, the temperature of the food often rises well above 200 degrees F. So wouldn't that make yogurt no better tolerated than any other milk product?
The answer is yes, as these yogurt websites attest:
The Cascade Yogurt FAQ page says:
Does cooking with yogurt affect the live cultures?
Temperature plays an important role with live active cultures. If yogurt is used in a recipe that requires even moderate heating the living yogurt bacteria may be detrimentally affected to some degree. However, the yogurt still is an excellent low calorie substitute for sour cream in recipes and offers valuable calcium, phosphorus, potassium and protein nutrients.
And Stonyfield Yogurt has similar advice:
Does cooking with yogurt destroy the cultures or alter their effectiveness?
Heating yogurt to a high temperature will destroy the cultures. To prevent this, never add yogurt directly to a hot mixture. Instead, stir a few tablespoons of the hot food into the yogurt, warming it gradually. Then stir the warmed yogurt into the hot mixture. Do this near the end of the cooking process, so the yogurt won’t be heated for a long time.
If you can adapt your recipes to keep the yogurt from prolonged high heating, you should do so.