Curd rice can become sour over time due to the fermentation process. Here is what you can do to prevent curd rice from becoming too sour: Use fresh curd or yogurt: Always use fresh curd or yogurt to make curd rice. If the curd or yogurt is old or past its expiration date, it is more likely to become sour. Use cold ingredients: Make sure that the rice and curd are cooled down to room temperature or refrigerated before mixing them together. This will slow down the fermentation process and prevent the curd rice from becoming too sour too quickly. Use less sour curd or yogurt: If you are using sour curd or yogurt, use less of it than you would normally use. This will reduce the overall sourness of the dish.