I do think it can be the kind of milk or curd earning system that makes it grainy. When there is large amount of whey in the curd, then it turns grainy once it can be heated. Crush the saffron marginally between your fingers and insert it to heat milk. https://aminaresidency61593.bloggip.com/32117491/the-smart-trick-of-chicken-biriyani-that-no-one-is-discussing