Query on how to prevent burning of dum

Since raisins are a type of dried fruit, they don't have a lot of moisture left in them. Heat from baking with just make them drier, eventually resulting in that chewy, not particularly pleasant lump. The internal raisins are more protected from the heat, so they stay the same consistency.

One solution may be to soak the raisins in water, fruit juice, or even wine (boozy scones!) before cooking. The extra liquid will help prevent that drying effect; you'll still have some consistency difference between "outside" and "inside" raisins, but none will be inedible. I soak raisins as well as dried cranberries and cherries before using them in muffins.

Some options:

1. Prepare the biryani in an oven (used when making biryani with uncooked meat) in a really thick, covered oven pan, with no foiling at 200 degrees Celsius for around 30 mins. It won't stick to the bottom or get burnt.
2. Keep a cast iron tawa underneath the pot the rice is in. Rice won't stick.
3. Add some ghee at the bottom of the rice before layering.
4. Keep the heat low. "Dum"ming the biryani is steaming it rather than cooking it with high heat. You'll also need to seal the container.

The sticky rice at the bottom is called "khurchan", and some people actually find it delicious. The burnt rice is because of the lack of liquid and the high heat. Happens mainly because the water evaporates while cooking.

There's a similar question asked here: How do I avoid dry meat and burned masala and rice in biryana?