You can use betel leaves to ferment idli or dosa batter. Actually, betel leaves contain natural enzymes and bacteria, which speed up fermentation. You simply place 2-3 betel leaves in the vessel containing the batter and close the lid on top. You will get the batter fermented in 8 to 10 hours. This method is very effective in cold. After fermenting, remove the betel leaf and throw it away.