You must avoid opening the lid frequently to check the fermentation. This practice helps you prevent temperature changes inside the vessel. You must use rice and urad dal in the exact correct ratio. You must also ensure the batter does not become too watery while grinding.
The batter shows clear signs when it ferments properly. The volume increases, small bubbles appear on the surface, and a mild sour smell comes out. You must immediately place the batter in the refrigerator once you notice these signs. You should not leave it outside for a long time. Adding a green chilli is a simple trick. However, the correct temperature and ingredient ratio remain the basic factors for perfect batter fermentation.