Adding a small amount of rice flour to urad dal batter for vadas helps to absorb excess moisture, resulting in crispier vadas that hold their shape better during frying.
Urad dal, when ground into a batter, can sometimes become too moist, leading to vadas that are soggy or prone to breaking apart in the hot oil. Rice flour acts as a binding agent and a drying agent. It absorbs some of the excess water in the batter, creating a more stable and manageable dough. This results in vadas that are crispier on the outside and have a better texture overall. The starch in the rice flour also contributes to the browning and crisping process during frying, enhancing the vada's appeal. The amount of rice flour added is typically small, usually just a tablespoon or two per cup of urad dal, as too much can make the vadas dense and hard. The ideal ratio ensures a balance between moisture absorption and maintaining the light, airy texture characteristic of well-made vadas.
Always soak the urad dal for the correct amount of time (usually 4-6 hours) before grinding. Over-soaking can lead to a batter that's too watery, requiring more rice flour and potentially affecting the vada's texture. Under-soaking will make it difficult to grind into a smooth batter.