A vendor making vada beats the ground lentil batter by hand for several minutes before frying. The beating is not mixing, and skipping it produces a visibly different snack.
The batter holds air the way an egg white does
Ground soaked lentils release proteins that can form films around air bubbles. Vigorous beating incorporates air and those proteins stabilize it in the thick paste.
The batter visibly lightens in color and increases in volume as this happens, which is the cue cooks use rather than a fixed beating time.
A common test is dropping a small amount into water. Aerated batter floats, because the trapped air has lowered its density enough to change how it behaves.
Trapped air becomes the interior texture
In hot oil, those bubbles expand and the steam generated inside enlarges them further. What was a dense paste becomes an open, spongy interior.
Without beating, the same ingredients fry into a solid, heavy lump. The exterior can still be crisp while the inside is unpleasantly compact.
The contrast between a crisp shell and a light interior is the entire appeal of the item, so the beating is closer to a structural step than a finishing touch.
Aeration also limits oil uptake
A batter that expands quickly in oil pushes steam outward through the surface, and that outward flow resists oil moving in during the early minutes of frying.
A dense batter generates less internal steam, so oil penetrates more freely and the finished piece is heavier and greasier for the same frying time.
This is why a badly made vada feels oily rather than merely rich. The fault is usually in the beating rather than in the temperature of the oil.
Grinding conditions decide whether beating works
The batter must be ground thick, with as little water as the grinder will tolerate. A thin batter cannot hold bubbles and simply spreads in the oil.
Heat is the other enemy, since a fast blender warms the paste and degrades the proteins that would otherwise stabilize the foam.
Traditional wet grinders turn slowly and stay cool for this reason. A home cook using a US countertop blender usually grinds in short bursts with ice water to compensate.
The window between beating and frying is short
Aerated batter starts losing air as soon as it is left standing, so vendors beat only what they are about to fry.
Batter held for an hour needs beating again, and batter left overnight has usually begun fermenting, which is a different preparation with a different flavor.
This is one reason vada is made continuously through a service rather than fried ahead, since the item depends on a batter state that does not survive being held.