Dosa and idli batter is built from rice and urad dal in a ratio that varies by household and region but almost never drops one of the two. Each ingredient performs a job the other cannot.
The dal provides the structure that holds gas
Urad dal contains proteins and a mucilaginous compound that, once ground, produce a viscous, sticky batter capable of trapping bubbles.
Grinding it with small additions of water incorporates air directly, so the batter is already aerated before fermentation begins.
Without that network the gas produced during fermentation simply escapes, which is why an all-rice batter goes sour without ever rising.
The rice provides the fuel and the finish
Rice contributes the bulk of the starch, which the microbes convert into the acids and gas that make the batter rise and sour.
It also determines the finished texture: a higher proportion of rice gives a crisper dosa, while more dal gives a softer, spongier result.
Different rice types behave differently here too, with parboiled rice giving body and raw rice giving crispness, which is why many recipes use both.
Fermentation is wild and therefore local
No culture is added. The bacteria and yeasts come from the ingredients themselves, from the equipment and from the surrounding air.
That is why a batter made from the same recipe behaves differently in different kitchens and why moving house changes a family's results.
It also means temperature governs everything, since warmth speeds the process dramatically and a cool room can stall it for a day or more.
Grinding method changes the outcome
A traditional stone grinder works slowly with a crushing action, incorporating air while keeping the batter cool.
A high-speed blender generates heat that can damage the proteins and drive off the aeration, which is why recipes written for one do not transfer cleanly to the other.
Cooks using a blender compensate with ice water and short bursts, essentially trying to restore the conditions the stone provided naturally.
Timing decides which dish it becomes
A young batter that has risen but not soured much makes better idli, where lightness matters more than tang.
An older batter is more acidic and thinner, which spreads well on a griddle and crisps more readily, making it better for dosa.
The same vessel therefore yields different dishes across successive days, which is why households manage the batter as a continuing process rather than a single preparation.