In the kitchens of South India, one bowl of fermented batter serves two quite different breakfasts, the soft steamed idli and the crisp, tangy dosa, and the souring is not a seasoning added for taste but the very thing that makes idli light and dosa lacy at the edges. Unlike a batter raised quickly with a chemical leavener, this one depends on bacteria and wild yeast that already live on the rice and dal, working slowly over many hours, so its behaviour shifts with temperature, humidity and even altitude. Most of the complaints cooks have about it, that it will not rise, that it tastes off, that it varies from week to week, trace back to a few variables that become easy to manage once you know what to watch.
Ratios and soaking
A common starting point is about 4 parts rice, usually idli rice or parboiled rice with a little raw rice mixed in, to 1 part whole urad dal, although some cooks move closer to 3:1 for a softer, more aerated idli. A small handful of poha, or a few fenugreek seeds soaked with the rice, is a frequent addition, since both are believed to help fermentation and texture, and fenugreek in particular is credited with a subtle depth and a slightly better rise. Rice and dal are soaked separately in plenty of water, usually for 4 to 6 hours or overnight, because both must hydrate fully before grinding; grains that are under-soaked grind coarse and gritty, which affects the texture of the batter and how evenly it ferments.
Grinding the dal for air
If one step decides the outcome, it is the dal grind. Urad is ground on its own, and the target is a whipped, foamy paste, not a heavy purée. Add cold water in small splashes while it turns; the mass should swell, lighten in colour and froth at the surface. Wet grinders manage this naturally, while a blender needs short pulses with pauses to scrape down, since a long continuous run warms the paste and drives out the air you have just built in. That trapped air is what later makes a steamed idli spongy, and a dense paste yields a flat, compact idli no matter how well the batter sours. Rice is ground separately and left a little grainy, because some tooth in the rice contributes to the crumb. Finally the two are combined and mixed thoroughly with clean hands, whose warmth and natural microbes are traditionally thought to help the batter along.
A warm place to work
Fermentation does best somewhere warm and free of draughts, roughly 25 to 30°C (77 to 86°F). Below that range it slows dramatically and can take 12 hours or more, and well above it the batter can over-ferment, turning very sour or watery. In a warm climate or in summer, a covered bowl left on the kitchen counter overnight usually ferments well without help. In a cold kitchen or in winter, cooks put the covered bowl in an oven with only the light switched on and no heat, use a yogurt maker or the low yogurt setting of an electric pressure cooker, or wrap the bowl in a thick towel and set it near a warm appliance. Whatever the container, leave room for the batter to roughly double, since a tightly packed or fully sealed vessel can overflow or hold back the rise.
Recognising batter that is ready
Look first for height: the batter should stand visibly taller, often doubled, with a gently domed, airy top and small bubbles on the surface and along the cut when a spoon breaks in. Then smell it, expecting a faint, pleasant sourness with a little yeast and nothing harsh. Finally fold a spoon through; it should feel light and springy rather than weighty. Colour usually lightens a shade as the process goes on. Once those signs appear, you can cook either dish, though some cooks find that batter at its first peak steams into the fluffiest idli, while batter held another day in the refrigerator spreads thinner and crisps better as dosa, having loosened and turned a bit more acidic.
Slow batter versus spoiled batter
A batter that is merely under-fermented has lacked time or warmth. It stays dense, barely rises, and smells of raw rice and dal instead of anything sour, and the remedy is a warmer spot and more hours, not extra salt or water. Spoilage is different and calls for throwing the batter away: the smell turns acrid or simply wrong, patches of grey, pink or dark colour appear in place of an even pale surface, and the texture goes slimy instead of airy. Warm weather without refrigeration for more than a day or two, or storage past about a week even in the refrigerator, is the usual cause. Never taste a doubtful batter to find out; trust the nose and eyes and start again.
When to add salt
Cooks disagree about salt. Put in before fermentation, it can slow the souring slightly, since it pulls out moisture and influences the microbes doing the work; put in afterwards, it leaves the speed alone but means stirring through a batter that has already thickened. A moderate pinch beforehand is common because a slower, more even ferment is easier to time than a fast one that over-sours before you are ready to cook. In very hot weather, where batter races ahead, salting only after fermentation or using slightly less dal against the rice can take some of the speed out.
Keeping the batter once it is done
Cold stops the clock, so put the finished batter in the refrigerator rather than leaving it out, where it would keep souring and become too sharp within a day or two. Chilled batter usually stays good for 4 to 5 days and grows tangier with time, which many dosa cooks welcome. Because it settles into layers, stir it gently before cooking, and let it lose its chill for a while, since cold batter spreads poorly on a hot tawa.