Households in Tamil Nadu, Karnataka, Kerala and Andhra have long kept a bowl of batter quietly working on the counter, because idli and dosa both start from the same fermented mixture. Fermenting is far more than a flavour step, since it is the reason idli turns out light and dosa gets its tang and crisp edge. Unlike a batter raised quickly with a chemical leavener, this one relies on bacteria and wild yeast that live on the rice and dal and produce lift and sourness over many hours, so it reacts to temperature, humidity and even altitude. Most of what home cooks grumble about, such as a batter that will not rise, tastes odd or changes from week to week, traces back to a few factors that are easy to manage once you know where to look.
Choosing ratios and soaking
A usual starting point is roughly 4 parts rice, being idli rice or parboiled rice with a little raw rice mixed in, to 1 part whole urad dal, though some cooks shift toward 3:1 for a softer, airier idli. Many add a small handful of poha (flattened rice) or a few fenugreek seeds to the rice soak, as both are thought to support fermentation and texture, fenugreek giving subtle depth and a slightly better rise. Keep the rice and the dal in separate bowls of generous water, about 4 to 6 hours or a whole night, because each must be fully hydrated before it meets the grinder; grains soaked too briefly come out coarse and gritty, which harms the texture of the batter and the evenness of its fermentation.
How the dal should be ground
Grind the urad dal on its own, aiming for a paste that is light, fluffy and full of air rather than a dense purée; of all the technical steps, this one counts most. Add cold water in small splashes as it turns, and the dal should swell visibly and go pale and foamy at the surface, like a thick batter with air whipped through it. A wet grinder does this best, or a strong blender run in short bursts with scraping and gradual water, in preference to one long grind that can warm the batter and knock the air out. Aeration matters because it is largely what gives steamed idli their light, spongy crumb, and a dal paste ground dense and heavy yields a flat, compact idli no matter how well it ferments afterward. The rice is ground to a slightly coarser, grainy paste rather than a perfectly smooth one, since some texture improves the crumb, and the two pastes are then combined and mixed thoroughly by hand before fermenting.
Locating a warm enough place
The best place for fermenting is warm and free of drafts, around 25 to 30°C (77 to 86°F). Colder than that and the process drags, sometimes past 12 hours; much hotter and the batter can over-ferment into something too sour or thin. Where the climate is warm, or in summer, batter left covered on the counter overnight usually ferments well unaided. In a cold kitchen or in winter, popular workarounds include setting the covered bowl in an oven with only the light on and no heat, using a yogurt maker or a pressure cooker's low yogurt setting, or swaddling the bowl in a heavy towel near a warm appliance. Whichever container you choose, allow space for the batter to about double in volume, because a tightly packed or fully sealed one can overflow or trap the rise.
Recognising well-fermented batter
A batter that has fermented well has clearly climbed, often to twice its starting height, with a gently domed, airy top instead of a flat one, and you will see small bubbles at the surface and again as a spoon goes in. It smells faintly sour and a little yeasty, a clean and appetising tang with nothing sharp or unpleasant, and it feels light and slightly spongy when a spoon is folded through, not dense and heavy. The colour often turns a touch paler as fermentation advances. At this stage the batter is ready for idli or dosa, although many cooks find that very fresh batter steams into slightly fluffier idli, while batter rested another day in the refrigerator often spreads and crisps better as dosa, having had longer to loosen and become a little more acidic.
Under-fermented versus spoiled
If the batter is under-fermented, it has just not had enough time or warmth: it sits dense, scarcely lifts, and carries the smell of raw rice and dal without any sourness; the cure is more time in a warmer place, not extra salt or water. Spoiled batter is another matter and belongs in the bin: it gives off an acrid or plainly unpleasant smell in place of a clean tang, may show grey, pink or dark patches instead of an even pale colour, and can turn slimy rather than light and airy. Spoilage usually follows batter left out of the refrigerator for over a day or two in warm weather, or kept past a week even when chilled. Do not taste a suspect batter to test it; let the smell and look decide, and begin again with a new batch.
When to add the salt
When to add salt is a real point of disagreement among home cooks. Salting before fermentation can slow the process slightly, because salt draws out moisture and affects the microbial activity behind the rise, whereas adding it afterward leaves the speed untouched but means seasoning a batter that has already thickened and must be mixed again. Many cooks salt modestly beforehand precisely because a slower, more controlled fermentation is easier to manage than a very fast one that over-sours the batter before it is convenient to use. In a very hot climate where the batter sours too quickly, holding the salt back until afterward, or using a little less dal relative to rice, can moderate the pace.
Keeping fermented batter
When the batter has reached the sourness you want, put it in the refrigerator to stop it souring further; left on the counter it keeps turning sharper and is too acidic to use after a day or two. Chilled, it usually stays good for 4 to 5 days, gaining a little more tang over time, a change that many dosa makers in fact like. Stir gently before use, since it separates a little on standing, and let very cold batter sit at room temperature briefly, because batter straight from the refrigerator spreads less easily on a hot tawa.