Morning in a South Indian kitchen: someone lifts a lid and the batter has pushed itself up into a dome, pocked with tiny bubbles, smelling gently of sour dough and yeast. That smell is the goal. Fermentation is what lets idli steam up soft and gives dosa its tang and lacy, crisp rim. It comes from bacteria and wild yeast that already live on the rice and dal, working through the night, so a cold evening or a humid week changes the outcome. When people complain that their batter will not rise, tastes odd or turns out different every week, the cause is nearly always one of a few things, and each is easy to control once you know the signs.
Ratios and soaking
Most households begin with roughly four measures of rice to one of whole urad dal. The rice is typically idli rice or parboiled rice with a spoonful of raw rice in it. People who like a softer, fluffier idli go closer to three to one. It is common to soak a handful of poha, or a pinch of fenugreek seeds, together with the rice. Both are believed to help the rise, and fenugreek adds a little depth. Keep rice and dal in separate bowls, cover each with generous water, and leave them for 4 to 6 hours or overnight. Everything needs to swell fully before it meets the grinder. Grains that were rushed grind up gritty, and gritty batter ferments unevenly and cooks into a coarse crumb.
Why the dal is ground the way it is
Of all the steps, this one makes the biggest difference. Grind the urad dal on its own, adding cold water by the spoonful, and keep going until the paste has swelled to a much larger volume and looks pale, glossy and foamy, almost like whipped cream. A wet grinder is ideal. A strong blender works too if you run it in short pulses, scrape the sides and add water slowly. A single long run heats the paste and beats out the air you are trying to trap. Trapped air is what gives a steamed idli its sponge. If the dal paste is thick and heavy, the idli will be flat and dense, however well the batter later ferments. The rice goes through separately and comes out finer than grits yet slightly coarse, with a bit of grain you can feel, which helps the crumb. Fold the two pastes together with your clean hand until no streaks remain, then set the bowl aside.
Finding a warm enough spot
Aim for a draft-free corner at about 25 to 30°C (77 to 86°F). Cooler than that and the batter creeps along, sometimes needing 12 hours or longer. Much hotter and it can run away from you, turning sharply sour and thin. In a warm summer kitchen a covered bowl on the counter overnight usually does the job unaided. In winter, try one of these: set the covered bowl in an oven with only the light on, never the heat; use the yogurt setting on an electric pressure cooker or a yogurt maker; or wrap the bowl in a heavy towel and park it beside something that stays warm. Pick a vessel big enough for the batter to double. If you fill it to the brim or clamp the lid on tight, it will spill or choke.
Reading properly fermented batter
Ripe batter tells you in three ways. It looks lifted, often twice its first height, with a rounded, bubbly top instead of a flat one and fine bubbles that show again when you scoop out a spoonful. It smells mildly sour and yeasty, a pleasant tang with nothing harsh in it. And it feels airy when you stir, never heavy or pasty. The colour usually fades a touch lighter. At this stage you can make idli or dosa. Many cooks say the freshest batter steams the fluffiest idli, while batter that has spent another day in the refrigerator pours thinner and crisps better as dosa, because it has had longer to loosen and sour.
Telling under-fermented from spoiled
Batter that is merely under-fermented is not bad, only unfinished. It looks flat, barely lifts, and smells of plain raw rice and lentil. Give it more time somewhere warmer. Extra salt or water will not help. Spoiled batter is a different problem, and it goes in the bin. Its smell is sharp, sour in a rotten way, or simply wrong. You may see grey, pink or dark patches, and the texture can turn slimy where it should be light. Most spoilage starts when batter sits out for more than a day or two in warm weather, or hangs around in the refrigerator for over a week. If it smells off, do not taste it to be sure. Believe your nose, and start again.
Salt timing and fermentation speed
Cooks disagree about when to add salt. Put it in before fermenting and it may slow the microbes a little by pulling out moisture. Put it in afterwards and speed is untouched, but you must stir salt into batter that has already thickened. Plenty of cooks salt early on purpose, because they prefer a calmer, more predictable ferment to a racing one that sours before dinner is ready. If you live somewhere hot and your batter turns too sharp too fast, either hold the salt until the end or trim the proportion of dal to rice slightly to slow things down.
Storing fermented batter
When the batter reaches the stage you like, chill it. At room temperature it goes on souring, and after a day or two it is too sharp for anything. In the refrigerator it generally keeps for 4 to 5 days and grows a little tangier with each one, which many dosa lovers actually enjoy. Stir it gently before use, because it separates as it stands. Let a very cold batch warm up for a short while before the pan, since chilled batter does not spread well on a hot tawa.
A week of batter in practice
A simple routine helps if you cook these dishes often. On the first evening, soak, grind and set the bowl somewhere warm. Next morning, check the dome, the smell and the bubbles, then chill whatever you will not use that day. Steam idli on the first day, when the batter is lightest. Save the next few days for dosa and uttapam, when the extra sourness helps the crust. By day five, taste the batter in a cooked spoonful and judge whether it still pleases you. If the tang has become too strong, thin the batter with a little water and use it for savoury pancakes that welcome that sharpness. Then wash the container, because old batter left in a bowl seeds the next batch with flavours you do not want.