If your idlis come out like small hockey pucks, the problem almost never begins at the steamer. It began the night before, in the bowl. Fermentation is what makes idli soft and dosa tangy and lacy at the edges, and it is not a decorative step. No baking soda is involved. Natural bacteria and wild yeast living on the grains do the work over many hours, so the same recipe gives different results in a cool flat in January and a warm kitchen in July. The usual complaints, a batter that will not rise, one that smells wrong, one that works one week and fails the next, all come down to a few controllable factors. Once you know them, you can stop guessing.
Ratios and soaking
Begin with roughly four measures of rice to one of whole urad dal. For the rice, idli rice or parboiled rice does best, with a little raw rice blended in. Cooks who want a softer, fluffier idli push the ratio toward three to one. Dropping a small handful of poha or a few fenugreek seeds into the rice is a common habit, and the fenugreek is credited with extra depth and a slightly better rise. Soak the rice and dal in separate bowls with plenty of water, anywhere from four to six hours, or overnight. Grains that are only half soaked grind into a gritty paste, and gritty batter ferments unevenly, so be patient here.
Why the dal is ground the way it is
This is the step that decides your idli. You are not making a purée. You are making foam. Grind the dal on its own, trickling in cold water as you go, and watch it swell, lighten and turn frothy until it looks like whipped batter. A wet grinder is ideal. If you only have a blender, work in short pulses, scrape the sides, add water a little at a time and rest between rounds. Grinding non-stop warms the paste and knocks out the air you just worked in. Why bother? Trapped air is what lets the steamed idli spring up soft. A dense, heavy dal paste gives a flat idli even if fermentation goes beautifully. Grind the rice separately into a paste that is a little grainy, since that texture helps the crumb. Then stir the two together with your clean hand and leave the bowl to ferment.
Finding a warm enough spot
Aim for a draft-free place between about 25 and 30°C (77 to 86°F). Cooler than that and the batter drags along for twelve hours or more. Much warmer and it can go over the top, turning very sour or watery. In summer, a covered bowl on the counter usually does the job overnight. In a cold kitchen, try one of these tricks: put the covered bowl in the oven with only the light switched on, use the yogurt setting on an electric pressure cooker or yogurt maker, or wrap the bowl in a heavy towel beside a warm appliance. Fill the container only partway. The batter should have room to climb to about twice its size, and a lid clamped tight on a full pot can end in a mess.
Reading properly fermented batter
You will know it is ready when the surface looks puffed and rounded and the volume has roughly doubled. Tiny bubbles show on top and appear again whenever you dig a spoon in. Smell it. Ready batter has a mild sourness with a hint of yeast, pleasant and clean, not sharp. Stir it and it should feel airy. Often the colour lightens slightly. From this stage you can make either idli or dosa. Plenty of cooks say that batter used right away makes the fluffiest idli, whereas batter held one more day in the refrigerator spreads thinner and crisps better on the tawa, because it has loosened and turned a bit more acidic.
Telling under-fermented from spoiled
A batter that is merely under-fermented is dense, hardly bigger than when you started and smells of raw rice and dal. Nothing is wrong with it. It needs more hours or a warmer shelf, not extra salt and not extra water. Spoiled batter is a separate matter, and you should discard it. Warning signs include a harsh or unpleasant smell instead of a clean tang, patches of grey, pink or dark colour, and a slimy feel. Batter left out for over a day or two in hot weather, or kept beyond about a week even in the refrigerator, goes bad most often. If something smells off, do not taste it to double-check. Trust your nose and eyes and make a new batch.
Salt timing and fermentation speed
Cooks argue about when to add salt. Early salting can slow the process a little, since salt pulls out moisture and affects the microbes. Salting after fermentation does not affect speed, but you then have to stir a batter that has already thickened. Many people salt modestly at the start for a gentler rhythm, because a fast fermentation can sour the batter before you are ready to cook. If your kitchen is so hot that batter sours too quickly, hold the salt until the end, or use slightly less dal compared with rice, and the pace will ease.
Storing fermented batter
When the batter looks and smells right, move it to the refrigerator to slow things down. Left on the counter, it keeps souring and soon becomes too sharp to enjoy. In the cold it should stay good for four to five days, getting tangier as it sits, which dosa lovers often prefer. Give it a gentle stir before use, because it separates a little. Let a very cold batch warm up on the counter for a short while, or it will refuse to spread on a hot tawa.