The same fermented batter underlies both idli and dosa, and fermentation here isn't some flavor extra bolted onto the base — it's where idli gets its lightness and dosa gets its tang and crisp edges. Unlike a quick chemically-leavened mix, this batter leans on wild yeast and naturally present bacteria living on the rice and lentils to build both lift and sourness across several hours, so results shift with temperature, humidity, and even elevation. When home cooks run into trouble with this batter — no rise, an off taste, results that vary week to week — the cause nearly always sits inside a short list of variables, each one manageable once you know what it's actually doing.
Getting the ratio and the soak right
A workable starting ratio runs about four parts rice — mostly idli or parboiled varieties, with a small portion of raw rice folded in — against one part whole urad dal; some cooks prefer leaning closer to a 3:1 split for an idli with more air and a softer bite. Many recipes also call for a small handful of flattened rice (poha), or a pinch of fenugreek seeds added to the soaking rice, since each is believed to help the ferment along and improve texture, with fenugreek especially credited for a faint depth of flavor and a touch more lift in the finished batter. Keep rice and dal soaking in separate bowls, usually for four to six hours, or left overnight, in a generous amount of water, because full hydration before grinding matters for both; grains that haven't had enough time underwater grind up coarse and gritty, and that unevenness carries through into both texture and fermentation speed.
The logic behind how the dal gets ground
Urad dal is ground alone, and the aim is a light, airy texture close to whipped cream, not a heavy, dense mash — likely the one step in this entire method that matters most on the technical side. While it grinds, with cold water worked in a little at a time, it should puff up noticeably and turn a pale, foamy white across the top, the way a batter looks once air has been beaten into it on purpose. A wet grinder, or a powerful blender pulsed on and off rather than run continuously, with the sides scraped often and water added only a splash at a time, gets this done far better than one long, uninterrupted grind, which warms the mixture and knocks that air straight back out. This trapped air is largely what gives a steamed idli its soft, spongy crumb; even a flawless ferment can't rescue a heavy, dense dal paste from baking into something flat and compact. Rice, meanwhile, gets ground on its own and left a touch coarse and grainy instead of perfectly smooth, since a little texture there helps the finished crumb — only once both pastes are ready do they join together, worked into one batter by hand, ahead of the ferment.
Finding warmth the batter can actually use
This batter rises best somewhere warm and free of drafts, with the sweet spot sitting between 25 and 30°C — that's about 77 to 86°F; drop below that band and things slow down dramatically, sometimes stretching past 12 hours, while climbing well above it risks a batter that turns excessively sour or watery instead of properly risen. During summer, or in a naturally warm climate, simply covering the bowl and setting it out on the counter overnight is typically all it takes. Cold kitchens, or winter months, call for a bit more engineering: an oven with just its inside bulb left on (never the heat), the low-yogurt program on an instant pot, or a bowl wrapped in a heavy towel set beside some other warm appliance. Whatever method gets used, always leave the container enough headroom for the batter to roughly double in size, since a tightly packed or sealed-up container risks overflowing or smothering the rise entirely.
Spotting batter that's fermented the way it should
Batter that's fermented properly looks visibly risen, often to about double its starting volume, with a surface that's slightly domed and airy rather than flat — and you should be able to spot small bubbles both on top and inside when a spoon cuts through it. The smell should land as faintly sour and a touch yeasty: a clean, appetite-friendly tang, not anything sharp or unpleasant. It should also feel light and a little springy when a spoon folds through it, never dense or heavy, and the color typically turns a shade paler as fermentation moves forward. Batter at this point suits both idli and dosa equally, although plenty of cooks notice a difference: used right after fermenting, it tends to steam up into a fluffier idli, whereas a second day resting in the fridge leaves it thinner and better suited to crisping as dosa, having sat long enough to loosen up and pick up a touch more acidity.
Telling under-fermented batter from batter that's actually gone bad
Batter that hasn't fermented enough simply hasn't had the time or the warmth it needs yet — it stays dense, shows little or no rise, and smells mostly like raw rice and lentils rather than anything sour. The fix is more time somewhere warmer, not extra salt or water. A batter that's genuinely spoiled is a different situation altogether, and should go straight in the bin: it carries an acrid, off, distinctly unpleasant odor rather than a clean sour note, it may show patches of grey, pink, or dark discoloration instead of an even pale tone, and it can turn slimy rather than staying light and airy. The usual culprits are batter left out unrefrigerated for a day or two too many in warm weather, or batter kept past a week even inside the fridge — and batter that smells wrong isn't worth tasting to double-check. Trust your nose and your eyes, and start a fresh batch instead.
Where salt belongs in the timeline
Salt is genuinely contested territory among home cooks: stirring it in before fermentation slows the process down a little, since salt draws out moisture and interferes somewhat with the microbial activity driving the rise, whereas adding it afterward leaves the speed unaffected but means remixing a batter that's already thickened up. Plenty of cooks still salt early on purpose, on the theory that a slow, steady ferment stays easier to control than a quick one liable to turn sour before it's convenient to cook with. In a particularly warm climate, where batter tends to turn sour too quickly, waiting to salt until the ferment's done — or trimming the dal back a little relative to the rice — helps slow things down.
Storing batter once it's fermented
Once the batter has fermented the way you want it, move it to the refrigerator to keep fermentation from running further; skip that step and it just keeps turning more sour, becoming too sharp to use within one to two days at room temperature. Chilled, it generally holds up for four to five days, building a bit more tang the longer it sits in there, a quality plenty of dosa cooks actually like. Stir it gently before you use it, since standing causes some separation, and give very cold batter a little time to warm up before cooking — cold batter straight out of the refrigerator won't spread evenly across a hot tawa.