Idli and dosa start from the exact same batter, and the fermentation it goes through isn't a flavoring step added on top — it's the actual source of idli's lightness and dosa's tang and crisp edge. That puts this batter in a different category from a quick-leavened one: it depends on live bacteria and wild yeast already present on the rice and dal, working over several hours, which means temperature, humidity, and even altitude all change how it behaves. When a cook says this batter is inconsistent from one week to the next, or won't rise, or tastes off, the cause is almost never random — it's one of a short list of variables that behave predictably once their roles are understood.
The ratio and soak set up everything that follows
Most recipes start around 4 parts rice — typically idli rice or parboiled rice with a little raw rice mixed in — to 1 part whole urad dal, though a softer, more aerated idli often comes from pushing that closer to 3:1. Adding a small handful of poha, or a few fenugreek seeds, alongside the rice is common practice; both are thought to support fermentation and texture, and fenugreek in particular adds a bit of depth along with a slightly better rise. Rice and dal soak separately, usually 4 to 6 hours or overnight, in plenty of water, because both need to hydrate fully before they're ground — grains that go into the grinder under-soaked come out coarse and gritty, and that affects the batter's texture as much as how evenly it ferments.
Why the dal gets ground the way it does
The single most important technical step in this whole process is probably the dal grind. Urad dal is ground on its own, and the goal is a light, fluffy, aerated paste — not a dense purée — because that aeration is largely what gives a steamed idli its soft, spongy crumb; a heavy, dense dal paste produces a flat idli no matter how well the batter later ferments. As it grinds with small, gradual additions of cold water, the paste should visibly gain volume and turn pale and foamy on top, almost like it's had air whipped in. A wet grinder or a high-powered blender run in short bursts, with frequent scraping and gradual water additions, achieves that far better than one long continuous grind, which generates enough heat to knock the air back out. The rice, in contrast, is ground to a slightly coarse, grainy paste rather than a smooth one, since that texture improves the final crumb, and the two pastes are mixed together thoroughly by hand once both are ready.
Finding the right warmth for the batter to work in
This fermentation runs best somewhere warm and draft-free, roughly 25 to 30°C (77 to 86°F); drop much below that and it can take 12 hours or more to finish, while running well above it risks a batter that turns excessively sour or watery before it's convenient to use. A kitchen counter left covered overnight is usually enough during a warm season or in a warm climate generally. In a cold kitchen, the usual workarounds include an oven with just the light left on (never the heating element), a yogurt-maker or an instant pot's low yogurt setting, or a thick towel wrapped around the bowl near a warm appliance. Whatever container holds the batter needs room to spare, since it should roughly double in volume as it ferments, and a tightly sealed or overfilled container risks a mess or a stalled rise.
Reading a properly fermented batter
A batter that's fermented correctly looks visibly risen — often doubled — with a slightly domed, airy surface rather than a flat one, and small bubbles show up both on top and when a spoon dips in. The smell is a clean, faint sourness with a slightly yeasty edge, appetizing rather than sharp, and folding a spoon through it should feel light and a little spongy rather than dense. Color tends to pale slightly as fermentation progresses. Batter at this point works for either idli or dosa, though many cooks notice freshly fermented batter steams into a fluffier idli, while batter that's spent another day in the refrigerator spreads more easily and crisps better as dosa, having had time to loosen and turn a touch more acidic.
Under-fermented is a timing problem; spoiled is not
A batter that simply hasn't had enough time or warmth stays dense, barely rises if at all, and smells mostly like raw rice and dal rather than anything sour — more time somewhere warmer fixes it, not more salt and not more water. Spoiled batter is an entirely different situation, and the only correct response is to discard it rather than try to use it: it develops an off, acrid smell instead of a clean sour tang, may show discoloration — grey, pink or dark patches rather than an even pale color — and can turn slimy instead of light and airy. The usual cause is batter left unrefrigerated for a day or two in warm weather, or kept refrigerated well past a week. Any batter that smells wrong doesn't need tasting to confirm the problem; the smell and look alone are reason enough to start over.
Where salt fits into the timing
Cooks genuinely disagree on when to salt this batter, and both sides have a point. Salting before fermentation slows the process down somewhat, since salt pulls moisture out of the batter and affects the microbial activity driving the rise — which is exactly why plenty of cooks prefer it, since a slower, steadier fermentation is easier to manage than a fast one that can over-sour before it's convenient to use. Salting afterward doesn't change the speed at all, but it means re-mixing a batter that's already thickened in order to season it evenly. In a particularly warm climate, where batter tends to sour too fast regardless, holding the salt back until after fermentation — or cutting back slightly on the dal relative to the rice — helps slow the pace down.
Keeping fermented batter once it's ready
Once the batter has reached the point a cook wants, it belongs in the refrigerator, since fermentation at room temperature keeps going and turns the batter unusably sharp within a day or two. Refrigerated, it typically holds for 4 to 5 days, picking up a slightly more pronounced tang as it sits, which plenty of dosa cooks actually prefer over a freshly fermented batch. It separates somewhat on standing, so a gentle stir before use is worth doing, and letting a very cold batch sit at room temperature briefly before cooking helps it spread properly on a hot tawa instead of seizing up.