Idli and dosa start from identical fermented batter, and the fermentation isn't decoration — it's the entire reason idli turns out light and dosa turns out tangy with a crisp edge. Unlike a chemically leavened batter, this one relies on naturally occurring bacteria and wild yeast living on the rice and dal to build lift and sourness over several hours, so it behaves differently depending on temperature, humidity, even altitude. Most of what frustrates people about this batter — it won't rise, it tastes off, it comes out differently every week — traces back to a small set of variables that are easy to manage once you know what's actually driving them.
Getting the rice-to-dal ratio and soak right
A common starting ratio is 4 parts rice — idli rice or parboiled rice with a little raw rice mixed in — to 1 part whole urad dal; some cooks push toward 3:1 for a softer, more aerated idli. Many also soak in a small handful of poha, or a few fenugreek seeds, alongside the rice, since both are thought to help fermentation and texture — fenugreek especially, which deepens flavor slightly and improves the rise. Soak rice and dal separately for 4 to 6 hours, or overnight, in plenty of water. Both need to hydrate fully before grinding; grains that haven't soaked long enough grind coarse and gritty, which hurts the batter's texture and its ability to ferment evenly.
The dal grind decides the texture
Urad dal gets ground alone, and the goal is a light, fluffy, aerated paste — not a dense purée — arguably the single most important step in the entire process. While grinding, add cold water in small amounts; the dal should visibly grow in volume and turn pale and foamy on top, almost like a batter whipped full of air. A wet grinder, or a high-powered blender run in short bursts with scraping and gradual water additions, gets there far better than one long continuous grind, which heats the batter and knocks the air straight back out. That aeration is largely responsible for an idli's light, spongy crumb — a dense dal paste produces a flat, compact idli no matter how well the batter ferments afterward. Rice gets ground separately into a slightly coarser, grainy paste rather than something perfectly smooth, since a bit of texture here improves the finished crumb. Combine the two pastes and mix thoroughly by hand before letting the batter ferment.
Finding warmth for fermentation
Fermentation runs best somewhere warm and draft-free, roughly 25 to 30°C (77 to 86°F). Drop below that and it slows sharply, sometimes taking 12 hours or longer; climb well above it and the batter risks over-fermenting into something excessively sour or watery. Warm climates and summer months usually need nothing special — batter covered and left on the counter overnight ferments fine on its own. Cold kitchens and winter call for a workaround: an oven with just the light bulb on and no heat, a yogurt maker, an instant pot's yogurt setting on low, or a bowl wrapped in a thick towel near a warm appliance. Whatever container you choose, leave room for the batter to roughly double — pack it too tightly or seal it completely and it can overflow or have its rise choked off.
What properly fermented batter looks like
Properly fermented batter rises visibly, often doubling, with a slightly domed and airy surface rather than a flat one, and small bubbles appear both on top and when you dip a spoon in. It smells faintly sour and a little yeasty — a clean, appetizing tang rather than anything sharp or off — and feels light and slightly spongy as a spoon moves through it, not dense or heavy. The color tends to pale slightly as fermentation progresses. Batter at this stage works for either dish, though freshly fermented batter tends to steam into fluffier idli, while batter rested an extra day in the fridge spreads and crisps better as dosa, having had more time to loosen and turn a touch more acidic.
Under-fermented is fixable; spoiled is not
Under-fermented batter just needs more time or more warmth — it stays dense, barely rises, and smells mostly of raw rice and dal rather than anything sour. More time in a warmer spot fixes it; more salt or more water does not. Spoiled batter is a different problem, and the only fix is to discard it: it develops an off, acrid or clearly unpleasant smell instead of a clean sour tang, may show discoloration — grey, pink or dark patches instead of an even pale color — and can turn slimy instead of light and airy. The most common cause is batter left unrefrigerated more than a day or two in warm weather, or kept past a week even in the fridge. If the smell seems wrong, trust that instinct and start a fresh batch rather than tasting to confirm.
When to add salt, and why it changes the speed
Home cooks genuinely disagree about salt timing. Add it before fermentation and it can slightly slow things down, since salt draws out moisture and affects the microbial activity driving the process. Add it after, and speed doesn't change, but you're now seasoning a batter that's already thickened and needs re-mixing. Plenty of cooks salt before fermenting on purpose, since a slower, more controlled rise is easier to manage than a fast one that turns over-sour before it's convenient to use. In a very warm climate, where batter tends to sour too fast regardless, holding the salt back until after fermentation — or using slightly less dal relative to rice — helps slow the whole process down.
Storing batter once it's fermented
Refrigerate the batter once it's fermented to your liking; left at room temperature, it keeps souring and turns unusably sharp within a day or two. Refrigerated, it holds well for 4 to 5 days, picking up a more pronounced tang the longer it sits — a change plenty of dosa cooks actually prefer. Give it a gentle stir before using, since it separates slightly while standing, and let very cold batter sit at room temperature briefly before cooking, since batter straight from the fridge doesn't spread as easily on a hot tawa.