Idli and dosa both start from the same fermented batter, and the fermentation is not a flavour garnish — it's what gives idli its lightness and dosa its characteristic tang and crisp edge. Unlike a quick-leavened batter, this one relies on naturally occurring bacteria and wild yeast on the rice and dal to produce lift and sourness over several hours, which means it behaves differently depending on temperature, humidity, and even altitude. Most of the frustration home cooks have with this batter — it won't rise, it tastes off, it's inconsistent week to week — comes down to a handful of variables that are straightforward to control once you know what you're looking for.
Ratios and soaking
A standard starting ratio is about 4 parts rice (a mix of idli rice or parboiled rice and a small amount of raw rice) to 1 part whole urad dal, though some cooks push closer to 3:1 for a softer, more aerated idli. A small handful of poha (flattened rice) or a few fenugreek seeds soaked in with the rice is common, since both are believed to help fermentation and texture, fenugreek in particular contributing a subtle depth and slightly better rise. Rice and dal are soaked separately, typically for 4 to 6 hours or overnight, in plenty of water, since both need to fully hydrate before grinding; under-soaked grains grind coarse and gritty rather than smooth, which affects both the batter's texture and how evenly it ferments.
Why the dal is ground the way it is
Urad dal is ground separately from the rice, and the goal is a light, fluffy, aerated paste rather than a dense purée — this is the single most important technical step in the whole process. As the dal grinds with small additions of cold water, it should noticeably increase in volume and turn pale and foamy on top, almost like a thick batter with air whipped into it; a wet grinder or a high-powered blender run in short bursts, scraping down and adding water gradually, achieves this better than one long continuous grind, which can heat the batter and knock the air back out. This aeration matters because it's largely what gives steamed idli their characteristic light, spongy texture; a dal paste ground dense and heavy produces a flat, compact idli regardless of how well the batter later ferments. The rice is ground separately to a slightly coarser, grainy paste — not perfectly smooth — since some texture here contributes to a better crumb, then the two pastes are combined and mixed thoroughly by hand before fermenting.
Finding a warm enough spot
Fermentation works best in a warm, draft-free environment, roughly 25 to 30°C (77 to 86°F); below that range, fermentation slows dramatically and can take 12 hours or more, while well above it, the batter can over-ferment and turn excessively sour or watery. In a warm climate or during summer, batter left covered on a kitchen counter overnight usually ferments well on its own. In a cold kitchen or winter, common workarounds include placing the covered bowl inside an oven with just the oven light on (not the heat), using a yogurt-maker or an instant pot's yogurt setting on low, or wrapping the bowl in a thick towel and setting it near a warm appliance. The batter container should always leave room for the batter to roughly double in volume, since a tightly packed or fully sealed container can overflow or trap the batter's rise.
Reading properly fermented batter
Properly fermented batter looks visibly risen, often doubled, with a slightly domed, airy surface rather than a flat one, and small bubbles are usually visible both on top and when you spoon into it. It smells faintly sour and slightly yeasty, a clean, appetising tang rather than anything sharp or unpleasant, and it has a light, slightly spongy feel when you fold a spoon through it rather than a dense, heavy one. The colour typically turns a touch paler as fermentation progresses. Batter at this stage is ready to use for either idli or dosa, though many cooks note that very freshly fermented batter steams into slightly fluffier idli, while batter that's rested another day in the refrigerator often spreads and crisps better as dosa, since it's had more time to loosen and turn slightly more acidic.
Telling under-fermented from spoiled
Under-fermented batter is simply batter that hasn't had enough time or warmth: it stays dense, doesn't rise much if at all, and smells mostly of raw rice and dal rather than anything sour. The fix is more time in a warmer spot, not more salt or more water. Spoiled batter is a different thing altogether and should be discarded rather than used: it develops an off, acrid, or distinctly unpleasant smell rather than a clean sour tang, may show discolouration — grey, pink, or dark patches — rather than an even pale colour, and can develop a slimy rather than light-and-airy texture. Batter left unrefrigerated for more than a day or two in warm weather, or kept well past a week even refrigerated, is the most common cause of spoilage, and any batter that smells wrong should not be tasted to confirm — trust the smell and appearance and start a fresh batch.
Salt timing and fermentation speed
Salt is a point of real debate among home cooks: adding it before fermentation can slightly slow the process, since salt draws out moisture and affects the microbial activity that drives fermentation, while adding it after fermentation has no effect on speed but means seasoning a batter that's already thickened and needs re-mixing. Many cooks add a modest amount of salt before fermenting specifically because a slightly slower, more controlled fermentation is easier to manage than a very fast one, which can over-sour the batter before it's convenient to use. If you're fermenting in a very warm climate where the batter tends to turn too sour too quickly, holding the salt back until after fermentation, or using slightly less dal relative to rice, can help moderate the pace.
Storing fermented batter
Once fermented to your liking, batter should be refrigerated to slow further fermentation; left at room temperature, it continues to sour and eventually turns unusably sharp within a day or two. Refrigerated, it typically keeps well for 4 to 5 days, continuing to develop a slightly more pronounced tang as it sits, which many dosa cooks actually prefer. Give refrigerated batter a gentle stir before using, since it separates slightly on standing, and let it sit at room temperature for a short while before cooking if it's been very cold, since batter straight from the refrigerator spreads less easily on a hot tawa.