Both idli and dosa start from the exact same fermented batter, and the fermentation step is doing real work, not just sitting there for tradition's sake — it's what makes idli light and gives dosa its tang and crisp edge. This batter depends on naturally occurring bacteria and wild yeast living on the rice and dal to build lift and sourness over several hours, which means it reacts to temperature, humidity, and even altitude in ways a quick-leavened batter never would. Nearly all the trouble home cooks run into here — a batter that won't rise, tastes off, or behaves differently from one week to the next — traces back to a small, learnable set of variables.
Ratios and soaking
Roughly 4 parts rice (commonly a blend of idli or parboiled rice with a small portion of raw rice) to 1 part whole urad dal is a typical starting ratio, 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 alongside the rice is also common, both thought to support fermentation and texture, with fenugreek adding a subtle depth and slightly better rise. Rice and dal soak separately, usually 4 to 6 hours or overnight in plenty of water, since each needs full hydration before grinding — grains that haven't soaked long enough grind coarse and gritty, which hurts both texture and how evenly the batter ferments.
Why the dal is ground the way it is
Arguably the single most important technical step in the whole process is grinding urad dal separately from the rice into a light, fluffy, aerated paste rather than a dense purée. Ground with small additions of cold water, the dal should visibly expand in volume and turn pale and foamy on top, almost like a batter with air whipped straight into it — something a wet grinder or a powerful blender run in short bursts, scraping down the sides and adding water gradually, achieves far better than one continuous long grind, which heats the batter and knocks the air back out. This aeration is largely what gives steamed idli their light, spongy crumb; a dense, heavy dal paste produces a flat, compact idli no matter how well the batter goes on to ferment. Rice, by contrast, gets ground into a slightly coarser, grainy paste rather than a perfectly smooth one, since a little texture there improves the final crumb, and only once both pastes are ready do they get combined and mixed thoroughly by hand before fermentation begins.
Finding a warm enough spot
Somewhere around 25 to 30°C (77 to 86°F), draft-free, is where fermentation performs best; fall below that range and it slows drastically, sometimes taking 12 hours or longer, while going well above it risks over-fermenting the batter into something excessively sour or watery. A warm climate or summer months usually mean batter left covered on the counter overnight ferments well without any help. In a colder kitchen or during winter, common workarounds include setting the covered bowl inside an oven with only the oven light on (never the heat), using a yogurt-maker or an instant pot's low yogurt setting, or wrapping the bowl in a thick towel near a warm appliance. Whatever container holds the batter needs room for it to roughly double, since a tightly packed or fully sealed container risks overflowing or trapping the rise.
Reading properly fermented batter
A visibly risen batter, often doubled, with a slightly domed, airy surface rather than a flat one, and small bubbles visible both on top and when a spoon dips in, signals proper fermentation. The smell should read faintly sour and slightly yeasty — a clean, appetising tang rather than anything sharp or off-putting — and the texture should feel light and a little spongy when a spoon folds through it, not dense or heavy; the colour typically turns a shade paler as fermentation proceeds. Batter at this stage works for either idli or dosa, though many cooks find freshly fermented batter steams into fluffier idli, while batter rested another day in the fridge spreads and crisps better as dosa, having had extra time to loosen and turn slightly more acidic.
Telling under-fermented from spoiled
Under-fermented batter simply hasn't had enough time or warmth: it stays dense, barely rises if at all, and smells mostly of raw rice and dal rather than anything sour, and the remedy is more time somewhere warmer, not extra salt or water. Spoiled batter is an entirely different problem and belongs straight in the bin — it carries an off, acrid, or outright unpleasant smell instead of a clean sour tang, may show discolouration in grey, pink, or dark patches rather than an even pale tone, and can turn slimy instead of light and airy. The usual cause is batter left unrefrigerated for more than a day or two in warm weather, or kept well past a week even in the fridge, and any batter that smells wrong should be discarded rather than taste-tested to confirm — trust the smell and start fresh instead.
Salt timing and fermentation speed
Whether to salt before or after fermenting is a genuine point of disagreement: salt added before the process can slow it down slightly, since it pulls out moisture and affects the microbial activity driving fermentation, while salt added afterward has no effect on speed but means re-mixing batter that's already thickened to season it evenly. Many cooks still salt before fermenting anyway, specifically because a slower, more controlled fermentation is easier to manage than a fast one that risks over-souring before it's convenient to use. In a particularly warm climate where batter tends to sour too fast, holding salt back until after fermentation, or cutting the dal slightly relative to rice, can help slow the pace.
Storing fermented batter
Refrigeration slows things down once the batter has fermented to the desired point; left at room temperature, it keeps souring and turns too sharp to use well within a day or two. Refrigerated, it typically holds 4 to 5 days, gradually developing a more pronounced tang that many dosa cooks actually prefer. A gentle stir before use is worth doing since it separates slightly on standing, and letting it sit out briefly before cooking helps if it's been very cold, since batter straight from the fridge doesn't spread as easily on a hot tawa.