Idli and dosa share one fermented batter, and the fermentation is central to both. It gives idli its softness and gives dosa its faint tang and crisp edge. The process depends on bacteria and wild yeasts already present on the rice and dal, which produce gas and acidity over several hours, so its pace shifts with temperature, humidity and even altitude. Most complaints about this batter, such as failure to rise, an off taste or results that vary from week to week, trace back to a short list of variables that are easy to control once identified. This reference covers ratios and soaking, grinding, warmth, how to recognise a good batter, how to tell weak fermentation from spoilage, the question of when to add salt, and storage.
Ratios and soaking
A common starting proportion is about four parts of rice to one part of whole urad dal. The rice is usually idli rice or parboiled rice with a little raw rice, and some cooks move toward three to one for a softer, airier idli. A small handful of flattened rice, or a few fenugreek seeds, is often soaked with the rice, since both are believed to assist fermentation and texture, with fenugreek said to add depth and a slightly better rise. Rice and dal are soaked in separate containers, generally for four to six hours or overnight, in generous water, because both must hydrate fully before grinding. Grains that are under-soaked grind coarse and gritty, which harms the texture of the batter and the evenness with which it ferments.
Grinding the dal for air
The dal is ground apart from the rice, and the aim is a light, foamy paste in place of a dense purée. This is the most important technical step in the whole sequence. As the dal grinds with small additions of cold water, it should visibly increase in volume and turn pale and frothy, resembling a thick batter with air beaten into it. A wet grinder, or a strong blender run in short bursts with scraping and gradual water additions, achieves this better than one long continuous run, which can warm the paste and knock the air out. Aeration matters because it is largely responsible for the sponge-like texture of a steamed idli; a dense, heavy dal paste gives a flat, compact idli however well the batter later ferments. The rice is ground separately to a slightly coarse, grainy paste, since some texture helps the crumb, and the two pastes are then mixed thoroughly by hand before fermentation.
Keeping the batter warm enough
Fermentation proceeds best in a warm, draft-free place at roughly 25 to 30°C (77 to 86°F). Below that range it slows sharply and may take twelve hours or more, while well above it the batter can over-ferment and become too sour or watery. In a warm climate or in summer, a covered bowl left on the counter overnight usually ferments without help. In a cold kitchen several workarounds exist. First, the covered bowl can sit in an oven with only the light switched on and the heat off. Second, a yogurt maker, or the yogurt setting of a multicooker on low, provides gentle warmth. Third, the bowl can be wrapped in a thick towel and placed near a warm appliance. Whichever method is used, the container should leave room for the batter to roughly double, since a tightly filled or sealed vessel may overflow or restrict the rise.
Recognising batter that is ready
Ready batter looks visibly risen, often doubled, with an airy, slightly domed top in place of a flat one, and small bubbles are visible at the surface and when a spoon is pushed into it. The smell is mildly sour and faintly yeasty, a clean and appealing tang, never sharp or unpleasant. The texture feels light and slightly spongy when folded, not dense and heavy, and the colour often turns a little paler as fermentation advances. Batter at this stage can be used for either idli or dosa. Many cooks observe that very freshly fermented batter steams into a softer idli, while batter rested a further day in the refrigerator often spreads and crisps better as dosa, because it has had longer to loosen and become somewhat more acidic.
Weak fermentation versus spoilage
Under-fermented batter has simply not had enough time or warmth. It stays dense, rises little or not at all, and smells chiefly of raw rice and dal, with no sour note. The remedy is more time in a warmer place, not more salt or water. Spoiled batter is a different matter and must be discarded. It develops an acrid or plainly unpleasant smell in place of a clean tang, may show grey, pink or dark patches instead of an even pale colour, and can turn slimy and not light and airy. The usual causes are batter left unrefrigerated beyond a day or two in warm weather, or kept well beyond a week even in the refrigerator. Suspect batter should never be tasted to confirm; the smell and the appearance are sufficient grounds to discard it and start again.
When to add salt
The timing of salt is debated. Added before fermentation, it can slow the process slightly, because salt draws out moisture and affects the microbial activity that drives the rise. Added afterward, it has no effect on speed, but the batter is already thickened and must be mixed again. Many cooks add a modest amount at the start precisely because a somewhat slower, more controlled fermentation is easier to manage than a fast one that over-sours the batter before it is convenient to use. In very hot climates, where batter tends to sour too quickly, holding back the salt until the end, or using a little less dal against the rice, helps to moderate the pace.
Storing fermented batter
Once the batter has reached the preferred point, it should go into the refrigerator to slow further souring. Left at room temperature it keeps becoming more acidic and turns unusably sharp within a day or two. Under refrigeration it usually stays good for four to five days, developing a slightly stronger tang that many dosa cooks prefer. Refrigerated batter separates a little on standing, so a gentle stir is needed before use. It also helps to let very cold batter sit out briefly before cooking, since batter taken straight from the refrigerator spreads less easily on a hot griddle. Batter that has been stored should be judged by smell before each use, using the same criteria as freshly fermented batter.