If your batter refuses to rise one week and tastes odd the next, you're not alone, and it usually isn't your fault. Idli and dosa begin from the same fermented batter, and the fermentation is no garnish. It is what makes idli light and gives dosa its tang and crisp edge. A quick-leavened batter uses baking powder or soda, but this one counts on bacteria and wild yeast already present on the rice and dal to create lift and sourness across several hours. As a result it responds to temperature, humidity and even altitude. Most of the problems home cooks report, such as no rise, an off taste or results that change from week to week, come down to a few variables that are easy to control once you know what to look for.
Ratios and soaking
Start with about 4 parts rice to 1 part whole urad dal. The rice is usually idli rice or parboiled rice with a small amount of raw rice mixed in, and some cooks move closer to 3:1 for a softer, airier idli. It is common to soak a small handful of poha (flattened rice) or a few fenugreek seeds along with the rice. Both are believed to help fermentation and texture, and fenugreek in particular adds a subtle depth and a slightly better rise. Soak rice and dal separately, typically for 4 to 6 hours or overnight, in plenty of water. Both must hydrate fully before grinding, because under-soaked grains grind coarse and gritty and not smooth, which hurts the texture of the batter and how evenly it ferments.
How and why the dal is ground
Grind the urad dal on its own, separate from the rice. You want a light, fluffy, aerated paste and not a dense purée, and this is the most important technical step in the whole process. Add cold water in small splashes while the dal grinds, and you should see the paste swell, lighten in colour and froth on the surface, like a thick batter that has had air whipped into it. A wet grinder does this well. So does a strong blender used in short pulses, with the sides scraped down and water added gradually. A single long grind is worse, since it warms the batter and drives the air back out. The aeration matters because it is largely what gives steamed idli their light, spongy crumb. A dal paste ground dense and heavy gives a flat, compact idli no matter how well the batter ferments afterward. Grind the rice separately to a slightly coarser, grainy paste, not perfectly smooth, as a little texture here contributes to a better crumb. Then combine the two pastes and mix them thoroughly by hand before fermenting.
Finding a warm enough spot
Fermentation does best somewhere warm and free of drafts, roughly 25 to 30°C (77 to 86°F). Below that range it slows sharply and can take 12 hours or more. Well above it, the batter may over-ferment and turn too sour or watery. In a warm climate or in summer, batter covered and left on the kitchen counter overnight usually ferments well by itself. If your kitchen is cold or it is winter, there are a few workarounds. You can put the covered bowl in an oven with only the oven light on (not the heat), use a yogurt maker or an instant pot's yogurt setting on low, or wrap the bowl in a thick towel and set it near a warm appliance. Whichever container you choose, leave room for the batter to roughly double, since a tightly packed or fully sealed container can overflow or choke the rise.
Recognising well-fermented batter
Look first at the surface. Good batter has visibly risen, often to double, with a slightly domed, airy top and not a flat one, and small bubbles show on top and as you spoon into it. Smell it next. It should be faintly sour and slightly yeasty, a clean and appetising tang and nothing sharp or unpleasant. When you fold a spoon through it, it should feel light and a little spongy, not dense and heavy. The colour usually turns a touch paler as fermentation goes on. At this point the batter is ready for idli or dosa. Cooks often notice a difference between the two. Batter that has only just finished fermenting tends to give a fluffier idli, whereas batter that has spent another day in the refrigerator usually spreads thinner and crisps better as dosa, having had longer to loosen and sharpen in acidity.
Under-fermented or spoiled?
Under-fermented batter has simply not had enough time or warmth. It stays dense, rises little if at all, and smells mostly of raw rice and dal and not of anything sour. The answer is more time in a warmer place, not more salt or more water. Spoiled batter is another matter, and you should discard it instead of using it. It gives off an off, acrid or plainly unpleasant smell instead of a clean sour tang, may show grey, pink or dark patches where you would expect an even pale colour, and can turn slimy and not light and airy. Spoilage usually follows batter that sat out of the refrigerator for over a day or two in warm weather, or that stayed in the refrigerator well beyond a week. If a batter smells wrong, don't taste it to find out. Trust your nose and eyes, and make a new batch.
Salt timing and the pace of fermentation
Cooks disagree about when to salt. Adding it before fermentation can slow the process a little, because salt draws out moisture and affects the microbial activity behind the rise. Adding it afterward doesn't change the speed, but you're then seasoning a batter that has already thickened and has to be re-mixed. Many cooks put in a modest amount of salt at the start precisely because a slightly slower, more controlled fermentation is easier to manage than a very fast one, which can make the batter too sour before you are ready to use it. If you ferment in a very hot climate where batter turns sour too quickly, hold the salt back until fermentation is done, or use slightly less dal compared with rice, to slow things down.
Storing fermented batter
Once the batter has fermented to your liking, put it in the refrigerator to slow further fermentation. Left at room temperature it keeps souring and within a day or two becomes unusably sharp. In the refrigerator it typically keeps well for 4 to 5 days, getting a more pronounced tang as the days pass, which many dosa cooks actually prefer. Stir it gently before you use it, because it separates a little on standing. Very cold batter is also harder to work with, so let it warm up on the counter for a short while, as batter straight from the refrigerator spreads less easily on a hot tawa.