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6 Min
27/01/2026
Anyone who's grown up eating idlis knows the difference between a good one and a mediocre one. The good ones are soft, spongy, and have just the right amount of tang. The disappointing ones are dense, flat, or too sour. What makes the difference? Turns out, there's actual science behind it.
Making idlis might seem straightforward – soak rice and dal, grind, ferment, and steam. However, even seasoned idli enthusiasts know that one misstep can lead to dense or gummy results. The secret lies in the details – ensure the rice and dal are soaked adequately, grind them to the right consistency, and ferment the batter in a warm environment. During the monsoon, for instance, using slightly less water while grinding and placing the batter in a warm spot can help achieve the desired rise. Understanding and controlling these factors will lead to consistently soft and fluffy idlis.
Idlis need just two main ingredients: rice (usually parboiled) and urad dal. Each does a specific job. The rice gives structure. The urad dal is what makes idlis fluffy. Proteins and carbohydrates present in black gram have the capacity to retain CO₂ released during fermentation, which results in ultra-soft and fluffy idlis.
The ratio in which they are used also matters. The perfect amount is 4:1 of rice:dal. Add a little extra rice, and your idlis will stiffen. Too much dal and you will have softer idlis than usual, that fall apart instead of holding shape. Then comes the grinding part, which is also crucial. The rice and dal should also be ground separately, because the way they react to water and being ground is quite different.
This is where the magic happens. The idli batters need to be left out overnight for the bacteria that naturally live on the grains to start multiplying. The predominant microorganism responsible for souring, as well as for gas production, was found to be Leuconostoc mesenteroides. This bacterium does two things – it produces lactic acid (which gives the tangy taste) and carbon dioxide (which aerates the batter).
But it’s not just one type of bacteria at work. As the batter ferments, more helpful bacteria like Weissella, Enterococcus, and Streptococcus join in. Some natural yeasts, like Saccharomyces, also help by making the batter rise and become light. Did you know urad dal (black gram) actually carries its own friendly bacteria? So, you don’t need to add any extra yeast or starter here at all to kickstart the fermentation process.
Temperature: The bacteria work best in warm conditions, around 25-30°C. Too cold and fermentation takes forever, or doesn't happen at all. Too hot and you might kill the bacteria.
Time: It is essential to soak rice and urad dal overnight. After soaking, grind separately and mix. After mixing, it should be left to ferment overnight. Usually, 8-12 hours is enough. Less than that and the batter won't rise properly. More than that, and it gets too sour.
Some people add a pinch of yeast to speed things up or make the batter fluffier, but traditionalists say it changes the flavour.
Knowing the science behind idlis might help you get the perfect batch, but it’s hard to keep track at a microscopic level. So, despite it all, your idlis might just turn out flattish, dense or watery. Here are some common mishaps and tips to deal with them:
If your batter fails to double or show bubbles after 8-10 hours, it’s likely the fermentation temperature is too low or the microbial activity is weak. It could also be hampered by using expired or compromised products. Place the batter in a warm spot (inside an oven with light on, wrapped in cloth) to speed up fermentation. Also, ensure you include fermentation boosters like soaked fenugreek seeds or a spoonful of previous batter.
Dense idlis usually result from poor batter aeration, incorrect rice:dal ratio, or overmixing after fermentation. Use the recommended 3:1 or 4:1 rice to urad dal ratio, grind urad till it’s light and fluffy, and mix gently so you don’t deflate bubbles. Make sure to steam for around 10-12 minutes only, as oversteaming can dry them out.
If the batter is overly thin, it struggles to trap gas and will yield flat idlis. Use less water during grinding, aiming for a thick but pourable consistency. If it’s already too runny, add soaked poha or a bit of rice flour or coarse rice to thicken.
There is a thin line between perfectly fermented versus overly fermented. Hot or humid conditions can make the batter over-ferment, producing a sharp sour smell and compromising the idli texture. To prevent this, reduce fermentation time and move the batter to refrigeration once it doubles. Avoid leaving it out too long, especially in warm weather, and monitor closely.
Getting your idlis right isn’t magic; it’s just knowing how moody your batter is. Once you understand what each stage needs, the rest falls into place. No more guessing, no more flops. Just soft idlis that hold their own next to the steady sambar and chutney. Whether you’re feeding a family or just yourself, it’s worth doing right.
