Idli: Why do we get different textures for different ratios?

Did you read my post on the effect of using different rice : urad dal ratios for making idlis? The main factor that leads to so much difference in their texture is the specific gravity of the batter. 

Role of each ingredient in the idli recipe

Before we understand specific gravity, let us take a look at the role of each ingredient for making idlis:

• Idli rice – this is the main ingredient which provides body and structure to the idli. 
• Urad dal / Black gram / Ulutham paruppu – its foaming property helps incorporate air and makes the idli spongy; its stickiness holds the rice particles together and it is also the source of the microbes that ferment the batter. For a detailed understanding, read my post on the role of urad dal in idli. 
• Water – it hydrates rice and urad dal and makes them easier to grind, it is a substrate (base) for dissolving some of the nutrients and for the fermentation action to happen, the amount of water can be adjusted to get different textures. Too little water will yield hard idlis; the right amount of water yields spongy idlis; too much water results in sticky flat idlis.
• Salt – for taste; also affects texture by controlling the rate of fermentation. We will explore this in detail in another post. 

When we process these ingredients by soaking, grinding, mixing, fermenting and steaming them, their physical and chemical properties undergo a lot of change from one step to the next. The key physical property in the mixing stage (when you mix rice batter and urad dal batter to get the final idli batter) is the specific gravity of the batter.

What is specific gravity?

Specific gravity of a liquid is an indicator of how dense or light the liquid is, compared to water.

Specific Gravity =Density of the liquid you are testing / Density of water

If the liquid is as dense as water, its specific gravity would be 1. If it is lighter than water, its specific gravity would be less than 1 and the liquid will float in water. If it is denser than water, its specific gravity would be more than 1 and it will sink in water. 

Why Does This Matter? 

The specific gravity of the idli batter matters because it determines how much and how quickly the batter would rise during fermentation and during steaming. 

If the batter is too heavy, it will not rise much during fermentation and during steaming. The air bubbles and steam will not be able to expand and push the batter much. So your idlis will turn out dense. 

If the batter is too light, the bubbles will expand too much during fermentation. Also, the batter will rise too quickly while steaming. And then you would see it collapsing soon after. That is, the air would expand and escape quickly, before the rice and urad dal cook and set in place. So the idlis will turn out flat. 

So, we want the batter to have moderate specific gravity. So that, while steaming, it can rise and hold the air bubbles while the rice and dal cook at the same time. After the rice and dal cook sufficiently, that is when the air should break out and escape, leaving behind a network of connected holes.

How does the specific gravity of the batter vary with rice: dal ratio?

There are many ways to change the specific gravity of idli batter. In this post, we explore only the effect of using different rice: dal ratios,  while keeping other factors the same. (other factors like the amount of water added, soaking time, grinding time, quality of ingredients, etc). The simplest way to keep these factors the same is by using the same batch of rice batter and urad dal batter, but just mixing them in different proportions to get different ratios. 

When I ground a big batch of rice and urad dal for the ratio experiment, the batch of rice batter had a specific gravity of 1.12. And the urad dal batter had a specific gravity of 0.408. And when I mixed the batters in different proportions, here is how the specific gravity of the final batter varied for different ratios:

And here is how the idlis turned out (the rice:dal ratio is specified in green and the specific gravity in white:

By checking the taste and the texture of the idlis, the ones with the ratios 3:1 to 5:1 were the best of the lot – they were soft, spongy and absorbent. They also had the characteristic smell and sour-sweet taste you would expect from a good idli. Note that the specific gravity of the batters for these ratios varies from 0.7 to 0.8. 

I think 0.7 to 0.8 is the ideal range for specific gravity of idli batter. If the specific gravity of the batter is below this range, the idlis are turning out flat and sticky. This is because there is too much urad dal in the batter – much more sticky substance than is needed to hold the rice in place. There is also too much air which escapes too soon and the batter caves in. When the specific gravity is above this range, the idlis are turning out firm and bouncy instead of being spongy. This is because there is too much rice, which is heavier. 

Why I recommend the  4:1 ratio for idli:

If you have noticed this while grinding idli batter,  in the case of 4:1 ratio, the amount of urad dal batter and rice batter mixed together is almost always equal. ie., their volumes are almost the same (say X). In this day’s experiment, since the specific gravity of the rice batter is 1.1 and that of urad batter is 0.4, when we mix them in equal volume, the specific gravity of the resulting batter would be (0.4X + 1.1X)/2X which is 1.5X/2X. This is 0.75, which falls right in the middle of the desired range. So 4:1 leaves a nice margin for adjusting uncontrollable factors (like the quality of urad dal, amount of aeration during natural fermentation, etc). That is one main reason why I strongly recommend the 4:1 ratio. 

How to adjust the specific gravity of your idli batter? How to check it without making any calculations?

After you have soaked rice and urad dal according to your preferred ratio, here is a great way to fine tune the specific gravity of the resulting batter – by controlling the amount of water you add while grinding the urad dal. 

And you can check for the right consistency without doing any calculations or measurements -by  simply doing the batter float test! Here is my post on how to do the batter float test and what to observe.

Do you know of other methods to adjust and check the consistency of the batter? Share through a comment!