What kind of urad dal makes the best idlis?

Urad dal (Black gram) is typically available in grocery stores in four forms – split deskinned, whole deskinned, split with skin and whole with skin. Some cooks start with the ‘with skin’ versions but they soak the dal and remove the skin before grinding. So you effectively have six possible versions of the dal to choose from!! Now, which of these works best for making idlis and why? Let’s find out!

I took half a cup of each form of urad dal and compared them at each step in making idlis. Here’s how things turned out:

Weighing and washing the dry grains:

I measured out half a cup of each type and weighed the dry grains. (Since it is difficult to exactly measure equal volumes for things like pulses, I did adjust the number of grains a bit to bring them to a round and equal number for each kind). You can observe that the samples with skin weigh lesser than the ones without skin. Hence, it looks like the skin is much less dense than the grains, but it still occupies a good amount of space. 

Type of black gram grain	Dry weight of half cup of grains (gm)	Weight of grains after a single wash (gm)
Split, without skin (SWS)	85	101
Whole, without skin (WWS)	85	96
Split, from which skin will be removed after soaking  (SSR)	80	97
Whole, from which skin will be removed after soaking  (WSR)	80	93
Split, from which skin will not be removed after soaking  (SSN)	80	101
Whole, from which skin will not be removed after soaking  (WSN)	80	88

From the last column, you can see that the split grains absorb water much more readily than the whole grains. This is expected, since a split grain would have more surface area than a whole grain. From the numbers, it also appears that the skin does not absorb water that readily compared to the grain.

Soaking:

To soak the grains, I added exactly 180 ml of water to each sample and left them to soak for 5 hours in separate, labelled glass jars. Since I have only one wet grinder, I had to grind the batches one after another. So I staggered the washing and soaking of each sample by 30 minutes to account for the grinding time (approx 25 min). This way, each sample soaked exactly for the same amount of time. 

Here are the samples after a 5-hour soak:

Notice how the split dal with skin has absorbed almost all of the water added. Whereas, the whole dal with skin has not absorbed much water or swollen in size much (compared to the whole dal without skin). 

Deskinning:

Some people prefer to remove the urad dal’s skin while making idlis, mainly because of three reasons:

1. Appearance – People prefer white idlis for some reason. And having the skin on would yield a dark brownish-grey idli.
2. Digestibility – The skin apparently has some anti-nutrients which make it difficult for us to fully absorb the proteins in the dal during digestion. 
3. Taste – Idlis made with the skin on tend to have a very mild bitter taste that puts off some folks.

 The way the skin is traditionally removed is to rub and mash the soaked dal (while still in water) with your fingers, vigorously and repeatedly. The skin then loosens off and floats in the water. It can be drained away by tilting the vessel. 

This needs to be repeated 3-4 times in order to remove most of the skin. Even so, you would still see some stubborn skins here and there! But that’s okay.

Also, it is a little harder to remove the skin from the whole grains than the split ones:

Grinding:

Next, I ground each batch of urad dal with its soak water + 90 ml more water added intermittently while grinding. Each batch was ground for 25 minutes. The weight and volume of each batter was different:

Type of black gram grain	Weight of grains after 5-hrr soak (gm)	Batter weight (gm)	Batter volume (ml)	Specific gravity
Split, without skin (SWS)	177	345	680	0.51
Whole, without skin (WWS)	180	349	710	0.49
Split, from which skin was removed after soaking  (SSR)	157 (after removing skin)	327	800	0.41
Whole, from which skin was removed after soaking  (WSR)	147 (after removing skin)	353	900	0.39
Split, from which skin was not removed after soaking  (SSN)	195	351	780	0.45
Whole, from which skin was not removed after soaking  (WSN)	139	320	700	0.46

From the last column on specific gravity, notice that the lightest batter of all is the one made from whole urad dal from which skin was removed after soaking. This is important as you may recall from one of my earlier posts that the specific gravity of the idli batter is a key factor in determining the final texture of the idli. 

Fermentation:

I took out 250ml batter of each sample and set it out for fermentation:

Here is how much the samples rose after fermenting for about ————-  hours:

For each pair from the left, you can see that the split grain batter has risen slightly more than the corresponding whole grain batter. Also, notice how the rise in volume is higher as the amount of skin in the batter increases. You can see it more clearly from this re-arrangement of the bottles:

Steaming:

Next I greased an idli plate with a few drops of oil and poured out two tablespoons of each sample to steam the idlis:

Here is how the idlis looked like after steaming for 7 minutes:

(I made one more set of these, just to check for consistency. They also came out the same). Notice how the batters with skin have cracks on top. Looks like these batters should have been stirred a bit to release the excess air before steaming. Nevertheless, since we are now just comparing the samples, it is good to have given them all the same treatment (of not stirring before steaming). 

Here is the cross sectional-view of the idlis:

Notice how the whole grain idlis are more fluffy than the split grain idlis. This is despite the fact that the split grain batters rose more in volume! Also observe how the idlis are increasingly fluffier as the amount of skin in the batter increases. Here are the idlis re-arranged, so you can see this more clearly:

What causes these differences?

Here is a quick summary picture, comparing the different kinds of urad dal, the resulting batter and the final idlis:

 The differences between the samples are mainly because of the properties of the skin and the processes that are commercially adopted to remove the skin. 

Dehulling (removing the skin of urad dal )is not an easy process because urad dal has sticky gums and mucilages, which bind the skin to the grain strongly. So, dehulling is done in a two-step process –  loosening of the skin, followed by its removal. At a  commercial scale, loosening the skin involves application of oil, heat and water. First, cleaned and graded urad grains are subjected to pitting (making small holes on the surface by scraping the grain) to increase soaking capability. Then these pitted grains are mixed thoroughly with edible oil and spread for sun drying in a thin layer for 2–3 days. At large scale, heat treatment is typically used instead of sun drying since it is supposed to make this process quicker and efficient, along with reduced wastage in the form of broken grains. At the end of drying, water is sprayed and mixed thoroughly. The grains are heaped overnight. Then the skin is removed by running the grains through a roller machine.

The problem is that, when heat is applied, some of the proteins in the urad dal get denatured (partially ‘cooked’). Also, some of the enzymes present in the dal get deactivated due to the heat. Since these enzymes and some of the proteins are actively involved in the fermentation process, the damage due to heat affects the fermentation of the batter and reduces the rise in volume. This is why commercially de-skinned versions of the dal result in batters with lesser volume, compared to the samples with skin. 

My recommendation:

Use Whole Urad Dal With Some Skin Removed After Soaking

Reasons:

1. It is more wholesome since the skin introduces more dietary fibre to the idlis. Also, some of the research papers mention that the antinutrients in the skin are eliminated during fermentation of the batter.
2. It is quite tasty! The idlis are very soft and fluffy because of the low specific gravity of the batter (Out of all the samples tested, the specific gravity is lowest for whole urad dal with skin removed after soaking). Also, I did not taste any bitterness. In fact, I loved the mild rustic flavour introduced by some of the skin left behind after soaking. 
3. More value for money – Observe that this form of urad dal yields the most batter volume. Also, since the dry weight of  grains-with-skin is lesser than the dry weight of commercially de-skinned dal, you would get more idlis for each kg of dal you buy. 
4. I also love the interesting colour the skin imparts to the idlis 🙂 Imagine serving these tinted idlis on a light pastel-coloured plate with bright white coconut chutney.. Would be a feast for the eyes too!

References:

1. Tiwari BK, Jagan Mohan R, Vasan BS. Effect of heat processing on milling of black gram and its end product quality. J Food Eng. 2007;78:356–360. doi: 10.1016/j.jfoodeng.2005.10.003. 
2. Kamani MH, Meera MS. Assessment of black gram milling by-product as a potential source of nutrients. J Food Sci Technol. 2021 Oct;58(10):3844-3852. doi: 10.1007/s13197-020-04845-0. Epub 2020 Oct 22. PMID: 34471308; PMCID: PMC8357861.
3. Joyner JJ, Yadav BK. Microwave assisted dehulling of black gram (Vigna mungo L). J Food Sci Technol. 2015 Apr;52(4):2003-12. doi: 10.1007/s13197-013-1182-9. Epub 2013 Oct 24. PMID: 25829580; PMCID: PMC4375172.