How to do the Batter Float Test for Idli?

Did you know, you can predict the texture of your idlis by observing the batter itself! Experienced cooks do this by dropping a small blob of urad batter in water, to check its consistency while grinding. This is called the ‘Batter Float Test’. But what does this test actually measure?  Why do we do it especially for urad dal? What should you observe while doing this test? Read this post to find out.

What Does The Batter Float Test Actually Measure?

When you drop a blob of batter in a bowl of water, you are basically checking how well that blob floats in water. By doing this, you are actually seeing how much lighter or heavier the batter is, compared to water. There is a scientific term for this relative density between the two liquids – it is called specific gravity. 

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

(When both are at room temperature). And Density = Mass / Volume. Which means, how many kilograms does 1 litre of the liquid weigh. (Or how many grams does 1 ml of the liquid weigh). 

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. 

So, by dropping the batter in water, you are checking if the batter is lighter or denser than water. 

Why Does This Matter? 

The specific gravity of the 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 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 and steam would expand and escape quickly, before the rice and urad dal cook and set in place. After they escape, the still wet and uncooked batter will then fall flat and cook. So the idlis will turn out flat. 

Ideally, 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 Can You Control The Specific Gravity Of Your Idli Batter?

There are four main ingredients in the idli batter that determine its consistency – rice, urad dal, water and air. So, you can tweak the specific gravity of the batter mainly by 3 ways:

• By changing the rice : urad dal ratio: Rice batter is denser than water and urad dal batter is lighter than water. So you can change the density of the final idli batter by mixing the two in different ratios. Here is my earlier post on why the 4:1 rice:dal ratio is very reliable for getting spongy idlis. Watch out for an upcoming post on how specific gravity varies for different rice:dal ratios. 
• By controlling the amount of water you add while soaking and grinding rice and dal. Too little water will get you hard and dense idlis.  And too much water will yield flat sticky idlis. I have found that 1.5X of water by volume (compared to the dry grains) works good for rice and 2.25X water by volume works good for urad dal. This is the total amount of water to be added – including soaking and grinding. Watch out for an upcoming detailed post on this. 
• By controlling the amount of air incorporated into the batter. This can be done in many ways – by soaking the urad dal thoroughly, by controlling the amount of water you add while grinding urad batter, how you grind the urad batter (in a mixie or wet grinder),  how much you mix the batter after fermentation (letting bubbles escape while mixing), etc

Why is the Batter Float Test done only for Urad Dal batter?

While all these above ways are available to tweak the specific gravity and bring it to the range we want, you would usually see that the batter float test is specifically done for checking the consistency of urad dal batter only. The other options for tweaking the specific gravity are usually not adopted by home cooks. I guess this is because of the following reasons:

• Everyone has their preferred rice:dal ratio. And in any case, after you have soaked and started grinding for a particular ratio, there is not much you can do to tweak it for that batch. 
• Rice batter is slightly heavier than water. It has a specific gravity of about 1:1, which is close to that of water. And it does not incorporate any air while grinding. So, even if you add a little less or more water to it, you cannot change its specific gravity too much. 
• But the specific gravity of urad dal batter changes wildly, since it incorporates air and foams up. This foaming is one of the reasons why we use urad dal only for making idlis and not any other pulses. How much air gets incorporated into the urad dal batter depends on how long you soak the dal, whether you grind it in a mixie or wet grinder, etc. But the most useful factor is that the amount of air incorporated into urad dal can be controlled by how much water you add while grinding it. Which is probably why experienced cooks advise adding water little-by-little while grinding. And also recommend checking its consistency from time to time by doing the float test.
• The float test is not done for the final mixed batter (rice batter and urad dal batter after mixing) because the final batter can have the same specific gravity for very different consistencies – depending on the ratio of the ingredients, how much water was added, etc. To confirm this, I made 3 batches of urad dal batter with different amounts of water added to each. And 3 batches of rice batter with different amounts of water added to each. Then I combined the two batters in all the 9 combinations possible (all of them were for 4:1 rice:dal ratio). Here’s how the batters and the idlis turned out:

Notice how the batters with lesser specific gravity have risen more during fermentation.

You can see that the fluffiness of the idlis varies so much for the same/similar specific gravity. So, the specific gravity of the final idli batter is not a simple clear indicator of the final idli texture. Maybe this is why the float test for the urad batter is a more reliable test (provided you take care to add the right amount of water to rice . My recommendation is 1.5X cups of water for every cup of rice, in total for soaking and grinding).

How To Do The Batter Float Test?

Soak urad dal in double its volume of water (ie, 2 cups of water for every cup of dry urad grains) for at least two hours. (Here’s a post on the right soaking time). When you grind urad dal, first add the soaked dal alone to the wet grinder and let it grind for a minute or two. Then add the remaining soak water – not all at once, but in 1-2 tablespoons once every 3-4 minutes. Once the urad dal starts fluffing up well, wet your fingers in water and take a small blob of the urad dal batter. (Wetting your fingers is so that the batter doesn’t stick to them).  Roll the blob gently so it forms a ball shape. Put it in a small glass bowl half-filled with water. 

Observe the following things:

• Does the ball float fully below water or does it rise slightly above the water surface?
• Does the shape of the ball remain intact or does it flatten?
• Does the urad paste disperse into the water quickly or does the water stay clear? 

Now add the blob back to the grinder. (The grinder can keep running while you do the test. Just remember to be safe or pause it while picking up the samples for the float test). If the blob does not rise above the surface of the water at all, then the specific gravity is still high. You can continue adding more fresh water and grind some more. Again, don’t add the water all at once. Add just 1-2 tablespoons every 3-4 minutes and let it grind for a couple of minutes. Then repeat the float test every time.

What Is the Ideal Consistency? When Should You Stop Adding Water While Grinding?

Just to demonstrate how much the specific gravity varies for urad batter, I soaked 1.5 cups of urad dal in just 2 cups of water for 4 hours. Then I added the soaked urad dal grains and the remaining soak water (~2tbsp) to the grinder and let it run for 25 minutes. I took out a sample and did the float test. Then I added 45ml (quarter cup) of water every 2 minutes and repeated the float test. At each stage, I also took out 50 ml of the batter and weighed it, so that I could calculate the specific gravity as well.

This is how the float test looks like over time, as you add water and keep grinding the urad dal: (The numbers indicate the specific gravity of the sample)

This is the same image, with just the mid-line of the blobs highlighted in green:

For the first 4 samples, notice how the midline of the blob falls below the surface of the water – these are dense and would yield hard and firm idlis. Notice how the last 3 samples have spread out and flattened – these would give flat sticky idlis. 

The ideal consistency is that of sample 6 and 7. Notice how the midline of the blob is right at the surface of the water and the blob holds its ball shape without flattening out. It even looks like an idli! Stop adding water at this stage. This is when the maximum amount of air has been incorporated and the specific gravity is minimum, while keeping the foam structure intact. So, these idlis would be springy and soft.

This stage is reached when you the total amount of water that you have used for soaking and grinding the urad dal reaches about 2.25X – 2.5X the original volume of the dry urad grains (ie., 2.25-2.5 cups of water for every cup of urad dal you started with). The recommended amount of water is a range (0.25X-0.5x) and not a single number because a lot of factors impact the foaming quality of the urad dal – its variety, freshness, processing conditions while it was deskinned, etc. So, whenever you grind every batch, a little mindfulness and observation is all you need to ensure your idlis turn out springy and soft.

Was this article helpful? Do you use other indicators for checking your batter? Let me know!