The Physics of Medu Vada

A Chemical Engineer’s Take on an Ancient Cuisine Art

In this relatively new scenario of #socialdistancing, #workfromhome, the suggestion is to #learnsomethingnew. Here goes…

Did not know before today that if Urad daal batter is mixed / blended at fast speeds (high shear), it gets progressively more and more difficult to blend! In techno it’s called Shear Thickening. To blend Urad daal batter, slow & steady wins.

As an aside, Ketchup is a Bingham Fluid which means that after a critical shear, it flows like water. Water is what we call in techno a Newtonian fluid. The trick to getting ketchup to flow is to (controlled) vigorously shake the bottle. This increases the shear near the walls of the tube to a level such that it is higher than the critical shear after which the ketchup starts flowing like water. More or less of it will flow out of the open mouth of the bottle as you tilt it more or less. Before ketchup starts to flow smoothly

it is critical to exceed it’s critical shear at the walls!

Anyway, back to the Urad Daal batter. In addition to it being shear thickening, it has a highly hydrophobic surface. What this means is that will have strong (and I mean STRONG) van der Waals force adhesion to your hand. Translation from techno to lingo:

it is very sticky!!

But now I have a clue…

So, I wet my hand with water (you see, I had to figure out how to make the donut hole!) and ergo!! The batter becomes easy to roll into a ball and flatten into a bun and to poke the donut hole through it! Water is the key. Not oil. Why? Because the batter is lipophilic / hydrophobic!!

In techno, somehow the polar water molecules prevent the small scale van der Waals force to take hold. Stickiness means van der Waals forces are happening near the surfaces that touch.

And then the frying! Takes medium to low heat. Why? Because its the balance between heating too quickly which will only cook the outer crust and this reduces the rate of heat penetration into the bulk which results in a crispy outer skin and an uncooked inner bulk. Slowing down the rate of heating allows the inside to cook and eventually the outer crust sets to a golden brown color.

#thereyouhaveit #chemicalengineering #atwork #inthekitchen #covid19