Sunday, November 27, 2011

How does a chapati or roti fluff up?


Many of us are familiar with the way to make a perfect chapati or roti:  (i) roll flat a piece of moist dough evenly, (ii) heat one side on a pan, (iii) flip and heat the other side, (iv) then put it directly onto the flames.  If done right, the chapati puffs up like a balloon, a giant bubble of water vapor contained within the intact surface of the roti.  Why and how does this happen? Centuries of chapati making have led, through trial and error, to this sequence of steps. Think through this, as we did through a dinner conversation with my younger son, and there are clear scientific reasons behind each of these steps.

When a chapati is heated, dissolved water in the dough evaporates and collects together to form small bubbles which then coalesce into one large bubble. As more dissolved moisture is converted to vapor, the bubble grows, pumping up the chapati and pushing the malleable dough outward.  Instead of growing large as it does, why doesn't the water vapor bubble simply escape by perforating the front or the back surface, as in the figure below?  These surfaces are readily available to the bubble less than a millimeter away!

The answer lies in the manner that the chapati is heated.  Take the hottest part of the chapati—the bottom surface that is in contact with the hot pan.   Water vapor that has formed within an escape depth of this surface leaks out through the back.  This water denuded outer layer of dough now hardens, losing its malleability and making it harder for a  water vapor bubble to punch through.
 
Now the top surface needs hardening as well—which is why in the best puffed chapatis you need to heat one side, then flip over to heat the other side.  Once both surfaces have been hardened, the escape routes from top and bottom have been sealed, the growing bubble is frustrated and forced to expand laterally, as in the figure below.

Why is there a single bubble and not many?  When a bubble is formed, a new surface is created (i.e. the circumference of the bubble).  This has an energy cost because atomic bonds have to be broken to create the surface (if you split a wooden log in two, you have expended energy to break atomic bonds to create two new surfaces that weren’t there).  Systems in nature always try to minimize the energy cost if they can. If two bubbles of the same radius coalesce to form a single bubble, the surface area reduces to 2-1/3 of the original area, if n bubbles coalesce, the area reduction is a factor of n-1/3.  Creating a single large bubble instead of many small ones therefore requires a smaller area of surface to be created, and a lower energy cost.  It is thus fated that the bubbles must give up their individuality.  This is why in a good chapatti, in the end, there is one huge bubble. 

We are still left with step (iv), that the best chapattis are made when—after heating each side on a pan--the chapatti is put directly onto the flames for a vigorous fluff up.  There is a reason behind this as well. The dough is not perfect.  Some water vapor can still leak out through the top and bottom surfaces.  There is therefore a small “leak rate”.  Now when the dissolved moisture is converted to vapor in a hot chapatti, there is a certain gas generation rate. If this generation rate is much larger than the leak rate, then we can expect a good puff, otherwise not. Sticking the chapatti directly onto the flames raises its temperature and rapidly converts the moisture into gas—greatly increasing the gas generation rate and making the leak rate negligible in comparison.


3 comments:

  1. What about Puris?

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  2. Puris are similar, except that in this case, the surfaces are hardened and sealed by heat conduction through the hot oil.

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  3. Nice explanation. Thanks a lot.

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