Why does papad increase in size when fried in oil or microwaved?

In both the cooking methods, frying in oil and roasting in microwave, the papad gets more or less uniformly heated throughout its volume. And during this, some chemical and physical changes occur.

While making papad, the dough is prepared by adding about 1per cent of baking soda — sodium bicarbonate (NaHCO3), to its major ingredients like urad dal or rice flour, salt, other flavouring agents and spices.

The baking powder, which is a mixture of suitable proportion of sodium bicarbonate and citric acid, undergoes a decomposition type chemical reaction on heating and produces a large volume of carbon dioxide gas — CO{-2}. In rice papads, the rice grains also release a large amount of water vapour and other gases during the frying or roasting process due to thermal decomposition. The release of these gases is responsible for making the fried papads crisp and swollen. An approximate estimate shows that the amount of gas produced in papad is about 10 times its volume. This gas does not get released out of the papad as it is made from practically solid dough. Instead, the gas renders the papad into a rather dry and porous mass; and we enjoy its crispness.

The gas, thus, fills small pockets or bubbles inside the papad. These pockets are more or less spherical in shape and have various sizes.

The ones near the surface pop out as blisters which can be commonly seen on the fried papads. Some of these blisters burst and release the CO{-2} and other gases.

But the bubbles located in the inner regions retain the gas leading to a swelling of the papad somewhat uniformly on all sides.

Assuming about half of the total amount of gas produced is retained, the increase in volume would be as much as five times giving a rough estimate of increase in diameter to be about one and half times the original size of the papad. A similar increase in thickness in regions away from the popped out blisters is realized.

Source: The Hindu