From starch to finish: Swetha Sivakumar on how grain gets puffed
What turns dry flour into a pillowy mass? What does it take to make a grain of rice balloon? Take a look, in this week’s Sound Bites.
Do corn starch and wheat flour have moisture in them?
In 1901, a botanist in New York named Alexander P Anderson decided to, once and for all, find out. So he packed a bit of each of these two flours in two test tubes, hermetically sealed them, and placed them in an oven set to 260 degrees Celsius.
After a while, he took them out and they seemed unchanged, until he shattered the tubes with a hammer.
Each flour instantly expanded and morphed into a “porous puffed mass, white as snow”, he wrote in his laboratory notes. Indicating? The presence of moisture.
We now know that most grain flours are 12% to 14% moisture. As the starch is heated in the oven, the moisture turns to steam. When Anderson shattered the tubes, there was a sudden drop in air pressure. The steam that had been contained in the containers was now free to expand and escape; but before it could, the starch molecules, which had been desperately seeking water to replace the moisture they had lost, clung to the steam.
The result was that the fleeing water vapour simultaneously cooked the starch (when raw starches are exposed to moisture and temperatures above 60 degrees Celsius, the bonds holding them in their compact, dry state are broken and they form a gel-like network) and expanded it (this network is stretched to its maximum limit by the expanding water vapour), creating the pillowy masses.
In 1902, Anderson was granted a patent for this “dry method of swelling starch materials of all kinds to render them porous”. While his process may have been new, the principles were already in use centuries earlier, in India.
In the traditional technique for making puffed rice, the element manipulated is temperature, not air pressure. The temperature of the grain is raised so rapidly that the moisture in it turns to steam. That steam contains so much energy that it expands the starches within the grain like a balloon. The technical term for this is high-temperature short-time (HTST) heating.
It is a process used by street-food vendors, intuitively and through hereditary know-how. As with the tapri recipe for masala chai, the makers may never know why they do things as they do them, but the science nonetheless moulds the process and the result.(Click herefor a peek into the incredible science of masala chai.)
For puffed rice, vendors use sand or salt to achieve the high temperatures required. Simple as this sounds, it takes hours of practice and considerable skill to get it right. Several parameters must be balanced. If the temperature of the mix falls too low (<200 degrees Celsius), the mumra will be chewy rather than crisp. Too high (>270 degrees Celsius), and the grains begin to get discoloured, going from a fresh white to shades of brown and grey.
Toss the rice for too long and too much moisture is lost, leaving the mumra hard rather than crunchy. Take it off the heat too soon and the vendor loses volume; the grain will not have expanded as much as it could have.
Machines replace human skill (and variability) in manufacturing plants. Gun-puffing and extrusion-puffing machines are used to create puffed-grain snacks such as Cheetos and Rice Krispies. The puffed grains come out of this process with moisture levels of 5% to 7%. Even fresh out of the machines, they don’t have the audible crunch that is so key to the experience of eating such foods.
So, they undergo a drying process, where moisture levels drop to the required 1% to 3%. They are then coated with sugars or oils, and sealed into nitrogen-filled packets, to keep moisture from degrading their crunch through their long shelf life. As a result of the many processing steps, some vitamins and amino acids are lost.
I say, enjoy everything in moderation. I am partial to fresh mumra; in bhel puri, or with just a sprinkling of peanuts and coriander. And to go with it, a cup of that other ancient treat: the perfectly balanced chai.
(To reach Swetha Sivakumar with questions or feedback, email email@example.com)