The physics of fried rice

Mechanical engineers have revealed the extraordinary complexities involved in cooking perfect fried rice.

Making an authentic stir fry requires quickly tossing ingredients in a wok over extremely high temperatures up to 1200 degrees Celsius – and fried rice is one of the most ubiquitous, and one of the most challenging, variants.

Doctoral student Hungtang Ko from the Georgia Institute of Technology in the US videotaped expert chefs making the dish at two restaurants in Taiwan. (He had to explain to the restaurant’s patrons that they weren’t going to be on TV; rather that the filming was part of a serious scientific experiment.)

Back in his lab, Ko and lead researcher David Hu analysed the videos in order to break down the kinematics of wok-tossing.

The results revealed a pattern of continuous cycles, each lasting about one-third of a second, extending over two minutes.

Each cycle comprised four phases, involving two alternating wok movements: toward-and-away from the chef, and a seesawing backward-forward tilt.

“The high-speed tossing motion … involves four independent stages, whose translation and rotation allows the wok’s rounded sides to act like a ski-jump for the fried rice,” says Ko.

The overall effect, explains Hu, is like “flipping pancakes or juggling with rice”.

Effectively, the chef continually tosses the rice into the air, catches it, mixes it then tosses it again over the blistering heat. This mixing and cooling effect allows the rice to brown nicely on all sides without burning.

Ko was astonished at what they discovered when they broke the movement down. “It’s very interesting because it’s counterintuitive,” he says.

He explains that you would expect both ends of the wok to go in the same direction, like when tossing a pancake in a flat pan.

“But it’s not,” he says, “If you look closely, one end is going clockwise and the other is going counter-clockwise. So, this isn’t trivial mechanics – it’s a lot more complicated.”

He suggests the process most likely creates a Maillard reaction, named after French chemist Louis-Camille Maillard. It is the chemical interaction between amino acids and carbohydrates under high heat that produces hundreds of different flavour compounds – and leads to the trademark browning of fried meat.

Wok-tossing stir fries is clearly a special art. And it’s not easy. “It’s hard and it’s fast and it takes a lot of strength,” says Ko.

However, the pan itself never actually leaves the stove top. “We show that the wok is always contacting the stove and getting support from it, so the chef doesn’t have to lift it,” he notes.

Previously, researchers have devised mathematical models that they say could help robots make a perfect stir-fry, following a growing trend of robochefs.

Putting them into practice, they succeeded in sometimes mixing the food with a shaking or rotating motion. “But none were able to throw the rice, which meant they couldn’t cook at the high temperatures needed to produce nicely carbonated grains,” explains Hu.

Ko recently presented his findings at the American Physical Society’s Division of Fluid Dynamics 71st Annual Meeting, held in Atlanta, Georgia.