The trick is a timely combination of side-to-side and see-sawing motions.
ried rice is a classic dish in pretty much every Chinese restaurant, and the strenuous process of tossing the rice in a wok over high heat is key to producing the perfect final product. There’s always chemistry involved in cooking, but there’s also a fair amount of physics. Scientists at the Georgia Institute of Technology have devised a model for the kinematics of wok-tossing to explain how it produces fried rice that is nicely browned but not burnt. They described their work in a recent paper published in the Journal of the Royal Society: Interface.
This work hails from David Hu’s lab at Georgia Tech, known for investigating such diverse phenomena as the collective behavior of fire ants, water striders, snakes, various climbing insects, mosquitos, the unique properties of cat tongues, and animal bodily functions like urination and defecation—including a 2019 Ig Nobel Prize-winning study on why wombats produce cubed poo. Hu and his graduate student, Hungtang Ko—also a co-author on a 2019 paper on the physics of how fire ants band together to build rafts—discovered they shared a common interest in the physics of cooking, particularly Chinese stir-fry.
Hu and Ko chose to focus their investigation on fried rice (or “scattered golden rice”), a classic dish dating back some 1,500 years. According to the authors, tossing the ingredients in the wok while stir-frying ensures that the dish is browned but not burned. Something about this cooking process creates the so-called “Maillard reaction”: the chemical interaction of amino acids and carbohydrates subjected to high heat that is responsible for the browning of meats, for instance.
But woks are heavy, and the constant tossing can take its toll on Chinese chefs, some 64 percent of whom report chronic shoulder pain, among other ailments. Hu and Ko thought that a better understanding of the underlying kinematics of the process might one day lead to fewer wok-related injuries for chefs.
In the summers of 2018 and 2019, Ko and Hu filmed five chefs from stir-fry restaurants in Taiwan and China cooking fried rice and then extracted frequency data from that footage. (They had to explain to patrons that the recording was for science and that they were not making a television show.) It typically takes about two minutes to prepare the dish, including sporadic wok-tossing—some 276 tossing cycles in all, each lasting about one-third of a second.
Ko and Hu presented preliminary results of their experiments at a 2018 meeting of the American Physical Society’s Division of Fluid Dynamics, publishing the complete analysis in this latest paper. They were able to model the wok’s motion with just two variables, akin to a two-link pendulum, since chefs typically don’t lift the wok off the stove, maintaining “a single sliding point of contact,” they wrote. Their model predicted the trajectory of the rice based on projectile motion, using three metrics: the proportion of the rice being tossed, how high it was tossed, and its angular displacement.
The authors found two distinct stages of wok-tossing: pushing the wok forward and rotating it clockwise to catch rice as it falls; and pulling the wok back while rotating it counter-clockwise to toss the rice. Essentially, the wok executes two independent motions: side to side, and a see-sawing motion where the left end moves in a clockwise circle and the right moves counterclockwise. “The key is using the stove rim as the fulcrum of the seesaw motion,” the authors wrote. Also key: the two motions share the same frequency but are slightly out of phase.
Hu compared the effect to “flipping pancakes or juggling with rice.” The trick is to ensure that the rice constantly leaves the wok, allowing it to cool a little, since the wok temperature can reach up to 1,200 degrees Celsius. That produces fried rice that is perfectly browned but not burned.
Based on their analysis, Hu and Ko recommend that chefs increase both the frequency of motion when tossing fried rice in a wok and the “phase lag” between the two distinct motions. This “may enable rice to jump further, and promote cooling and mixing.”
The mathematical model Hu and Ko developed isn’t just a fun curiosity; it should also prove useful for industrial robotic designs. One goal for the authors is to develop a wearable exoskeleton or similar device to reduce the rate of shoulder injury among Chinese chefs. But there has been interest in automating cooking since the 1950s to perform such basic functions as cutting, boiling, frying, and pancake flipping—the latter task usually relying on reinforcement learning algorithms.
There have also been attempts to automate stir-frying fried rice in large batches, with limited success. Prior robotic designs have included a rotating drum to mix ingredients, and a see-sawing wok to flip ingredients, augmented with an automated spatula. These could mix ingredients via rotation or shaking but could not toss the rice and, thus, could not produce the ideal carbonated grains. “If there was an automated way of doing this, it could be very useful [for chefs],” said Hu.