Georgia Tech physicists unlock the key to good wok-tossed fried rice

Enlarge /. Wok throwing has long been suspected of causing the high shoulder injury rate among Chinese chefs.

Hunt Ko and David Hu / Georgia Tech

Fried rice is a classic dish in pretty much every Chinese restaurant, and the arduous process of throwing the rice into a wok over high heat is key to making the perfect end product. Cooking is always about chemistry, but there is also a lot of physics. Scientists at the Georgia Institute of Technology have developed a model for wok-throwing kinematics to explain how to make fried rice that is nicely browned but not burned. They described their work in a recent article in the Journal of the Royal Society: Interface.

This work comes from David Hus's laboratory at Georgia Tech, which is known to study phenomena as diverse as the collective behavior of fire ants, water striders, snakes, various climbing insects, mosquitoes, the unique properties of cat tongues, and animal body functions such as urination and defecation – Including an Ig Nobel Prize 2019 study on why wombats produce dung. Hu and his PhD student Hungtang Ko – also co-author of a 2019 paper on the physics of how fire ants form rafts – found that they had a common interest in the physics of cooking, especially in Chinese pan dishes.

Hu and Ko's research focused on fried rice (or "scattered golden rice"), a classic 1500 dish. According to the authors, throwing the ingredients into the wok while stirring ensures that the dish is browned but not is burned. The so-called "Maillard reaction" creates something about this cooking process: the chemical interaction of amino acids and carbohydrates that are exposed to high heat and are responsible for browning meat, for example.

But woks are heavy, and constant throwing can affect Chinese cooks, 64 percent of whom report chronic shoulder pain, among other things. Hu and Ko believed that a better understanding of the underlying kinematics of the process could one day result in fewer wok injuries for cooks.

In the summers of 2018 and 2019, Ko and Hu filmed five chefs from pan restaurants in Taiwan and China, cooked fried rice, and then extracted frequency data from this footage. (They had to explain to the customer that the recording was for science and that they were not doing a television program.) It usually takes about two minutes to prepare the dish, including sporadic wok throwing – a total of about 276 throwing cycles, about a third of a second.

• Image sequence showing the wok throwing process, modeled as a two-link pendulum.

  Hungtang Ko and David Hu / Georgia Tech

• Schematic representation of a mathematical model for wok throwing.

  Hungtang Ko and David Hu / Georgia Tech

• Graphic with the metrics for evaluating wok throws.

  Hungtang Ko and David Hu / Georgia Tech

Ko and Hu presented preliminary results of their experiments at a meeting of the Fluid Dynamics Department of the American Physical Society in 2018 and published the full analysis in this latest article. They were able to model the movement of the wok with just two variables, similar to a two-link pendulum, since cooks don't normally lift the wok off the stove and maintain "a single sliding contact point," they wrote. Her model predicted the trajectory of the rice based on projectile movement using three metrics: the amount of rice thrown, how high it was thrown, and its angular displacement.

The authors found two different stages of wok throwing: pushing the wok forward and rotating it clockwise to catch rice as it fell; and pull back the wok while turning it counterclockwise to throw the rice. In essence, the wok performs two independent movements: side by side and a rocking movement in which the left end moves clockwise and the right end counterclockwise. "The key is the use of the furnace rim as the linchpin of the rocking motion," the authors wrote. Also important: The two movements have the same frequency, but are slightly out of phase.

Hu compared the effect to "flipping pancakes or juggling rice". The trick is to make sure that the rice constantly leaves the wok and can cool down a bit, as the wok temperature can reach up to 1,200 degrees Celsius. This results in fried rice that is perfectly browned but not burned.

Based on their analysis, Hu and Ko recommend that the chefs increase both the frequency of movement when tossing fried rice into a wok and the "phase lag" between the two different movements. This "can allow rice to jump further and promote cooling and mixing."

The mathematical model developed by Hu and Ko is not just a funny curiosity. It should also prove useful for industrial robot designs. One goal of the authors is to develop a portable exoskeleton or similar device to reduce the rate of shoulder injuries in Chinese chefs. However, since the 1950s, there has been an interest in automating cooking to perform basic functions such as cutting, cooking, frying, and flipping pancakes – the latter task is usually based on gain learning algorithms.

Attempts have also been made to automate the roasting of fried rice in large quantities with limited success. Earlier robot designs included a rotating drum for mixing ingredients and a rocking wok for inverting ingredients, supplemented with an automated spatula. These could mix ingredients by rotating or shaking, but not throwing the rice and thus not producing the ideal carbonated grains. "If there was an automated method, it could be very useful (for cooks)," said Hu.

DOI: Journal of the Royal Society: Interface, 2020. 10.1098 / rsif.2019.0622 (About DOIs).