The physics of tossing fried rice

This paper touches on so many of my esoteric passions - Chinese cooking, ergonomics, robotics, automation, freaking assistive exoskeletons... I don't even know where to begin, other than to bless my spouse for sending it along for my birthday.

Fried rice is a 1500-year-old dish that is prepared using wok tossing, a technique that enables food to undergo temperatures of 1200°C without burning. Tossing of the heavy wok at high speed may be one contributor to shoulder pain, which is reported by 64.5% of Chinese restaurant chefs. In this combined experimental and theoretical study, we report the wok tossing kinematics of five professional restaurant chefs. The wok toss has a period of 0.3 s and involves two directions of movement: translation, which slides the rice along the wok, and rotation, which throws the rice into the air. We report the chosen kinematics of the chefs and use a theoretical model to predict the trajectory of rice based on projectile motion. Using our model, we rank all possible kinematics in terms of three metrics: the proportion of the rice that is tossed, its flight height and the angular displacement of the rice. We identify an optimal regime for making fried rice and suggest ways that wok tossing may be improved. This study may inspire the design of stir-fry robotics and exoskeletons to reduce the rate of muscle strain injury among professional chefs.

1. Introduction
   Stir-frying was used originally for drying rather than cooking food in the Han dynasty (206 BC to AD 220) [1]. One of the most popular dishes to emerge from this technique was scattered golden rice, which originated 1500 years ago in the Sui dynasty (581–619 AD) [2]. Stir-frying provides the long-sought wok hei flavour or the breath of the wok [3]. Wok hei is a type of browning, enabled by the Maillard [4] chemical reaction that takes place only when the wok surface reaches 1000°C. Under such high temperatures, chefs [5] surmise that the rapid motion of the wok enables browning without burning. This idea, however, has not yet been tested quantitatively. The goal of this study is to determine the mechanics underlying wok motion that makes browning without burning possible.

By studying wok tossing kinematics, we hope to improve the process of frying rice and inspire exoskeletons to reduce injuries in the restaurant industry. Fried rice is made in most of the Asian cuisines. In Chinese restaurants, it is made with the wok, the weight and speed of which have long been suspected to play a role in the high injury rate among chefs. Wok tossing is frequently mentioned in the literature concerning musculoskeletal disorders among Chinese chefs [6–8]. In 2014, wok tossing was highlighted as one of several critical risk factors that call for investigation and immediate change [6]. The precise muscles that are used in wok tossing remain unknown, but it is likely that the shoulder is heavily relied upon. The shoulder is used in a number of restaurant activities, and, as a consequence, discomfort of the shoulder is the most common disorder among Chinese restaurant chefs, with a prevalence of 64.5% [9]. Shoulder discomfort has forced two out of 18 chefs to leave their jobs [7]. Understanding the kinematics of the wok is an important first step in understanding how wok tossing affects the body.

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