The science behind 'wok hei'—the key to crafting the perfect bowl of fried rice

Chef Kwok Keung Tung expertly tosses the wok with one hand while stirring with a metal spatula in the other.

With both hands occupied, he cleverly uses his knee to adjust the gas stove’s lever, controlling the fire fan and occasionally igniting a third of the wok with flames.

In just three minutes, the once-sticky mound of white rice transforms into a golden bowl of fried rice, ready for service.

“This is the magic you’re after—wok hei (the breath of the wok),” says Danny Yip, co-founder of Hong Kong’s The Chairman restaurant, to Dinogo Travel.

“The wok is the heart of Chinese cooking in South China, and Cantonese chefs are the true masters of both fire and wok.”

When it comes to wok hei, there’s no one more qualified than Yip.

The Chairman was named the best restaurant in Asia for 2021 and holds the title of the highest-ranking Chinese restaurant on the World’s 50 Best Restaurants 2019 list (with no ranking for 2020 due to the pandemic).

For those raised in a Cantonese family, visiting a Chinese restaurant without hearing the terms 'gau wok hei' (enough wok hei) or 'ng gau wok hei' (not enough wok hei) is nearly impossible. These phrases are used to gauge how authentic a restaurant really is, often voiced by older patrons.

Hei (also spelled 'hay') is the Cantonese term for 'chi,' meaning the flow of energy. Once a mystical and abstract idea primarily associated with the South China region, it wasn't widely emphasized in other parts of China or Asia, even though woks were commonly used.

The concept of wok hei didn’t gain global recognition until the renowned American Chinese food writer Grace Young poetically coined it as 'the breath of a wok' in her 1990s book, 'The Wisdom of the Chinese Kitchen: Classic Family Recipes for Celebration and Healing.'

“Wok hei isn’t just about hot food; it’s that fleeting, seared flavor that lasts only for a minute or two,” wrote Young.

In essence, wok hei is the combination of the fragrant steam you inhale and the near-burning sensation on your tongue that elevates the dish’s flavor.

How a wok functions

In recent years, a growing number of food writers and scientists have been exploring the roots of Chinese cooking, delving deeper into its history and examining key elements like wok hei.

After realizing the lack of scientific research on Chinese cuisine, Hung-tang Ko, a doctoral student at the Georgia Institute of Technology, co-authored a paper titled 'The Physics of Tossing Fried Rice' with David Hu—a scientist known for his studies on fire ants and his Ig Nobel Prize-winning research on why wombats produce cube-shaped feces.

“Wok hei and the Maillard reaction both require intense heat. The commercial Chinese stoves generate an astonishing amount of heat,” says Ko, who spent months studying the mechanics of how chefs toss fried rice with a wok and simulating the rice’s trajectory.

The Maillard reaction is a chemical process between amino acids and reducing sugars in food subjected to high heat. It’s responsible for browning food and releasing delightful aromas and flavors.

But why is it necessary to cook at such high heat and with such speed?

“The key is to extract the maximum wok hei in the shortest time. That way, the aromas released during the Maillard reaction stay locked in,” explains Yip from The Chairman.

Therefore, an essential part of achieving wok hei—beyond the fire and the wok itself—is the chef's tossing technique.

The proper technique for tossing a wok

Tossing a wok is an art that requires time and practice to master.

At The Chairman, a novice chef spends over a year perfecting their wok skills by preparing meals for the staff before they’re trusted to cook for customers.

“Why don’t other chefs use a wok? It’s heavy, and the fire is tough to manage—it’s no wonder Chinese chefs have no arm hair left,” says Yip, only half in jest.

Why isn’t stirring enough? In fried rice, each time the rice leaves the hot wok surface, it cools down and avoids burning, as shown in the video above.

“Tossing the wok ensures better distribution, which is crucial when cooking at extremely high heat. Stirring at such temperatures usually leads to burning,” says Ko.

In his study, Ko found that chefs often pivot their woks using the edge of the stove, rather than lifting the entire wok, to conserve energy and speed up cooking.

Each toss involves two simultaneous motions: pushing and pulling back and forth, along with tilting and rotating the wok in a see-saw movement.

What makes the round-bottomed, highly conductive wok such a special tool for cooking?

“Other utensils might work, but you need to mix at incredible speeds to ensure the heat is distributed evenly through your ingredients,” says Ko.

On average, the chefs in the study tossed their woks at a rate of 2.7 times per second.

This intense activity is also why many Chinese chefs experience muscle strain and injury.

One of the objectives of Ko’s research was to explore the possibility of developing a robot to assist chefs in tossing their woks, easing the physical strain on their bodies. He believes the findings could have broader applications in everyday life.

“Imagine a clothes dryer that uses the same tossing technique as the wok – I think it would be more efficient, and probably a lot funnier,” Ko muses.

How to achieve the perfect fried rice.

Fried rice gained widespread attention in July after a viral YouTube video titled 'Uncle Roger DISGUSTED by this Egg Fried Rice Video' took off.

The video features 'Uncle Roger,' a character created by Malaysian-born, UK-based comedian Nigel Ng, as he reacts to a BBC tutorial on making egg fried rice.

In the video, Uncle Roger points out every mistake in the original egg fried rice demonstration, a reaction that has since racked up over 17 million views. One of the key issues? Watery rice.

This topic strikes a nerve with chefs in Hong Kong.

'Fried rice and fried beef noodles are the two dishes commonly used to evaluate the wok hei of a restaurant,' says Yip. 'Achieving the perfect balance of slightly toasted rice or noodles mixed evenly without burning is a real challenge.'

Ko concurs.

'Fried rice is a deeply iconic dish,' he says. 'It may appear simple, but making the perfect fried rice is actually quite challenging. The key is to keep the heat up—avoid adding watery ingredients that cool everything down—and stir frequently to prevent sticking or burning.'

Ko recommends using rice that was cooked the day before.

'It all comes down to the high heat. When you add leftover rice that's dried out, the moisture is minimal, which helps keep the wok hot and stops the rice from clumping together,' says Ko, the scientist.

At The Chairman, however, they take a slightly different approach.

'We know that most people use leftover rice because it’s drier. But we don’t. Our goal is to keep the rice moist on the inside while preserving its aroma. The secret? Eggs,' explains Yip.

Kwok, the chef, steps in to show how it's done.

He begins by quickly stir-frying the finely chopped ingredients in the wok, drying them out before setting them aside. Then, he adds the oil, egg mixture, and rice separately.

'Eggs cook faster than rice. The chef needs to be quick and mix everything together. Notice how you don’t even see the egg anymore?' Yip urges this writer to take a bite before the aroma fades.

And it’s true. The rice is slightly toasted and steaming, with a dry exterior, each grain coated in a perfect layer of golden yellow. The egg is invisible now, but the flavors are rich and the rice still steaming with every bite.

'Can you taste that?' Yip asks. 'That’s wok hei.'