Chefs in Japan dip a wet chopstick in hot oil and listen to the sizzle, to know when it’s ready for tempura. A physicist investigated that technique in the lab to zero in on the perfect fry frequency.


Thanksgiving is just 10 days away, and perhaps you’re planning to brave the deep fryer this year. Well, there are plenty of examples on YouTube of what not to do, like this frozen turkey disaster.


UNIDENTIFIED GROUP: Whoa (laughter).



So how do you know when the oil is ready? Well, a thermometer is an indispensable tool. Researchers in the field of fluid dynamics have an additional trick – listen to the sound of the oil.


CHANG: The scientists were inspired by a classic kitchen hack used to test tempura frying oil. You wet the pointy end of a wooden chopstick. You stick it into the oil. And then you listen.

TADD TRUSCOTT: And if you hear really loud popping or crackling, it’s probably too hot.


TRUSCOTT: And then if you don’t hear anything, it’s usually too cold.

CORNISH: Tadd Truscott is with the King Abdullah University of Science and Technology in Saudi Arabia. He says, when you get the oil just right…

TRUSCOTT: There’s sort of this nice bubbling sound. It almost feels kind of like a song (laughter) to some people – is how it was described to me once (laughter).

CHANG: His team wanted to see what kinds of bubbles form at different temperatures and how those bubbles, in turn, create this sizzling song. So they repeated the chopstick test at different oil temperatures while recording audio and high-speed video.

CORNISH: Truscott’s collaborator, Rafsan Rabbi of Utah State University, says that when you dip a wet chopstick into hot oil, the water starts to vaporize, forming water bubbles that rise to the surface. Air bubbles form, too.

RAFSAN RABBI: Now, that water bubble and those air bubbles would be different in shape and size, and that would dictate the amount of noise that you’re actually hearing, and that would degrade the frequency of the noise that you’re hearing.

CHANG: Now, if at this point you’re wondering why scientists care so much about fried foods, Truscott says the phenomena they’re studying go way beyond the fryer.

TRUSCOTT: In fact, even in your car, every time your piston – every time there’s combustion, a lot of these exact same behaviors are happening. And so all of this is important in our daily lives. We just don’t realize it. It’s all happening behind the scenes.

CORNISH: Real-world applications are fine, but how many cooks have seen their work? Collaborator Akihito Kiyama a postdoctoral researcher at Utah State, says he looked close to home for a willing chef.

AKIHITO KIYAMA: Oh, yeah, my wife. Yeah (laughter).

TRUSCOTT: Yeah, Aki’s wife was our go-to chef (laughter).

CHANG: Kiyama will present the results next week in Phoenix at the 74th Annual Meeting of the Division of Fluid Dynamics. No word yet on how many fry chefs will be there.


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