How our brains analyze the sound of food to determine crispness.

Can you hear how crispy-crunchy it is? You could…if it weren’t a picture. Credit: Martin Workman

Perhaps it’s not the most common dinner party compliment, but according to a recent study out of the University of Leeds, it may be an appropriate one.

The paper, published in the Journal of Texture Studies reports a dual discovery: A bite into a crispy type of food emits loud ultrasound pulses, and in just milliseconds, our ears and mouths analyze that sound to decide just how relatively crunchy the food is.

“It is the [sound] pulses which actually create the sensation of crispy-crunchiness,” said Malcolm Povey, professor of food physics and the lead author of the study.

The researchers used a microphone that records over a wide range of frequencies—from a few Hz to the very ultrasonic 200 kHz, a higher pitch than even dolphins can hear—to measure the sound of different cookies as they broke under a controlled force. The researchers found the cookies emitted short pulses of sound, and the crunchier the cookie, the more frequent the pulses.

Additionally, test subjects bit into the cookies and rated each one on its “crispy-crunchiness.” Povey said the ratings directly corresponded to the sounds that rang out when the participants bit into the cookies.

“We found an incredibly high correlation between the rate at which these pulses arrived in the microphone and people’s assessment of how crispy-crunchy the cookies were. I mean, we’re talking about correlations of over 90%,” he said. “What this tells me is that this is how the brain is assessing this particular sensory aspect of foods.”

Povey noted that each sound pulse corresponds to a single fracture in the cookie, and that crisp cookies fracture more frequently than less crisp ones. The breaking does not create a tone of a single frequency, but rather each pulse consists of many frequencies, some audible and some supersonic. Povey said he was shocked by the large amplitude of these pulses.

“They were right up to the point which, if they were tones, they’d destroy human hearing,” he said. “Most of them were above the threshold of pain, if they were tones. But the energy’s spread over all these frequencies, you see, so although there’s an awful lot of energy in the pulses, they don’t damage human hearing.”

Were the frequencies closer together or all within hearing range, we might experience severe hearing loss every time we bit into a crisp cookie. Thankfully, that’s not the way the cookie crumbles.

Originally published February 20, 2006

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