When Curious City listener Kaycee Mendoza heard about a looming popcorn shortage earlier this summer, she was worried.

Kaycee is a popcorn fanatic — and she’s particularly fond of the cheese and caramel crisp combination made by the Garrett Popcorn shops in Chicago.

“I feel like it’s the perfect mix of salty and sweet.” she said.

To Kaycee, caramel and cheese doesn’t necessarily seem like an intuitive pairing. So she wanted to know why this iconic Chicago popcorn mix works so well.

Chicago-based Garrett Popcorn makes nine different flavors, but the Garrett Mix — that combination of caramel and cheese — is the company’s best seller, said Garrett co-owner Megan Chody. It’s popular enough that locals refer to it as “Chicago Mix.”

To learn more about why this particular sweet and salty blend can seem so irresistible, we turned to food scientist Kantha Shelke, principle at Corvus Blue LLC, a food science and nutrition research firm, and senior lecturer at Johns Hopkins University. Shelke said it has to do with how our bodies are wired.

So just why is that cheesy, caramel-y combo so compelling?

The first — and perhaps unsurprising — reason, Shelke said, is that humans are programmed to like different flavors paired together.

“Humans are the only species that loves variety and it is for the fun of it and also for the health of it,” she said.

While caramel and cheese popcorn may not seem like it’s got much in the way of health benefits to offer, the sweetness of the caramel and saltiness of the cheese activates primal nutrition-seeking instincts, according to Shelke.

“Sugar means energy,” Shelke said. “It’s also sort of a reward. ‘Ah, that fruit has become ripe. It’s sweet. It’s good for me,’” she added.

In other words, our brains recognize sugar as an easy energy source, and reward us for finding it out in the world. According to Shelke, we gravitate towards sweet foods because our brains rely on glucose to function.

As for the cheese, Shelke said our brains recognize salt as providing minerals our bodies need to regulate nerve and muscle function.

“We look at salt as a way to get the electrolytes,” Shelke said. “And so when we get something that is salty, we know we are replenishing because we sweat it out.”

Salt also enhances flavor, making foods taste better — not necessarily saltier. Shelke said this enhancement blocks foods’ bitterness, spiciness and sourness, making them sweeter.

“If you’ve got a sour Granny Smith apple, dip it in salt and eat it,” Shelke advised. “It will suddenly taste very sweet because it cuts the sourness.”

This same technique can be applied by adding a dash of salt to coffee or salted butter to your marmalade toast to neutralize the bitterness.

By the same token, salt can enhance the sweetness of even non-bitter and non-sour foods.

And that, Shelke said, is why salty and sweet combinations like the famous Garrett Popcorn mix are so tempting.

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