The scientific reason kimchi is best made in earthenware onggi

For more than a thousand years, the spicy, pungent Korean cabbage dish known as kimchi was fermented in earthenware vessels called "onggi." Now, a pair of mechanical engineers have unraveled why these ancient Tupperware, made of mud slapped and pressed by hand and spun on a pottery wheel, are exquisitely suited to fostering the growth of probiotic microbes that transform humble cabbage into a culinary superstar.

Kimchi has come into vogue as a superfood, due in part to the gut-healthy bacteria that help deliver its distinctive punch and sour taste. But onggi have not followed suit, perhaps because they are bulky, heavy and fragile, and are not really designed to be portable or fit onto modern kitchen countertops. Instead of using onggi that are buried in the ground, many Koreans fill a second, specialized refrigerator with modern glass, steel or plastic containers of kimchi.

Previous studies had shown that kimchi made in onggi has greater acidity and antioxidant activity, higher numbers of lactic acid bacteria and slower growth of bad-tasting bacteria. But why?

"I thought there's still a big missing link," said David Hu, a mechanical engineer at Georgia Institute of Technology who previously unpacked the "wok tossing kinematics" behind making fried rice. "Everyone believed pores are important. Kimchi grown in these conditions have more [lactic acid] bacteria, and people didn't understand why."

The new study, published in the Journal of the Royal Society Interface, reveals that the walls of onggi are stippled with tiny micro-pores. Salty water can seep through and evaporate, leaving "salt flowers" on the outside of the vessel. The pores also allow the pottery to exhale some of the carbon dioxide produced by fermentation, creating an ideal environment for lactic acid bacteria to flourish.

Many professional chefs have an intuitive understanding of the benefits of using an onggi. Maangchi, a Korean chef and social media star, said that she owns several onggi, which she uses for different purposes — making jangajji (salty Korean fermented pickles), doenjang (fermented soybean paste), gochujang (fermented hot pepper paste) and homemade fermented alcohol.

She lives in an apartment in New York City with large windows, where she stores more than a dozen of the bulky pieces of pottery in the window so that they catch the sunlight. She remembers her grandmothers kept the outsides of their onggi very clean, perhaps because they understood, even without precise scientific measurements, that the pots needed to breathe.

Soohwan Kim, a mechanical engineer at Georgia Tech who led the experiments with an assist from his mother — whom he calls his "kimchi mentor" — said the study sought to unpack how those pores work, with the science of fluid mechanics.

Kim grew up on South Korea's Jeju-do, a volcanic island where onggi are still crafted today. He traveled back home to do much of the research, buying an onggi. He used a scanning electron microscope and a CT scan to zoom in and measure the pores. Those measurements allowed Kim and Hu to construct models to understand the complex fluid mechanics that transpire to make kimchi.

The scientists whipped up a standardized and de-spiced version of salted cabbage for experiments that allowed them to make measurements of carbon dioxide, oxygen and pressure within the onggi, to understand how it breathed. They found that even without specialized valves, sensors or monitors, the vessel generated an environment that was conducive to the growth of probiotic bacteria.

Maangchi said that she once did an experimental taste test, making one batch of kimchi in an onggi and one batch in a jar in the refrigerator. She invited her readers to try the two kinds at a tasting in Bryant Park.

For the most part, she said, they couldn't taste the difference.

Hayun Surl, an assistant professor of ceramics at Angelo State University in San Angelo, Tex., makes onggi from local clay. He acknowledged that many people still use onggi as a piece of culinary equipment, but he said that it is also transitioning from a purely functional object into an art piece with cultural and symbolic significance.

"The kimchi from the kimchi fridge is not bad, to be honest," Surl said. "We want to save space — most people living in apartments have no yard to put the big onggi jars out there."

Eunjo Park, a star chef based in New York, said that there isn't room for onggi in her small apartment. She said other members of her family, who live in the suburbs, have kimchi refrigerators, which help keep the temperature more constant. Park said it's possible to make kimchi with modern tools, but she still has fond memories of eating her grandmother's kimchi from a buried onggi, which had a special fizz.

"With the [lactic acid bacteria] fermentation, it gets very poppy, like with soda. It builds that," Park said. "If you try to buy regular kimchi in a store, it is sour and doesn't have the pop."