Kimchi, an ancient staple of Korean cuisine, is traditionally fermented in handmade earthenware jars called onggi. But when it comes to mass production, companies use jars made of glass and other materials to ferment kimchi in large quantities. Now a new study demonstrates why some kimchi makers still swear by the traditional technology: onggi provide great environments for the growth of lactic acid bacteria, the “good” microbes that give kimchi its signature sour flavor.
“Having higher bacterial proliferation is nutritionally beneficial and generates [the] unique taste of fermented food,” says Soohwan Kim, a mechanical engineering Ph.D. student specializing in fluid mechanics and biophysics at the Georgia Institute of Technology. For a study published last month in the Journal of the Royal Society Interface, Kim and his colleague compared an onggi with a glass jar and found the ceramic vessel produces kimchi with higher levels of lactic acid bacteria over the same fermentation time period. “There is a cultural belief that onggi used in fermented food makes the food better, but there isn’t good science on it,” Kim says. His work aims to change that.
Creating delicious kimchi is an artistic, scientific and culinary endeavor in which the fermentation process is key. Because many different factors affect that process, kimchi and other fermented foods are finicky, notes Maria L. Marco, a food microbiologist at the University of California, Davis. “Even within one particular food [made] using the same ingredients and same recipe, how can there be different flavors and outcomes to that fermentation? That’s a big question we don’t have all the answers to right now,” says Marco, who wasn’t involved with the new study.
In order to study the fermentation process, Kim and his doctoral adviser, Georgia Tech fluid dynamicist David Hu, used a combination of experimentation and mathematical modeling. They concluded that producing kimchi in an onggi enables more bacterial growth than doing so in a sealed glass jar. More importantly, the researchers uncovered what makes onggi ideal vessels for this process.
“I thought it was an interesting approach to try to begin to untangle how making fermented foods in different ways can affect the food that’s made,” Marco says.
The difference between the containers is related to their permeability, or how quickly liquid and gas can pass through them. Producing outstanding kimchi requires a Goldilocks situation, Kim says: fermentation vessels need to be semipermeable rather than overly permeable or impermeable.
Permeability is important because it influences the delicate dance between carbon dioxide and lactic acid bacteria. These microbes naturally migrate from soil to the vegetables growing in it, including the cabbage and other ingredients that form the base of kimchi. During the salty fermentation process, lactic acid and other types of bacteria flourish and expel carbon dioxide, which begins to build up in the container. The researchers’ results demonstrate that the lactic acid bacteria “proliferate in a moderate carbon dioxide level,” Kim says. But excessive carbon dioxide needs somewhere to go; otherwise it will grow to a high concentration that will eventually hinder the all-important bacterial growth. Conversely, if the container is too permeable, the carbon dioxide will escape too rapidly, reducing bacterial growth.
After Kim purchased an unglazed onggi from his hometown on Jeju Island, South Korea, he and Hu measured carbon dioxide levels as brined cabbage fermented in both the onggi and a tightly sealed glass jar. They carried out three trials per jar, sterilizing the containers in an autoclave before each trial. For accurate measurements, the researchers covered the onggi with a custom-made lid that could accommodate sensors for pressure, carbon dioxide and oxygen. They then used a thin plastic film to seal any remaining gaps between the sensors and the lid. Kim and Hu noted in the study that the glass jar leaked a little, albeit less than the onggi.
For two days per trial, the cabbage-filled onggi and glass jar rested in an oven set to 25 degrees Celsius. “I put that higher-than-usual [temperature] to accelerate the fermentation process,” Kim says. While kimchi can be fermented at much lower temperatures, such as five degrees C, Kim notes that he and Hu assumed that the only change resulting from the higher temperature “is the speed of the process.”
The researchers also used electron microscopy and computed tomography scans to probe the porous structure of the onggi and calculate its “gas permeability coefficient,” or how easily gas could pass through it. They discovered that this value was more than double that of the glass jar. Because the onggi allowed carbon dioxide to escape more rapidly than its glass counterpart, lactic acid bacteria thrived. In the glass jar, where carbon dioxide wasn’t released as quickly, the gas stifled bacterial growth. “The porous structure of the onggi mimics the loose soil where lactic acid bacteria [are] naturally found,” the researchers wrote in their paper. This helps ensure that food fermented in the vessel teems with the microbes.
The researchers also demonstrated the onggi’s permeability with an experiment drawn from Korean culture: they filled the vessel halfway with saltwater and showed that within eight hours, salt deposits began coating its outer surface. “Korean culture calls it a ‘salt flower,’” Kim says. This phenomenon is usually observed when Korean chefs ferment soy sauce (or ganjang) in onggi. When these salty blooms appear on the outside of an onggi, they are thought to signify that that particular vessel is permeable enough to produce top-notch sauces.
“Even in the ancient culture, [kimchi makers used] regulation of gas level to optimize the fermentation process,” Kim says. Although large-scale kimchi producers combine glass, plastic or metal jars with a gas regulation system, Kim and Hu’s work demonstrates that onggi can accomplish the same goal on their own.
Perhaps as a result of that ability, the traditional vessels are still in demand today: artisans continue to create onggi that kimchi enthusiasts can use to achieve quality fermentation.