Fermented foods such as kimchi have been an integral part of Korean cuisine throughout history.
In combined experimentaland theoretical research Hu and Kim measured carbon dioxide levels in onggi during kimchi fermentation and developed a mathematical model showing how the gas is formed and passes through the porous walls of the vessel. “So we measured how gases are released while kimchi is fermenting inside the onggi. No one had done this before,” explains the professor.
The porous structure of clay vessels imitatesloose soil with lactic acid bacteria, which are known for their beneficial probiotic nature. Although previous studies had shown that kimchi fermented in onggi contained more beneficial microorganisms, no one knew for sure how this phenomenon related to the unique properties of the container material.
The experiment was carried out using a traditional jarHandmade onggi from an artisan in Jeju, South Korea. The scientists first tested the onggi's permeability by observing how water evaporates through the container over time. Then, carbon dioxide and pressure sensors were installed both in the vessel and in an ordinary hermetically sealed glass jar. They prepared the salted cabbage themselves and placed it in both containers. The scientists used sensors to measure and compare changes in carbon dioxide, a sign of fermentation.
Hu and Kim also developed a mathematical modelbased on the porosity of the onggi. The model allowed them to infer the rate of carbon dioxide formation as the vessel gradually released it. They concluded that the porous walls of the container allow carbon dioxide to escape from the onnga, speeding up fermentation.
The porosity of the container also serveda “safety valve,” which caused carbon dioxide levels to increase more slowly than when fermented in a glass jar, blocking the entry of external particles. The data showed that CO₂ levels in onggi were half that of glass containers. As a result, scientists were able to uncover the ancient secret of ideal, high-quality kimchi - it’s all about traditional vessels and their effect on fermentation.
"Ongs were developed without modern knowledgein the field of chemistry, microbiology or fluid mechanics, but they work surprisingly well,” the scientists write. The semi-porous nature of Onggi is unique compared to other forms of earthenware. It is not easy to make a clay vessel that “leaks”, but only slightly. Terracotta containers, for example, drain water too quickly.
“It’s amazing that for thousands of years peopleThey built these special containers out of mud, but in many ways they are very high-tech. We found that the right degree of porosity allows kimchi to ferment faster, and onggs ensure this,” the study authors added.
Some artisans still use the ancient onesmethods for making onggi, but their number is decreasing. The market is now flooded with inauthentic versions of the vessels. “We hope this research will draw attention to this traditional craft work and inspire energy-efficient methods of fermentation and food storage. In addition, onggs are quite beautiful,” the scientists conclude.
Kimchi is a Korean dish that representsspicy pickled vegetables, primarily Chinese cabbage. In general terms, these are pickled heads or leaves of cruciferous vegetables seasoned with red pepper, green onions and onion juice, garlic and ginger.
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