Spicy fermented cabbage known as kimchi is a staple of Korean cuisine, traditionally made in earthenware vessels called onggi. Nowadays, most Korean households have kimchi refrigerators for this purpose, while on a commercial scale, kimchi is made by mass fermentation in glass, steel, or plastic containers. But is the kimchi made with these modern contraptions equal in quality to the traditional fermentation method? Lots of kimchi aficionados seems like it isn’t, and now the pro-onggi faction has scientific evidence to back up that claim.
It turns out that the porosity of the walls of the onggi helps the most wanted bacteria to proliferate during the fermentation process, according to a recent article published in the Journal of the Royal Society Interface. “We wanted to find the ‘secret sauce’ for how onggi make kimchi so good,” said co-author David Hu of Georgia Tech. “So we measured the evolution of gases during the fermentation of kimchi inside the onggi, which no one had done before.”
Handmade clay vessels known as onggi have long been used by Korean chefs to ferment foods, including odd (I am willow), gochujang (red chilli paste), and doenjang (soybean paste), as well as kimchi. The cabbage or daikon is cut into small uniform pieces, which are coated in salt as a preservative. Salt sucks up water and inhibits the growth of many unwanted microorganisms. Then the excess water is dried and seasonings are added, often sugar, which further serves to bind any remaining free water. Finally, the pickled cabbage is placed in an airtight canning jar, where it remains for the next 24 to 48 hours at room temperature. The pot is “burped” from time to time to release the carbon dioxide formed during the fermentation process.
There are many varieties of kimchi, however, and the winter variety was traditionally made in large quantities to last the entire season, stored in the ground in large pots. Previous studies have shown that kimchi fermented in onggi for a month had significantly higher growth of salt-loving lactic acid bacteria than kimchi fermented in plastic or steel containers, while slowing the growth of unwanted aerobic bacteria that may impart a bad taste to the final product. According to Hu et al.
Hu and his team wanted to know more about the connection between the material properties of onggi and the growth of bacteria during kimchi fermentation, and they turned to fluid mechanics for guidance. They purchased a large onggi in a village on Jeju Island in Korea, tall and wide enough to accommodate the addition of airborne carbon dioxide sensors. Onggi was made in the traditional way by hand pressing and pounding raw mud (composed of water, silt and clay) and removing the pebbles. The clay was then shaped into long rods and the final pot shaped on a spinning wheel before drying. The final step was to sinter the onggi in a kiln for a day before it was cooled. This particular onggi was not glazed, which the authors said could impede permeability.
Hu et al. first used a scanning electron microscope and a CT scanner to examine the pore structure of their onggi. Next, they tested the onggi’s permeability by observing how water evaporated through the container over time. Next, they experimented with the actual fermentation process by making their own kimchi – three trials each using the onggi they had purchased and an airtight glass jar (to better capture the process with time-lapse video footage ). Both vessels were equipped with carbon dioxide and pressure sensors, which measured and compared changes in carbon dioxide, as it is a key signature of fermentation.
Time-lapse video of the water evaporation experiments showed “fleurs de sel” forming on the outside of the vessel for eight hours, traditionally considered a sign of a high-quality fermentation vessel. “We assume that the salt water passes through the wall and evaporates on the outer surface, leaving behind salt crystals,” the authors wrote. “The evaporation of water on the outer surface is replenished by water seeping through the wall.”
SEM analysis of a cross-section of onggi showed quite a large pore diameter (most in the 20-150 micron range), formed as a result of all of the manufacturing process variables. Porcelain, in comparison, has uniformly fine particles that are largely impermeable to gas. The scanner confirmed that the onggi had high porosity. And carbon dioxide measurements showed that the onggi is continuously “breathing” CO2 during fermentation, which has long been hypothetical but not measured until now.
The fact that the porous walls of the obggi are permeable to CO2 helps reduce gas levels inside the tank. These lower levels, in turn, are favored by the desired lactic acid bacteria, which can proliferate in greater numbers under such conditions. Hu et al. even developed a mathematical model to show how CO2 was generated and moved through these porous walls.
“The Onggi were designed without modern knowledge of chemistry, microbiology, or fluid mechanics, but they perform remarkably well,” said co-author Soohwan Kim, a graduate student in Hu’s lab. “It’s very exciting to get this new insight into old technology through the prism of fluid dynamics. We hope this study will draw attention to this traditional craftsmanship and inspire energy-efficient methods for fermenting and preserving food. Also, the onggi are quite beautiful.
DOI: Journal of the Royal Society Interface, 2023. 10.1098/rsif.2023.0034 (About DOIs).