The very best kimchi is made in conventional clay jars (onggi) – Ars Technica

The fermented spicy cabbage often called kimchi is a staple of Korean delicacies, historically made in earthenware vessels often called omg. Today most Korean households have kimchi fridges for that objective, whereas on a industrial scale, kimchi is made by way of mass fermentation in glass, metal, or plastic containers. However is the kimchi made with these fashionable contraptions of equal high quality to the normal fermentation technique? Many kimchi aficionados would argue that it’s not, and now the pro-onggi faction has some science to again up that assertion.

It seems that the porosity of the onggi’s partitions assist essentially the most desired micro organism proliferate throughout the fermentation course of, in line with a recent paper revealed within the Journal of the Royal Society Interface. “We needed to search out the ‘secret sauce’ for a way onggi make kimchi style so good,” said co-author David Hu of Georgia Tech. “So we measured how the gases developed whereas kimchi fermented contained in the onggi—one thing nobody had finished earlier than.”

The handmade clay vessels often called onggi have lengthy been utilized by Korean cooks to ferment meals, together with odd (I’m willow), gochujang (crimson pepper paste), and doenjang (soy bean paste), in addition to kimchi. The cabbage or daikon is sliced into small uniform items, that are coated with salt as a preservative. The salt attracts out the water and inhibits the expansion of many undesirable microorganisms. Then the surplus water is dried and seasonings are added, typically together with sugar, which additional serves to bind any remaining free water. Lastly the brined cabbage is positioned into an hermetic canning jar, the place it stays for the following 24 to 48 hours at room temperature. The jar is “burped” often to launch carbon dioxide fashioned throughout the fermentation course of.

There are numerous types of kimchi, nonetheless, and the winter selection was historically made in giant portions to final the season, saved within the floor in giant pots. Prior research have found that kimchi fermented in onggi over the course of a month had considerably greater development of the specified salt-loving lactic acid micro organism than kimchi fermented in plastic or metal containers, whereas slowing the expansion of undesirable cardio micro organism that may impart a foul style to the ultimate product. Onggi have additionally been proven to extend the acidity and antioxidant activity of kimchi, in line with Hu et al.

Hu and his workforce needed to study extra in regards to the connection between the fabric properties of the onggi and the expansion of micro organism throughout kimchi fermentation, and so they turned to fluid mechanics for steerage. They bought a big onggi from a village on Jeju Island in Korea, sufficiently tall and huge to accommodate the addition of airborne carbon dioxide sensors. The onggi was made within the conventional means by hand-pressing and slapping uncooked mud (made up of water, silt, and clay) and selecting out any pebbles. The clay was then fashioned into lengthy rods and the ultimate jar formed on a spinning wheel earlier than being left to dry out. The ultimate step concerned sintering the onggi inside a kiln for a day earlier than it was cooled down. This specific onggi was not glazed, which, the authors famous, might hamper permeability.

Hu et al. first used a scanning electron microscope and CT scanner to look at the pore construction of their onggi. Then they examined the permeability of the onggi by observing how water evaporated by way of the container over time. Subsequent they carried out experiments of the particular fermentation course of by making their very own kimchi—three trials every utilizing each the onggi they’d bought and a hermetically sealed glass jar (the higher to seize the method with time-lapsed video footage). Each vessels had been outfitted with carbon dioxide and stress sensors, which measured and in contrast the adjustments in carbon dioxide, since that may be a key signature of fermentation.

The time-lapsed video from the water-evaporation experiments confirmed the formation of “salt flowers” on the surface of the container over the course of eight hours—historically considered an indication of a high-quality fermentation vessel. “We surmise that salt water flows by way of the wall and evaporates on the outer floor, leaving salt crystals behind,” the authors wrote. “Evaporation of water on the outer floor is replenished by water depraved by way of the wall.”

The SEM evaluation of an onggi cross-section confirmed pretty giant pore diameter (most within the vary of 20-150 microns), fashioned because of all of the variables within the manufacturing course of. Porcelain, by comparability, has uniformly high quality particles which might be largely impermeable to fuel. The CT scan confirmed that the onggi had excessive porosity. And the carbon dioxide measurements confirmed that the onggi constantly “breathes” CO₂ throughout fermentation—one thing that has lengthy been hypothesized however not measured till now.

The truth that the obggi’s porous partitions are permeable to CO₂ helps scale back the degrees of the fuel contained in the vessel. These decrease ranges, in flip, are favored by the specified lactic acid micro organism, that are in a position to proliferate in higher numbers below such situations. Hu et al. even developed a mathematical mannequin to indicate how the CO₂ was generated and moved by way of these porous partitions.

“Onggi had been designed with out fashionable information of chemistry, microbiology, or fluid mechanics, however they work remarkably properly,” said co-author Soohwan Kima graduate scholar in Hu’s lab. “It’s very attention-grabbing to get these new insights into historic know-how by way of the lens of fluid dynamics. We hope this examine attracts consideration to this conventional artisan work and conjures up energy-efficient strategies for fermenting and storing meals. Additionally, the onggi are fairly stunning.”

DOI: Journal of the Royal Society Interface, 2023. 10.1098/rsif.2023.0034 (About DOIs).