How Tiny Pores Make Korea's Onggi a Fermentation Tool

The reputation of Korean onggi as 'breathing' pottery sounds poetic, but its function can be described in physical terms. A 2023 study in the Journal of the Royal Society Interface examined the permeability of onggi walls and how that property changes the gas environment during fermentation. [1][2]

The researchers fermented salted napa cabbage in traditional onggi and in hermetically closed glassware. Rather than evaluating flavor, they measured carbon dioxide, a product of microbial activity, while characterizing the jars' porous structure with imaging and fluid-mechanics methods. [1]

The onggi contained interconnected pores ranging across microscopic sizes. Its permeability allowed carbon dioxide to move outward through the clay, keeping the internal concentration below the level found in glass. A mathematical model was then used to estimate how quickly the fermenting cabbage generated gas. [1]

According to that model, the cabbage in onggi generated about 26% more carbon dioxide during the 48-hour experiment. The authors interpreted this as evidence of greater bacterial proliferation under the conditions created by the jar. [1][2]

The outward flow may have another useful effect. The paper proposes that positive pressure inside the jar and continuous gas movement can reduce the entry of an external contaminant larger than the pores, functioning somewhat like a passive safety valve. [1]

The study does not establish that every traditional jar will behave identically. Onggi permeability depends on manufacturing details such as clay composition, firing temperature and glazing. The experiment also used salted cabbage rather than the full mixture of ingredients found in finished kimchi. [1]

Earlier food-science research had already reported differences in kimchi fermented in onggi compared with other containers. The newer work adds a possible physical mechanism by connecting microbial activity with the movement of gas through the jar itself. [4]

Onggi therefore offers an interesting meeting point between craft and engineering. Generations of potters created a useful fermentation vessel without measuring pore diameter or modelling gas flow; modern experiments can now describe some of the material behavior that helped make the vessel valuable.