Kimchi Fermentation in Permeable Korean Earthenware Onggi

Since ancient times, Korean chefs have fermented foods such as kimchi, gochujang, and soy sauce in traditional fermentation vessels called Onggi. Onggi is purported to provide gas permeability which facilitates the growth of lactic-acid-forming bacteria. In this experimental study, we perform time-lapse videography and carbon dioxide measurement in various fermentation vessels, including Onggi. Counter-intuitively, we find that the permeable wall of Onggi does not restrict the growth rate of bacteria but can limit their maximum number. The porosity thus acts as a safety valve for bacteria growth by blocking the entry of external contaminants without mechanical components.

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“Onggi” is a traditional vessel to ferment foods

^{© 2014, Ministry of Culture, Sports and Tourism and Korean Culture.}

According to ancient texts, Koreans have enjoyed fermented foods since the Goguryeo Kingdom (37 B.C. – A.D. 668). Before the advent of refrigeration, Korean chefs used handmade clay vessels called onggi to ferment their foods. Onggi were designed without modern knowledge of chemistry, microbiology, or fluid mechanics.

Onggi can be found in numerous historic records. Anak Tomb No. 3 is a Goguryeo mural tomb located in Anak-gun, Hwanghaenam-do, North Korea. It was excavated in 1949 and built in AD 357. This cultural heritage is considered one of the most well-represented murals in the life of Goguryeo at the time, with various screen compositions and elegant expressions. One mural depicts a wellside where water is drawn from a bucket using the principle of a lever alongside multiple onggi. Since almost 2000 years ago, Onggi culture has been part of Korean life.

^{© 2020, Northeast Asian History Foundation}

The scenery of Onggi markets

^{Top – Jeju Onggi Village, Bottom – Seoul Jinggwang Onggi
© 2021, Soohwan Kim.}

Lactic acid bacteria Everywhere

^{© Lactobacillus species (Arasu et al., Ann Microbiol, 2015)}

^{© Leuconosto species (Fred Breidt, North Carolina State University)}

Lactic Acid Bacteria (LAB) live in the gut and on the skin of all living organisms. A variety of LAB, including Lactobacillus species and Leuconostoc species are commonly found in every soil and on every leaf surface. Their abundance indicates their importance. For decades, LAB has been used in agricultural systems for improving soils, controlling diseases, and promoting plant growth. Just as Lacto-fermented food is free of pathogens, lactic acid in soil appears to provide protection from disease organisms.

Micro-pores in Onggi wall

Manufacturing process

Collect mud → Press & slap mud → Pick out pebbles → Divide into broad-long pieces → Shape on spinning wheel → Dry pot → Glaze → Sinter slowly in kiln (~1200°C)