Conversations with Tae-Woon Kim: On the safety of yeast colonies during kimchi fermentation

On the surface, kimchi can be considered a Korean-style pickle. However, under the hood, there’s a lot more going on than simple brining. It’s the process of fermentation that really makes the popular Korean side dish hum. A strain of Lactobacilli is responsible for the preservation process. And while this goes on, other colonies of microorganisms are known to set up shop as well. One well known phenomenon is the formation of a white film on the surface. Questions regarding the safety have arisen from time to time. Now, a scientific study has been done to surmise exactly what the film is and whether it displays any toxicity.

SCIENTIFIC INQUIRER: For people who are used to eating kimchi but know nothing about how it’s produced. Can you briefly describe the general process?

TAE-WOON KIM: here are many ways to make kimchi as can be seen on Google or Youtube.

The amount of cabbage and other ingredients used depends on amount of kimchi
being made. This is a general recipe.

1) Remove the outer leaves of kimchi cabbage, cut it in four quarters
2) Soak the cabbages in the salted water for 8-10 hours.
3) Put glutinous rice flour and water in a pan and boil it well.
4) Allow the glutinous rice paste to cool sufficiently.
5) Cut onion, scallion, leeks, water parsley, carrot and radish
6) Grind garlic and ginger
7) Add glutinous rice paste, grinded garlic, grinded ginger, shrimp, anchovy sauce, red pepper powder, sugar, sesame seeds into a large bowl and mix well to make seasonings
8) Put the sliced vegetables in the seasoning you made.
9) Wash the salted kimchi cabbages three times in the water, then drain the water for long time.
10) Put the seasonings evenly between kimchi cabbage leaves

SI: How common is the formation of the white substance (white scum) on kimchi?

When kimchi is exposed to air for a long time, it occurs in most of the kimchi. It occurs quickly at
room temperature and slowly at low temperatures (below 4°C).

SI: Why did it take so long for someone to formally identify the colony? Specifically, why did your lab choose to undertake it?

The major strains involved in fermentation of kimchi are lactic acid bacteria. Yeast has received less attention than lactic acid bacteria. However, studies related to yeast have been performed for a long time.

People wondered its safety. It was a kind of specialized task in the World Institute of Kimchi.

SI: Prior to testing, did you have suspicions what the white substance (white scum) might be?

No, not specifically. We assumed it would be yeast. Previously, there were some papers that analyzed these microorganisms.

SI: How did you identify the mysterious white colony? What did you discover?

We identified white colony-forming yeast using deep sequencing and culture dependent method. Five yeast strains belonging to Hanseniaspora uvarum, Pichia kluyveri, Yarrowia lipolytica, Candida sake, and Kazachstania servazzii were isolated.

SI: Are the same mold common to all types of kimchi or were they specific to certain types?

Different types of white colony-forming yeast appear depending on the storage temperature even in same kimchi. Some researcher analyzed yeast community structure in seven different commercial kimchi products and found that yeast community depends on the manufacturer.

SI: On a microbial level, what is occurring that allows the formation of yeast colonies?

Important factors determining white colony-forming yeast are its fermentation temperature and extent of exposure to oxygen. Storage of kimchi in high temperature and exposure to air for a long time accelerate formation of white colonies on the surface of kimchi.

SI: The big question. Are the yeast colonies safe?

We have been eating kimchi after removing yeast colonies and washing for a long time. Our study indicates that the five yeast strains isolated in our study don’t have toxin or antimicrobial resistance genes. However, additional studies such as toxicity tests need to be done on animals in order to verify the safety of the identified yeasts are needed.

SI: Finally, can you explain the significance of your study?

This study is significant in that it is a step forward to the alleviation of the anxiety regarding the hygienic safety of kimchi through scientific proof.

IMAGE SOURCE: Creative Commons

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