Kimchi-Making and Microbiology
by Janie
Kimchi-making is a matter of tossing cabbage leaves and spices together, followed by a trust exercise with lactic acid bacteria like Lactobacillus and Leuconostoc. But when it comes to preparing South Korea's national dish, there are myriad modi operandi and many subtleties even within those. Each region in the country has its own trademark pickled pizzazz. The further you stray from Seoul, the more pungent the extra ingredients – a heftier dose of fish sauce, or shucked oysters, or raw fish slices if you get as far south as Gyeongsang-do. Off-putting to some, but piquant and multidimensional to others.
Figure 1. Baechu kimchi. Source. Frontispiece
The variety doesn't end there. Beyond classic cabbage baechu kimchi, there is also kkakdugi, which is made with cubed Daikon radishes; chonggak kimchi, which translates to "bachelor" kimchi, because the particular radish variety has tufts of stems on top that resemble the long braids once worn by young men in old Korea; baek kimchi, which translates to "white" kimchi because it has no red chili flakes and isn't spicy; mul kimchi aka "water kimchi," which comes in a cold broth evocative of hot summer days. There is also kimchi made of green onions, cucumbers, mustard leaves, sesame leaves… Not to mention unfermented kimchi varieties.
Roberto's recent piece on the myth around San Francisco sourdough starters piqued my curiosity about this other fermented food near and dear to my heart. With this sheer variety of kimchi, it doesn't seem a stretch to imagine that different microbial compositions would thrive on different vegetables after different methods of preparation. So, kimchi from different regions must have characteristic microbial compositions. Natural products from the different vegetables must surely sway the microscopic residents of the kimchi? With these questions, I did some peeking into the different steps in kimchi-making in which microbes shine.
My starting point was the book A Journey in Search of Korea's Beauty by Bae Yong Joon. With that title, kimchi is a must-have feature! One section describes the author's observation of a gimjang, the annual wintertime preparation of kimchi. He writes:
"[She] showed us a kimchi jar that she had prepared in advance. She had added bamboo leaf on top of the kimchi. She explained that she brought the leaves from Gongju, a few hours drive south of Seoul, and that the leaves had to be plucked while the bamboo trees were in blossom. I did not know her exact reason for this at the time, but later, while leafing through a book on trees at home, I read that bamboo leaves contain natural antibacterial and preservative properties. In the old days, people used the leaves of the Northern Bamboo (Sasa borealis) tree to wrap rice cake. The leaves helped to keep the rice cake from spoiling too fast."
When the author gets into making his own kimchi a few pages later, he is sure to remember the bamboo leaves: "We pressed bamboo leaves firmly onto the top of the packed kimchi and closed the lids."
This was interesting – does the addition of bamboo leaves select for particular microbial species? Are the fermenting lactic acid bacteria relatively unaffected by the bamboo leaves' antibacterial compounds, while other microbes that may cause food spoilage are susceptible? Bamboo leaves were used often in the old days without modern refrigeration, and it's a tradition that has carried over into present-day. According to two studies (here and here), volatiles found in bamboo species include alpha-ionone and beta-ionone, which have well-demonstrated antimicrobial properties.
The other curiosity from the passage is the requirement for plucking the leaves while the bamboo is in bloom. My first guess was that the antimicrobial content maybe varies seasonally, the spring being ideal. (As a precedent, the antioxidant content of bamboo leaves varies season to season.) But a bit of Googling revealed that a bamboo plant only flowers once in its lifetime, which typically spans a few decades... In any case, into the earthenware jars go the mystical bamboo leaves.
Figure 2. Onggi gathered outside of a traditional Korean house (architecture originating from the fourteenth century Joseon Dynasty). Source
Those earthenware jars are a microbiological wonder, too. They are called onggi, and are used for fermenting various foods including kimchi, doenjang (soybean paste, used often as a broth base), and gochujang (chili pepper paste). The art of onggi-making first calls for iron-rich clay. The resulting kiln-fired jars have high microporosity, which can be further controlled with the surface glaze that is applied, which can in turn affect the taste of the resulting fermented product. The red-brown vessels are also called soomshineun hangali, which translates to "breathing earthenware pots."
It's unclear how the high iron content and/or microporosity of the jars affects the fermenting bacteria themselves: lactic acid bacteria across genera have long been known not to produce siderophores and to grow blithely unaffected by the presence or absence of iron (but Lactobacilli, including those isolated from kimchi, are apparently still capable of chelating iron). Kimchi fermented in onggi has higher concentrations of Lactobacillus and Leuconostoc species than kimchi fermented in plastic, glass, or steel containers (Fig. 3). It also has lower overall proliferation of aerobic bacteria, which the authors suggest is beneficial in preventing over-ripening and spoilage.
Figure 3. Bacterial content in kimchi fermented in each type of container. (a) Total aerobic bacteria (b) Leuconostoc sp. © Lactobacillus sp. The filled circles indicate nonglazed onggi containers, the filled triangles glazed onggi, the filled squares polyethylene plastic, the open circles polypropylene, the open triangles stainless steel, and the open squares glass. Source
Even onggi produced in different regions of Korea vary in particle size and porosity, introducing yet another variable. Not to mention all the variables within the kimchi itself. From the type and amount of garlic or ginger or green onion added, the variation in edible ingredients affects the fermentation rate as well. One study found that adding fermented anchovy sauce (a common ingredient) to kimchi resulted in increasing levels of glutamate (aka MSG, aka the embodiment of that oh-so-exalted umami) throughout the fermentation period not seen in kimchi made sans anchovies.
Presumably, kimchi fermented in onggi tastes better, too. Chemical composition varies between kimchi made in onggi and kimchi made in other containers – a more objective measure – but the "sensory evaluations" of kimchi involve a randomized panel of participants who judge based on color, smell, texture, flavor. (I wonder how the researchers recruit these people? A Seoul native's taste buds might not agree with the fishier flavor beloved to someone from a more southern locale…).
Despite all the contention, here is at least one consensus of opinion: most Koreans agree that any good kimchi has an effervescent feeling on the tongue. This is called siwonhanmat, which translates to "refreshing taste," or saida-gateun-mat, "a soda-like taste." It never occurred to me before to think about this, but this is literal carbonation, since lactic acid fermentation produces carbon dioxide.
In a post-pandemic future, I'll go beyond the in silico research and watch the culinary mastery in situ.
