The Kimchi Succession
The sharp, effervescent tang of kimchi is not from a single microbe, but a complex, multi-stage takeover by specialized bacteria. Initially, diverse microbes from the raw ingredients are present, but the salting process quickly selects for salt-tolerant lactic acid bacteria (LAB). In the early phase of fermentation, species like Leuconostoc mesenteroides and Weissella koreensis dominate. These heterofermentative bacteria convert, converting sugars in the cabbage into a mix of lactic acid, acetic acid, ethanol, and a significant amount of carbon dioxide. This CO2 production creates the anaerobic (oxygen-free) environment necessary for later stages and gives kimchi its characteristic fizz. Leuconostoc species also produce mannitol, a sugar alcohol that contributes a mild, refreshing sweetness, balancing the accumulating acids.
As fermentation progresses over days and weeks, the environment becomes increasingly acidic, with the pH dropping significantly. This acidic brine is inhospitable to the early-stage microbes. A new cast of more acid-tolerant, homofermentative LAB begins to flourish. Species such as Lactobacillus plantarum and Lactobacillus sakei take over. Unlike their predecessors, these bacteria primarily produce lactic acid, which sharply increases the sourness and further preserves the vegetables. This microbial succession is a predictable ecological shift, shifting the flavor profile from fresh and sweet to sour and complex.
Masters of the Meju
The foundational flavor of Korea's essential pastes—doenjang (soybean paste) and gochujang (chili paste)—originates with a block of cooked and dried soybeans called meju. The microbiome of meju is a different world from that of kimchi. Its fermentation is driven initially by molds, primarily from the genus Aspergillus. Specifically, Aspergillus oryzae (yellow koji mold) populates the drying soybean blocks. This mold secretes powerful enzymes, including proteases and amylases, which drive flavor development. Proteases break down the complex proteins in soybeans into smaller peptides and amino acids, creating the savory umami taste. Amylases break down starches into simpler sugars.
When the fermented meju is mixed with brine to make doenjang, a second fermentation stage begins, this time dominated by salt-tolerant bacteria. The primary microorganism in this phase is often Bacillus subtilis. This bacterium, along with various yeasts like Zygosaccharomyces, continues to break down the soy components, producing a complex array of volatile flavor compounds. In gochujang, the meju powder is mixed with chili powder, rice, and salt, creating a habitat where Bacillus species like B. subtilis and B. licheniformis thrive, contributing to its unique sweet and savory flavor profile.