The Microbial Engine of Korean Flavor
Meju (메주) is a brick of dried, fermented soybeans that is the base for some of Korea's most essential foods, including soybean paste (doenjang) and soy sauce (ganjang). The process begins with boiling soybeans for six to eight hours, crushing the hot beans, and pressing them into dense blocks. These blocks are then dried for several days before undergoing a wild fermentation that can last for months. This is not a sterile process; the blocks are colonized by a complex succession of microorganisms from the environment, particularly from the rice straw used to tie and hang the blocks. The air itself, inside and outside the fermentation rooms, contributes to the microbial population, with fungal concentrations inside fermentation rooms averaging around 1,524 CFU/m3.
The fermentation is a multi-stage microbial relay. Initially, bacteria from the genus Bacillus, especially Bacillus subtilis, dominate the process. These bacteria are often introduced via the equipment used to mold the meju and are necessary for producing enzymes like proteases and amylases. These enzymes begin breaking down the soybeans' proteins and carbohydrates into simpler compounds. As fermentation progresses, the microbial community shifts. Lactic acid bacteria such as Enterococcus and Lactococcus appear, which can lower the pH early on. Fungi, primarily from the phylum Ascomycota, also establish themselves. While yeasts like Candida may be present early, molds like Aspergillus, Scopulariopsis, and Monascus become dominant later in the process. The species present vary significantly by region and even by household, creating different flavor profiles.