Microbial communities in the fermentation of Meju, a Korean traditional soybean brick

Fermentative metabolic features of doenjang-meju as revealed by genome-centered metatranscriptomics

Fermentative features of doenjang-meju, a traditional Korean soybean brick, were investigated over 45 days via genome-centered metatranscriptomics. The pH value rapidly decreased within 10 days and successively increased after 20 days, along with an initial bacterial growth, including lactic acid bacteria, and subsequent fungal growth, suggesting their association. Polysaccharides and lipids underwent degradation, and amino acids, free sugars, and organic acids increased during the early stage. Metagenome analysis identified Aspergillus, Bacillus, Enterococcus, Staphylococcus, and Leuconostoc as major microbes, which were isolated and genome-sequenced. Metatranscriptomic analysis revealed the major roles of Bacillus and Enterococcus during the early period, shifting to Aspergillus dominance after 10 days. Metabolic pathway reconstruction and transcriptional analysis reveal that Aspergillus primarily decomposed polysaccharides to free sugars; Aspergillus and Bacillus metabolized lipids, free sugars, and organic acids generated by Enterococcus; and Aspergillus and Bacillus were instrumental in amino acid metabolism: their contributions varied by compounds and pathways.

Correlation between monosaccharide, oligosaccharide, and microbial community profile changes in traditional soybean brick (meju) fermentation

Meju is essential for making diverse traditional fermented soybean foods in Korea. To understand the changes in carbohydrates during fermentation, we aimed to identify autochthonous microorganisms from spontaneously fermented meju and compare the alterations in monosaccharides and oligosaccharides throughout the fermentation process. Microbial diversity was determined using a metabarcoding approach, and monosaccharide and oligosaccharide profiles were obtained by HPLC-Q-TOF MS and HPLC-MS/MS analyses, respectively. The dominant bacterial genera were Weissella, Lactobacillus, and Leuconostoc, while Mucor was highly abundant in the fungal community. The total monosaccharide content increased from Day 0 to Day 50, with the highest amount being 4.37 mg/g. Oligosaccharide profiling revealed the degradation of soybean dietary fiber during fermentation, and novel oligosaccharide structures were also discovered. Correlation analysis revealed that the fungus Mucor was positively related to pentose-containing oligosaccharides, galactose, and galacturonic acid, indicating that Mucor may degrade soybean dietary fibers such as xylogalacturonan, arabinogalactan, and rhamnogalacturonan. The negative relationships between the abundances of Weissella and oligo- and monosaccharides suggested that the bacteria may utilize saccharides for fermentation. These findings provide insights into the mechanisms underlying carbohydrate degradation and utilization; the key components involved in saccharide transformation that contribute to the characteristics of traditional meju were subsequently identified.