Abstract
Beijing Douzhir is a traditional Chinese fermented drink produced by the natural fermentation of mung beans as the raw material. Ma tofu is an edible by-product of Douzhir processing. Douzhir microbiota, particularly bacteria involved in the natural fermentation process, has not been clearly established, resulting in limited industrial Douzhir production. Here, three uncooked Douzhir samples (D group) and three uncooked Ma tofu samples (M group) (two replicates per sample) were collected from three manufacturers in different locations in Beijing. The composition and diversity of the bacterial communities in each sample were analyzed by high-throughput sequencing. In total, 637 operational taxonomic units (OTUs) were revealed in the D group through database alignment, and 656 OTUs were found in the M group. The Chao, ACE, and Shannon indices were not significantly different in Douzhir samples from different manufacturers (p > 0.05). Representatives of six phyla were found in all 12 samples. Dominant bacteria were isolated and identified using mung bean juice as the growth medium. In both Douzhir and Ma tofu samples, dominant bacteria belonging to Firmicutes and Proteobacteria comprised > 94% of the total microbiota. The dominant bacteria included members of the Lactococcus, Acetobacter, Streptococcus, and Lactobacillus genera. Considering the dominant-microbiota information, we employed a plate-separation technique and isolated two strains of acid-producing bacteria from the Douzhir and Ma tofu samples with starch-flocculating activity: Acetobacter indonesiensis and Lactococcus lactis subsp. lactis. Such strains can serve as a foundation for the standardized industrial production of Douzhir.
Highlights
Douzhir has been consumed for at least 1000 years and is currently the most popular fermented snack in Beijing
We identified 637 operational taxonomic units (OTUs) in the D group through database alignment by BLAST analysis in QIIME
Douzhir and Ma tofu samples exhibited similar numbers of OTUs, but samples from different manufacturers varied with respect to the number of unique OTUs
Summary
Douzhir (fermented mung bean juice) has been consumed for at least 1000 years and is currently the most popular fermented snack in Beijing. Douzhir comprises residual material left after starch production from mung beans. This sour liquid is rich in proteins, vitamin C, and dietary fiber (Ding et al, 2010). Adding sour liquid is a key step in Douzhir processing that serves two purposes. This treatment accelerates starch settling: when sour liquid is added to mung bean starch milk, starch rapidly coagulates into floc aggregates and rapidly settles. Sour liquid is added to the mixture to ferment the bean juice and make it taste sourer. The top layer comprises gray-green raw bean juice (Liu and Shen, 2007a,b)
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