Untargeted metabolomics revealed the effect of soybean metabolites on poly(γ-glutamic acid) production in fermented natto and its metabolic pathway

BACKGROUND

Natto mucus is mainly composed of poly(γ-glutamic acid) (γ-PGA), which affects the sensory quality of natto and has some effective functional activities. The soybean metabolites that cause different γ-PGA contents in different fermented natto are unclear.

RESULTS

In this study, we use untargeted metabolomics to analyze the metabolites of high-production γ-PGA natto and low-production γ-PGA natto and their fermented substrate soybean. A total of 257 main significantly different metabolites with the same trend among the three comparison groups were screened, of which 114 were downregulated and 143 were upregulated. Through the enrichment of metabolic pathways, the metabolic pathways with significant differences were purine metabolism, nucleotide metabolism, fructose and mannose metabolism, anthocyanin biosynthesis, isoflavonoid biosynthesis and the pentose phosphate pathway.

CONCLUSION

For 114 downregulated main significantly different metabolites with the same trend among the three comparison groups, Bacillus subtilis (natto) may directly decompose them to synthesize γ-PGA. Adding downregulated substances before fermentation or cultivating soybean varieties with the goal of high production of such substances has a great effect on the production of γ-PGA by natto fermentation. The enrichment analysis results showed the main pathways affecting the production of γ-PGA by Bacillus subtilis (natto) using soybean metabolites, which provides a theoretical basis for the production of γ-PGA by soybean and promotes the diversification of natto products. © 2023 Society of Chemical Industry.

Preparation of Type-A Gelatin/Poly-γ-Glutamic Acid Nanoparticles for Enhancing the Stability and Bioavailability of (-)-Epigallocatechin Gallate

(-)-Epigallocatechin gallate (EGCG) has gained considerable attention owing to its beneficial properties. However, its application as a functional food is restricted due to its instability and low bioavailability. In the present study, a food-derived nanoparticle system based on type A gelatin/γ-PGA was developed to preserve and deliver EGCG. The EGCG/gelatin/γ-PGA nanoparticles had a particle size of 155.1 ± 7.3 nm with a zeta potential of -23.9 ± 0.9 mV. Moreover, the EGCG/gelatin/γ-PGA nanoparticles enhanced the long-term storage stability and sustained antioxidant activity of EGCG compared to EGCG/gelatin nanoparticles. The nanoparticles protected EGCG in simulated gastric fluid containing pepsin while releasing it in simulated intestinal fluid. Additionally, the amount of EGCG transported in the Caco-2 monolayers treated with EGCG/gelatin/γ-PGA nanoparticles was three times higher than that of free EGCG, which might be related to the paracellular pathway and endocytosis. These results suggest that EGCG/gelatin/γ-PGA nanoparticles might be an effective delivery vehicle for EGCG, enhancing its potential applications in the functional food field.

Effects of a high-γ-polyglutamic acid-containing natto diet on liver lipids and cecal microbiota of adult female mice

Natto is a traditional Japanese fermented soy product high in γ-polyglutamic acid (γ-PGA), whose beneficial effects have been reported. We prepared high-γ-PGA natto and compared the dietary influence on liver lipids and cecal microbiota in mice fed a diet containing it or a standard diet. The mice were served a 30% high-γ-PGA natto diet (PGA group) or standard diet (Con group) for 28 days. Liver lipids, fecal lipids, and fecal bile acids were quantified. Cecal microbiota were analyzed by PCR amplification of the V3 and V4 regions of 16S rRNA genes and sequenced using a MiSeq System. Additionally, the cecal short-chain fatty acid profile was assessed. The results revealed that the liver lipid and triglyceride contents were significantly lower (p<0.01) and amounts of bile acids and lipids in the feces were significantly higher in the PGA group than in the Con group. The cecal butyric acid concentration was observed to be significantly higher in the PGA group than in the Con group. Principal component analysis of the cecal microbiota revealed that the PGA and Con groups were distinct. The ratio of Firmicutes/Bacteroidetes was found to be significantly low in the PGA mice. The results revealed a significantly higher relative abundance of Lachnospiraceae (p<0.05) and significantly lower relative abundance of Coriobacteriaceae (p<0.01) in the PGA group. Analysis of the correlation between bacterial abundance and liver lipids, cecal short-chain fatty acids, fecal lipids, and fecal bile acids suggested that intestinal microbiota can be categorized into different types based on lipid metabolism. Hepatic lipid accumulation typically facilitates the onset of nonalcoholic fatty liver disease (NAFLD). Our findings suggest that high-γ-PGA natto is a beneficial dietary component for the prevention of NAFLD.

γ-PGA-Rich Chungkookjang, Short-Term Fermented Soybeans: Prevents Memory Impairment by Modulating Brain Insulin Sensitivity, Neuro-Inflammation, and the Gut-Microbiome-Brain Axis

Fermented soybean paste is an indigenous food for use in cooking in East and Southeast Asia. Korea developed and used its traditional fermented foods two thousand years ago. Chungkookjang has unique characteristics such as short-term fermentation (24–72 h) without salt, and fermentation mostly with Bacilli. Traditionally fermented chungkookjang (TFC) is whole cooked soybeans that are fermented predominantly by Bacillus species. However, Bacillus species are different in the environment according to the regions and seasons due to the specific bacteria. Bacillus species differently contribute to the bioactive components of chungkookjang, resulting in different functionalities. In this review, we evaluated the production process of poly-γ-glutamic acid (γ-PGA)-rich chungkookjang fermented with specific Bacillus species and their effects on memory function through the modulation of brain insulin resistance, neuroinflammation, and the gut–microbiome–brain axis. Bacillus species were isolated from the TFC made in Sunchang, Korea, and they included Bacillus (B.) subtilis, B. licheniformis, and B. amyloliquefaciens. Chungkookjang contains isoflavone aglycans, peptides, dietary fiber, γ-PGA, and Bacillus species. Chungkookjangs made with B. licheniformis and B. amyloliquefaciens have higher contents of γ-PGA, and they are more effective for improving glucose metabolism and memory function. Chungkookjang has better efficacy for reducing inflammation and oxidative stress than other fermented soy foods. Insulin sensitivity is improved, not only in systemic organs such as the liver and adipose tissues, but also in the brain. Chungkookjang intake prevents and alleviates memory impairment induced by Alzheimer’s disease and cerebral ischemia. This review suggests that the intake of chungkookjang (20–30 g/day) rich in γ-PGA acts as a synbiotic in humans and promotes memory function by suppressing brain insulin resistance and neuroinflammation and by modulating the gut–microbiome–brain axis.

The bio-processing of soybean dregs by solid state fermentation using a poly γ-glutamic acid producing strain and its effect as feed additive

Soybean dregs are restricted as feed additives because they contain anti-nutrient factors. Herein, soybean dreg was bio-transformed by solid-state fermentation (SSF) using a poly γ-glutamic acid (γ-PGA) producing stain Bacillus amyloliquefaciens NX-2S. The maximum γ-PGA production of 65.79 g/kg was reached in a 5 L fermentation system while the conditions are 70% initial moisture of soybean dregs with addition of molasses meal, 12% inoculum size, 30 °C fermentation temperature, initial pH of 8, and 60 h fermentation time. Meanwhile, continuous batch fermentation was proved feasible. After SSF, the anti-nutritional factors such as trypsin inhibitor, phytic acid and tannin were reduced by 98.7%, 97.8%, and 63.2%, respectively. Compared with unfermented soybean dregs, adding fermented soybean dregs to feed increased the average weight gain of rats by 15.6% and reduced the ratio of feed to meat by 11.3%. Therefore, this study provided a feasible strategy for processing soybean dregs as feed additive.

Trends in the application of Bacillus in fermented foods

Bacillus species such as Bacillus subtilis and Bacillus amyloliquefaciens are widely used to produce fermented foods from soybeans and locust beans in Asian and West African countries, respectively. Genomic information for B. subtilis strains isolated from Asian Bacillus-fermented foods (BFFs) has been gathered, and the chemical components of fermented products were defined with metabolomic approaches, facilitating the development of new starter strains and the evaluation of health claims. On the other hand, although advanced studies have been performed for some commercially produced BFFs, home-manufactured products still remain to be characterized in rural areas. In West Africa, the microbial flora of BFFs was examined in detail, leading to the isolation of candidates of the starter that produced bacteriocin against Bacillus cereus contaminating the products. These studies may provide a choice of Bacillus strains in food application and increase opportunities for further usage of Bacillus in foods.