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

Abstract

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.

Fermented Soy Products: Beneficial Potential in Neurodegenerative Diseases

“…Recent studies demonstrated that the consumption of cheonggukjang fermented by Bacillus subtilis or Bacillus amyloliquefaciens increased the abundance of Bifidobacteriales and Lactobacillales in the gut. However, the population of Enterobacteriales, which are considered harmful bacteria, were lowered by a cheonggukjang-containing diet [126,127].…”

“…The intake of cheonggukjang fermented with Bacillus species prevents and alleviates the memory impairment observed in patients with AD and cerebral ischemic condition. In particular, cheonggukjang, which contains a high poly-L-γ-glutamic acid (γ-PGA), exhibited better efficacy for improving glucose metabolism and neuronal cell survival than did a low level of γ-PGA [126], although the neuroprotective effect and related mechanism(s) of γ-PGA remain unclear.…”

Anti-Inflammatory and Immunomodulatory Properties of Fermented Plant Foods

“…Besides, recent evidence suggested that short-term fermented soybean intake can be protective against memory impairment and Alzheimer’s disease in humans by suppressing insulin resistance in the brain, preventing neuroinflammation, and modulating the gut–microbiome–brain axis [ 200 ].…”

Current Perspectives on the Physiological Activities of Fermented Soybean-Derived Cheonggukjang

“…Cheonggukjang (CGJ) is a traditional Korean dish produced by fermenting boiled soybeans rice straw, which naturally contains Bacillus subtilis . Fresh CGJ is prepared by spreading rice straw on boiled soybeans and keeping them warm at 40–50 °C for 2–3 days ( Figure 1 A) [ 1 , 2 , 3 , 4 , 5 ]. During the fermentation process of CGJ, soy protein is decomposed into amino acids by potent proteolytic enzymes produced by B. subtilis ( Figure 1 B), thus improving digestibility and increasing the vitamin B 2 and calcium contents of the final product [ 6 ].…”

“…Currently, raw and powdered forms of CGJ are being developed. Unlike natto, which is a traditional Japanese dish consisting of fermented soybeans, and other soybean-fermented pastes in Asia that are eaten raw, CGJ does not contain salt, but can include ingredients such as crushed green onions, garlic, and red pepper powder for flavor, and salt can be added to the boiled soybeans to extend the product’s shelf life [ 3 , 4 , 5 ]. This provides an advantage over kimchi and other soybean paste products that contain high salt concentrations, a factor that is associated with gastric cancer and high blood pressure [ 11 , 12 ].…”