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 1A) ^{[1][2][3][4][5]}[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 1B), thus improving digestibility and increasing the vitamin B₂ and calcium contents of the final product [6]. The sticky mucous substance produced during fermentation contains poly-γ-glutamic acid (γ-PGA), the main component, and altered isoflavone compounds. Polyglutamic acid is beneficial to health, as it aids in the absorption of calcium ^[7][8][7,8]. Isoflavone, the main physiologically active substance in soybeans, improves the absorption and bioavailability of nutrients from the fermented soybeans ^[2][9][10][2,9,10].

Rich in microorganisms, enzymes, and various physiologically active substances, CGJ has gained attention as a functional food to help maintain and promote health by improving intestinal motility and blood circulation ^[4][5][4,5]. 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][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][11,12]. Moreover, CGJ can be produced within 2–3 days, whereas doenjang, another soybean paste, takes several months to manufacture ^[2][10][13][2,10,13]. CGJ, with its unique flavor, is recognized as the most nutritionally and economically effective way to consume soybeans.

Studies on CGJ have mainly focused on the fermentation process and biological activities of CGJ. Consumption of soy-based foods has been shown to prevent cancer, particularly reducing the incidence of breast and prostate cancers [14]. CGJ contains large amounts of various physiologically active substances, including γ-PGA, isoflavones, phytic acid, saponins, trypsin inhibitors, tocopherols, unsaturated fatty acids (i.e., conjugated linoleic acid (CLA)), dietary fiber, oligosaccharides, itutin A, bacillomycin D, pharmaceutical and industrial enzymes (including protease, amylase, and cellulase), and possesses antibacterial activity (Table 1) ^{[2][4][5][10]}[2,4,5,10]. CGJ is known to have thrombolytic effects owing to its protease activity. It also contains daidzein, an isoflavone (Figure 2), which has immunomodulatory effects that involve stimulation of estrogen receptor β (ER-β) ^[2][4][5][2,4,5]. Cell signaling mediated by ER-β is associated with regulation of antioxidant gene expression, the immune response, apoptosis, blood pressure, and inhibition of breast, prostate, and colon cancer cell proliferation ^[10][15][16][10,15,16]. In addition, oligosaccharides released from CGJ by the action of β-glucanases have various physiological activities, including diabetes prevention ^[10][17][18][10,17,18]. Free radicals generated in the body due to excessive drinking, smoking, overeating, and mental stress—which are linked to cancer, atherosclerosis, accelerated aging, and inflammation ^{[19][20][21][22]}[19,20,21,22]—can be countered by antioxidants (such as amino acids, chlorogenic acid, caffeic acid, and browning substances) and isoflavones (such as genistein and daidzein) found in CGJ ^{[2][4][5][23]}[2,4,5,23]. In addition, CGJ reportedly contains higher antioxidant levels than non-fermented soybeans [24]. CGJ also contains peptides that can lower blood pressure by acting as angiotensin I-converting enzyme (ACE) inhibitors ^{[4][5][10][25]}[4,5,10,25]. ACE converts angiotensin I to angiotensin II, increasing blood pressure in vivo [26]. Thus, CGJ contains physiologically active substances that have anticancer, antioxidant, and blood pressure-lowering activities, as well as hypertension and osteoporosis prevention properties.

The people of Asia have been producing special foods with unique flavors by adding different microbial strains (e.g., yeast or Bacillus spp.) to steamed soybeans for thousands of years ^[1][2][3][4][1,2,3,4]. The process of microbial fermentation not only imparts a unique flavor, but also enhances the nutritional value, shelf life, and bioactive substance content ^[10][27][10,27]. Research and development of food products is often based on the traditional foods of the in country in which the research is conducted. Asian countries, including Korea and Japan, have mainly investigated soybean-based fermented foods, whereas Europe and North America have actively conducted research on fermented foods such as cheese, wine, and beer ^[10][28][10,28]. The European Union organized a working group to provide extensive support to researchers investigating traditional foods to determine safety control measures, enhance their nutritional value, and promote marketing of these foods [29]. While continuous efforts have been directed toward researching fermented foods in Korea, a unified support system for such research has yet to be established at the national level ^[10][27][28][10,27,28]. At present, research is performed sporadically without an integrated database to converge study findings and there is a general lack of comprehensive discussion regarding the health functionality of traditional fermented foods ^[10][30][10,30]. In this review, we examine recent information on the novel physiological activities of CGJ, including anti-obesity, anti-diabetes, anti-inflammatory, and antimicrobial activities as well as immunostimulatory, neuroprotective, and skin improvement effects based on the following aspects: (1) nutritional properties of CGJ, (2) identification and characterization of microorganisms used in fermentation, and (3) health functionality assessment. Furthermore, this review aims to suggest future directions for study through the examination of CGJ characteristics, with a special focus on health functionality assessment.

CGJ is a well-known fermented food mainly produced between fall and early spring in Korea, as shown in Figure 1. Soybeans, the base ingredient of CGJ, comprises approximately 40% proteins and 20% lipids, resembling meat rather than grains in terms of nutritional values. Soybeans also contain 12% dietary fibers (2.3% soluble dietary fibers and 9.7% insoluble dietary fibers), which is not present in beef, and lacks cholesterol (Figure 3A) ^[1][2][10][1,2,10], which is associated with various diseases [31]. Aside from its unique flavor, CGJ ensures the most effective intake of soybeans, easily providing protein that may be deficient in Korean diets that are mainly composed of cereal grains [4]. Additionally, CGJ has higher protein and fat contents than doenjang (soybean paste) or gochujang (red chili paste) ^[24][31][24,31]. Hence, it has been an essential source of protein for a long time, along with other paste-based foods, for Koreans with a relatively low protein intake.

Different enzymes secreted by CGJ-producing bacteria during fermentation mediate degradation of the soybean outer coat, cell membrane fibers, and intracellular sugars and proteins to improve digestibility and increase the free amino acids contents [32]. During fermentation, proteins are degraded into peptone, polypeptides, dipeptides, and amino acids through protease activities (Figure 3B) [33]. β-amylase mediates the degradation of carbohydrates into glucose [34]. The sticky substance generated during CGJ fermentation is the combined product of the polysaccharide fructan and the amino acid polyglutamic acid ^[4][5][33][4,5,33]. There is a substantial increase in the vitamin B₂ level during fermentation; the vitamin B₂ content is 5- to 10-fold higher in steamed than in raw soybeans. Among the substances produced by CGJ bacteria, vitamin K (menaquinone; types K₁ and K₂) (Figure 3C) is required for the synthesis of proteins involved in blood coagulation ^{[2][6][35][36]}[2,6,35,36]. In raw soybeans, vitamin K₁ is present in trace amounts and the vitamin K₂ level is negligible. However, CGJ contains low levels of vitamin K₁ but 5- to 10-fold higher levels of vitamin K₂ than other vegetables ^{[1][2][3][4][10]}[1,2,3,4,10]. As vitamin K₂ is directly engaged in osteogenesis, vitamin K deficiency may increase the risk of fracture ^{[6][24][31][35][36]}[6,24,31,35,36]. In a study in Japan, the use of B. subtilis for natto fermentation was reportedly the most effective method of improving vitamin K₂ intake ^[2][10][37][2,10,37]. Thus, the bioactive effects of CGJ can be predicted from the main microbial strain (Bacillus) used for fermentation.