## 1 Introduction  
Douchi is a traditional fermented soybean product in China with both desirable taste and functional effects [1]. Shuidouchi is a type of bacterial Douchi, which is a type of seasoning that is fermented by bacterial action, which has a history of use as seasoning or as a dish for thousands of years. It was used not only as a simple food but also as a type of medicine in ancient Chinese medical books. Shuidouchi was also used as a traditional Chinese medicine with appetite-stimulating effects that strengthens the spleen, eliminates heat, and exerts detoxifying effects [2]. Modern scientific research methods have identified many functional ingredients in Shuidouchi, which confer antioxidant, antimutagenic, and anticancer effects of soybean products (e.g., Douchi, Shuidouchi, and soy sauce) [3]. However, studies on the physiological activity of Shuidouchi have rarely been conducted. Chinese Shuidouchi, Japanese Natto, and Korean Koji sauce are typical examples of food from bacterially fermented soybeans. Japan was the first to isolate and identify the fermentation strain of Natto and simultaneously develop purely fermented Natto products after an in-depth study of the physiological activities of Natto. The study reported that Natto exhibits antioxidant, anti-inflammatory, and anticancer effects [4; 5; 6]. These three bacterially fermented soybean foods (Chinese Shuidouchi, Japanese Natto, and Korean Chungkukjang) are all produced with similar technology. Japanese Natto and Korean Chungkukjang have been fully studied and reported to date, including process improvement, functional effects, and extraction of active ingredients [7; 8]. In China, only a few preliminary studies have been conducted on Shuidouchi, indicating its anti-inflammatory and anti-cancer effects [7]. The specific mechanism and process improvement of Shuidouchi have not been explored sufficiently. Previous studies showed that the quality of soybean sauce was related to numerous factors, such as the fermentation bacteria, the amount of water for soaking the soybeans, the fermentation time, fermentation temperature, and the time after fermentation [9; 10]. In addition, the fermentation container is also one of the factors to affect the quality of fermented soybean products [11]. A study confirmed that the quality of Korean Chungkukjang was related to these fermentation factors [7]. In China, the effects of single factors, such as fermentation temperature, fermentation time, and fermentation container on the quality of Shuidouchi were partially confirmed [2; 9; 12]. The optimal combination of these factors and the optimal fermentation process is the focus of this study. Intestinal health plays an important role for maintaining high quality of life. Digestive absorption, metabolism, and mucosal immunity can decline with age, and the number of probiotics in the digestive system can rapidly decrease. Intestinal canal aging can simultaneously lead to difficulty in defecation, flatulence, dysppepsia, and accumulation of vivotoxins, which, among other issues, influence human health [13]. Almost 95% of the Chinese suffer from intestinal diseases, which severely influences their health and quality of life. Constipation, colitis, and colon cancer are critical intestinal diseases posing considerable risks for human health [14]. Shuidouchi is rich in thrombol ingredients, soybean polypeptides, soybean oligosaccharides, protein melanin, soybean saponins, complex digestive enzymes, active isoflav, vitamin B1, microbial B2, vitamin C, vitamin E, amino acids, and further bioactive substances [15; 16]. The active ingredients of soybean polypeptides, soybean oligosaccharides, protein melanin, soybean saponins, complex digestive enzymes, active isoflav in Shuidouchi have been suggested to alleviate constipation and prevent both colitis and colon cancer [2; 3]. More active substances could be produced by improving the fermentation process of Shuidouchi.