###### Abstract  
There are millions of tons of fresh soy sauce residue (SSR) by-products created by China's soy sauce industry every year. Most of the SSR is directly discarded; this not only wastes resources, but also pollutes the environment. As it is rich in dietary fiber, which is beneficial to human health, skimmed SSR was used as a raw material to obtain soluble dietary fiber (SDF) in this study. Firstly, the process of ultrasonic-assisted enzymatic extraction of SDF was optimized through single factor experiments and a response surface test. The extraction rate of the SDF from SSR reached 76.8 \(\pm\) 0.8% under the optimum extracting conditions of a cellulase/hem  
used for the extraction of DF [6]. DF has multiple beneficial effects on the intestinal flora, such as anti-obesity, lowering blood glucose, etc., and thus can be widely used as a functional food component [7]. DF can be divided into soluble dietary fiber (SDF) and insoluble dietary fiber (IDF), according to its different levels of solubility [8]. IDF does not participate in the blood circulation of the human body, but it can stimulate intestinal peristalsis [9; 10] and have a laxative effect. SDF is highly valued for its solubility, gelling potential, oil-/water-retention capacity, and surface properties. SDF has attracted much attention for its physiological health care activities [11], such as the prevention of hypertension [12], diabetes [13], coronary heart disease [14], cardiovascular disease [15], and certain gastrointestinal diseases [16]. Different extraction methods and conditions greatly influence SDF's physicochemical properties. The common methods used for the preparation of DF mainly include chemical, physical, and biological methods. The chemical method is widely used for the extraction of SDF from by-products due to its simplicity of operation, low cost, and high SDF yield. It breaks the glycosidic bonds in the macromolecular polymers of DF using acid or alkali, generating reductive ends, producing small molecules with low polymerization and high solubility, and partially altering the microstructure of the original DF. Enzymatic extraction firstly removes starch and protein using amylase and protease to obtain high purity DF, and then cuts the \(\beta\)-1,4-glycosidic bond of IDF using cellulase to make IDF effectively transform into SDF [17], improving the yield of SDF [18] and enhancing its physicochemical properties. The ultrasonic method for preparing DF mainly utilizes the cavitation effect generated by ultrasound in liquid and its mechanical effect on the target substance; this breaks down the plant cell wall, making the originally tight plant tissue loose and promoting the dissolution of SDF in plant cells. Moreover, the vibration generated during the ultrasonic process can also accelerate the dissolution of SDF in cells, thereby improving the yield of SDF [19]. The ultrasound-assisted enzymatic extraction combines the advantages of enzymatic and ultrasound methods, and also compensates for the high cost of enzymatic methods and the low purity of ultrasound products. In the extraction process for SDF, ultrasound is firstly used to destroy the structure of plant cell walls and fully release cell contents, and then enzymes are used to hydrolyze the main component of cell walls, i.e., cellulose, into small-molecule water-soluble polysaccharides. Meanwhile, the IDF in cell contents can be modified into SDF to improve the yield of SDF [20]. It is a trend in current research to recycle and utilize by-products from agricultural and industrial production [21]. Many studies are dedicated to extracting SDF with different functions from fruit or vegetable residues such as Litchi pomace [22], grapefruit peel [23], potato pomace [24], and tomato pomace [25]. There are only a few reports on extracting DF from SSR. Research on SSR mainly focused on the recycling and utilization of its protein, oil, and soy isoflav, and it is an urgent need to find new ways to reuse SSR.