In order to obtain soluble dietary fiber, it is usually necessary to separate and extract it from natural raw materials. Different extraction methods, such as different extraction temperature, extraction time, PH value and solvent, will have different yields, chemical compositions, physical and chemical properties and functional properties of the extracted soluble dietary fiber products. Common soluble dietary fiber extraction methods include chemical methods, physical methods, enzymatic methods, fermentation methods, and combined methods. The basic principle is to destroy the structure of the connection between soluble dietary fiber and other substances or the glycosidic bond between polysaccharides to promote the conversion of insoluble dietary fiber to soluble dietary fiber, release the soluble dietary fiber free, increase the contact area between soluble dietary fiber and water or other solvents. Then  the soluble dietary fiber is extracted.

1. Chemical methods.

The chemical method refers to natural raw materials that are dried and crushed, treated with corresponding chemical reagents to remove impurities (except fat, starch and protein), and then extracted under acid or alkali conditions combined with high temperature to obtain the soluble dietary fiber. It is currently the most widely used extraction method and is often used to obtain soluble dietary fiber from various sources. The chemical method can be divided into acid method, alkali method and flocculant method according to different processes. Alkaline method is the commonly used extraction method, and acid hydrolysis method can be used to prepare soluble dietary fiber from raw materials with high pectin content.

2. Physical methods.

The physical method refers to the extraction method in the early days, and now refers to the method of extracting soluble dietary fiber by physical methods such as ultrafine pulverization, ultrasonic treatment, extrusion cooking, etc. Among them, the soluble alcohol precipitation method is the most convenient. The treated crude raw material is dissolved in warm water or hot water, and then its aqueous solution is precipitated with 4 times the volume of ethanol to obtain soluble dietary fiber, which is widely used in industry. With the development of science and technology, many emerging technologies are gradually being used in combination with soluble alcohol precipitation to extract soluble dietary fiber. The advantage of the physical method is that the extraction time is short, and the yield and purity of the obtained soluble dietary fiber are high. The disadvantage is that the equipment used is expensive and the maintenance cost is high.

3. Enzymatic methods.

The enzymatic hydrolysis method uses a variety of enzymes to remove other components (mainly protein, fat, reducing sugar, starch, etc.) in the raw materials other than dietary fiber, so that the soluble dietary fiber is released and extracted. Enzymes mainly include a-amylase, protease and the like. Some active ingredients can also be prepared by using cellulase, hemicellulase, and pectinase. Cellulase can decompose water-insoluble dietary fiber to produce small molecular weight monosaccharides or oligosaccharides, thereby improving the extraction rate of soluble dietary fiber. Enzymatic extraction has mild conditions, does not require high temperature and high pressure, can significantly reduce the damage to the structure of raw materials, and is easy to operate, which not only saves energy, but also is suitable for the extraction of soluble dietary fiber in raw materials with high starch and protein content. However, due to the high price of enzyme preparations, the cost of preparing soluble dietary fiber is too high, and it is difficult to realize large-scale industrial production.

4. Fermentation methods

The fermentation method is a new type of extraction method. Using microbial fermentation to consume protein, starch, etc. in food raw materials, and the secreted cellulase, hemicellulase, etc. destroy the glycosidic bonds between polysaccharides to make polysaccharides change from large molecules to small ones, so as to prepare soluble dietary fiber. A large number of studies have shown that the yield, color, and texture of soluble dietary fiber extracted by fermentation are better, and the water-holding and oil-holding properties of the extracted soluble dietary fiber are also significantly improved, and most of the fermentation raw materials are byproducts of food processing which are cheap. The disadvantage of this method is that the operation process is relatively complicated, and certain environmental conditions are required for fermentation.

5. Combined methods.

To sum up, both physical and biological methods have their own advantages and disadvantages, and the scope of applicable food processing is also different. Combining two or more of these methods can avoid the defects of a single method on the one hand, and on the other hand, can make multiple methods cooperate with each other to more effectively improve the yield and quality of soluble dietary fiber; usually a combination of multiple methods works better than a single approach. At present, the common combination methods include the combination of chemical method, enzymatic method, ultrasonic assisted method, mechanical pretreatment method and so on. The multi-method combined extraction of soluble dietary fiber has a higher yield and higher purity, and can also reduce the expensive cost of single application such as enzymatic method and extrusion method, which has high research and utilization value.