Abstract Studies have shown that modified dietary fiber has better physical and chemical properties and functions， and can better play its physiological effects. This paper systematically and comprehensively expounded the application progress of chemical separation method， microbial fermentation method， physical method， enzyme method and steam explosion method in dietary fiber modification， and expounded the development prospect of modified dietary fiber， so as to provide reference for further study on development and application of dietary fiber.
　　Key words Dietary fiber; Modification technology; Physiological function
　　
　　 In recent years， with the improvement of living standards， peoples requirement for quality of agricultural products has been increasingly strong. Dietary fiber is increasingly favored and concerned by people because of its important physiological functions[1-3]. Dietary fiber has unique physicochemical properties but no nutritional value. However， its healthcare function is quite high. According to its solubility， dietary fiber in nature is mainly divided into two classes： soluble dietary fiber （SDF） and insoluble dietary fiber （IDF）[4-8]. IDF mainly acts on the intestinal tract to produce mechanical peristalsis， while SDF can exert its metabolism in physiological functions as it can promote the growth of intestinal probiotics and has special effects in preventing diabetes， obesity， coronary heart disease， arteriosclerosis and hyperlipidemia[9-15]. Therefore， the proportion of SDF in dietary fiber is an important factor affecting the physiological functions of dietary fiber.
　　The current traditional modification methods have their limitations. For example， the cost of the chemical method is low， but the acidbase treatment causes the molecular structure of dietary fiber to be destroyed， and the quality characteristics of the product are poor. In the dietary fiber production by the enzyme method， due to the characteristics of the raw materials， they are not conducive to the action of related enzymes （amylase， protease， cellulase） on substrates and the release of watersoluble substances， so the yield of dietary fiber is relatively low， and the cost is relatively high， which is not conducive to the application of the method in actual production[16]. Meanwhile， many kinds of natural dietary fiber have the characteristics of low quality and the SDF content only of 3.0%-4.0%， and cannot meet the requirement of SDF content ≥10.0% for highquality dietary fiber and does not have good physiological activity and healthcare function. As a result， they cant meet the requirements of modern food nutrition， food development and processing. Consequently， it is inevitable to find suitable modification methods and improve the modification efficiency[17-19]. 　　Classification of Dietary Fiber
　　Soluble dietary fiber
　　SDF refers to the part of dietary fiber that is not digested by human digestive enzymes but soluble in warm water or hot water and whose aqueous solution can be reprecipitated by 4 times volume of ethanol. It mainly refers to the storage substances and secretions in plant cells. It also includes some microbial polysaccharides and synthetic polysaccharides such as pectin， guar gum， alginate， dextran and fungal polysaccharides. Barley， beans， carrots， citrus， flax， oat and oat bran in common foods are rich in watersoluble fiber. SDF not only plays an important role in maintaining dietary balance， but also has important physiological functions in preventing colon cancer and cardiovascular disease， lowering cholesterol， lowering blood lipid levels and delaying the absorption of glucose in the small intestine， thereby preventing diabetes[20]. SDF has a very wide range of applications in the food industry and is often used as a food additive in food industry. Adding dietary fiber to the pastry can enhance the water absorption of dough， prolong the settling time and enhance the elasticity of dough[21]. The addition of soluble dietary fiber to meat products can achieve the effects of enhancing mouthfeel， increasing yield and reducing costs， and has lipidlowering effect， so the produced meat products are especially suitable for diabetic patients[22]. Adding soluble dietary fiber to the beverage can make the particles in the beverage evenly distributed， and make the beverage longlasting without precipitation and gelatinous effect， thereby playing a health care role[23].
　　Insoluble dietary fiber
　　IDF refers to the portion of dietary fiber that is not digested by human digestive enzymes and is insoluble in hot water. It is a main constituent of cell wall， including cellulose， hemicellulose， lignin and vegetable wax. It is often found in the roots， stems， stems， leaves， bark and fruits of plants. The best source of IDF is whole grains， including wheat bran， oat， whole wheat flour and brown rice， oatmeal whole grains， beans， vegetables and fruits. The role of IDF in human body is firstly to promote gastrointestinal motility， accelerate food to go through the gastrointestinal tract， reduce absorption and prevent obesity and constipation. IDF extracted from wheat bran can be added to German burgers to replace fat， to lower cholesterol levels and improve their quality and taste[24]. Garcia et al.[25]added grain and fruit IDF， especially 1.5%citrus fiber， to fermented dry sausages， which thus obtained similar characteristics to highfat products in sensory indicators. 　　Dietary Fiber Modification Technology
　　The dietary fiber modification technology is to properly treat DF to change the molecular structures and relative contents of cellulose polymers， decompose macromolecular materials into small molecular components， promote the conversion of IDF to SDF and enhance or strengthen the original weak functions or the newly endowed functions， so as to make the SDF content increase. The principle is that the IDF macromolecular linkage bond， i.e.， glycosidic bond， is broken by modification， so that the dense network structure is loosened， and the physicochemical properties and biological activity of the dietary fiber are thereby changed， giving the modified dietary fiber with a larger specific surface area and higher water holding capacity and expansion force， which is thus more functional. At present， there are four main methods for dietary fiber modification： chemical method， physical method and biological method. In general， the combination of multiple methods is better than using one method alone[26].
　　Dietary Fiber Modification Methods
　　Chemical separation method
　　The chemical separation method refers to a method in which a crude product or a raw material is dried and ground， and then extracted with an acid or an alkali to prepare various dietary fiber. The alkali method is a commonly used extraction method in which a crude product or a raw material is dried and ground and treated with an alkali solution to give the filtrate， which is subjected to pH adjustment with an acid， bleaching and centrifugation， and then the supernatant is adjusted to the neutral pH with an alkali and precipitated with alcohol， from which the resulting precipitate is a watersoluble dietary fiber and the filter residue is a waterinsoluble dietary fiber. Chen et al.[27]treated Eucheuma with KOH， which not only can significantly improve the gelling property of carrageenan in alga and reduce the loss of carrageenan， but also can better remove nondietary fiber ingredients such as protein， fat， starch and pigment in alga. The acid hydrolysis method can be used to prepare dietary fiber from raw materials with high pectin content. Fissore et al.[28]studied the preparation of dietary fiber from sugar beet and white walnut by hydrolysis using citric acid. The results showed that the extraction rate increased with the decrease of pH and the increase of extraction time. Alpine et al.[29]used hydrochloric acid to modify Avena nuda bran insoluble dietary fiber. When acid hydrolysis was carried out at the pH of 4 and the temperature of 50 °C for 150 min with the materialtoliquid ratio of 1∶20 （g/ml）， the SDF yield was the highest at 11.07%. 　　Microbiological fermentation method
　　The fermentation method is a method for decomposing the macromolecular components of dietary fiber into small molecular compounds by microbial fermentation to increase the content of soluble dietary fiber in the dietary fiber， thereby improving the quality of the dietary fiber. The main mechanism of microbial fermentation modification of dietary fiber is that strains produce organic acid metabolites continuously during the long fermentation time， which cause the dietary fiber to be in acidic conditions for a long time. Acids are a good donor of protons， which can break the glycosidic bond of cellulose and produce a new reducing end， which leads to the increasing content of soluble dietary fiber[30]. Lamboo et al.[31]discussed the physicochemical properties of dietary fiber in oats fermented with different lactic acid bacteria. It was found that the change in the apparent viscosity of dietary fiber after fermentation was not obvious. Tu et al.[32]studied the preparation of highactivity dietary fiber by fermenting bean dregs with selfmade mixed strains， and found that the fermented soybean dreg dietary fiber had improved water retaining and water absorption capacities and the SDF/DF ratio as high as 13%. Mold can produce substances such as cellulase， which can effectively degrade insoluble dietary fiber and by which the glycosidic bond of insoluble dietary fiber is broken， and produces a new reducing end， which increases soluble dietary fiber content， so the physicochemical properties of the insoluble dietary fiber are changed. The fermentation method is also commonly used in the fermentation of bean dregs.
　　Physical method
　　Extrusion cooking technique
　　Extrusion cooking is a modern technique that forces materials to flow under different conditions and pass through a shaped hole （die） at a predetermined rate to obtain different products[33]. The extrusion technique integrates various unit operations such as conveying， mixing， heating and pressurization. The materials are subjected to high temperature and high shear in extruder barrels， and a part of macromolecular polymers is directly or indirectly converted into soluble fiber in a short time[34]. The extrusion treatment can improve the color and flavor of dietary fiber， and passivate part of the decomposition enzymes inducing bad flavor， so the stability and flavor of the products after extrusion are significantly improved[35]. Ye et al.[36]used a twinscrew extruder to perform extrusion cooking on tomato skin. The results showed that the soluble dietary fiber content in tomato skin increased from 3.40% to 5.86% after extrusion cooking. Zhou[37]studied the modification of mung bean skin dietary fiber and found that the soluble dietary fiber content in the mung bean skin residue after extrusion treatment increased to 9.27%. 　　Ultrahigh pressure treatment
　　UltraHigh Pressure （UHP） is a kind of physical cold processing technique. Generally， the food sealed in a flexible container is placed in a pressure system using water or other liquid as a pressure transmitting medium， and 100 MPa or more is used to treat the food， to achieve sterilization and improve food function characteristics[38]. Huang et al.[39]studied the modification effect of high pressure treatment on insoluble dietary fiber of bean dregs. The optimum yield of SDF was 7.56% under the condition of 400 MPa. MateosAparicio[40]studied the dietary fiber of bean dregs and found that as the treatment pressure increased， the ratio of soluble dietary fiber to insoluble dietary fiber and the physical and chemical indicators of dietary fiber increased to varying degrees.
　　Ultrafine pulverization
　　The ultrafine pulverization technique generally refers to a pulverization technique in which material particles are pulverized to a particle diameter of 100 μm or less. When the material is in an ultrafine state， its particle size is amongatoms， molecules and particles， and sometimes， it is referred to as the fourth state of matter. The ultrafine pulverization technique applied in the development of natural product resources generally achieves micronscale pulverization to destroy the cell wall structure of tissues and obtain desired material characteristics. Due to the micronization of particles， the surface area and porosity increase， and the ultrafine powder has unique physicochemical properties[41]. Studies have shown that ultrafinely pulverized foods can be absorbed faster in human body. After the ultrafine pulverization of raw materials， the cell wall is broken， and active components in cells are fully exposed， so the release rate and release amount thereof are greatly improved compared with conventional pulverization， and human body can absorb them more easily. Meanwhile， the particle volume becomes smaller after pulverization， the surface area increases sharply， and the solubility is improved[42]. Ronkart et al.[43]studied the effects of high pressure microjet on the chemical composition， rheology and structure of inulin. It was found that with the increase in the treatment times and pressure of high pressure microjet， the viscosity increased， the particles decreased， and the surface structure changed. Chen et al.[44]ultrafinely pulverized highester pectin with highpressure microjet， and found that the average molecular weight， apparent viscosity and particle size of the modified pectin decreased significantly， while the soluble solid content increased with the increase of treatment pressure. 　　Enzyme method
　　The enzyme method is to remove other components in raw materials except dietary fiber by using various enzymes， mainly proteins， fats， reducing sugars， starch and the like， so as to obtain dietary fiber. The enzymes used in the enzyme method mainly include three kinds： αamylase， protease and amyloglucosidase. Some active ingredients can also be obtained by treatment and preparation using cellulase， hemicellulase， pectinase. Cellulase can decompose insoluble dietary fiber to produce small molecular weight monosaccharides or oligosaccharides， thereby increasing the extraction rate of watersoluble dietary fiber. Due to the strong specificity of enzymes， the enzymatically prepared dietary fiber has a higher purity， which is also the main advantage of enzymatic extraction of dietary fiber[45]. In addition， the enzymatic extraction method goes with mild conditions with no need for high temperature and high pressure， and has easy operation， which saves energy and part of the process and equipment， so it is conducive to environmental protection and particularly suitable for the preparation process using raw materials with high starch and protein contents. Wang et al.[46]used 180 U/g of cellulase and 90 U/g of xylanase to modify the dietary fiber of bamboo shoots. At pH 5.0 and the enzymatic hydrolysis temperature of 50 ℃， the best modification effect was achieved at 2 h， giving a soluble dietary fiber content reaching 12.1%. Jiang et al.[47]used cellulose enzymatic hydrolysis to modify wheat bran dietary fiber at a ratio of material to liquid of 1∶10， an enzyme amount of 20 U/g， pH 4.8 and a temperature of 60 ℃， and the SDF yield was 12.67% after 2 h of hydrolysis.
　　Xiaoran DUAN et al. Research Status and Progress on Modification of Dietary Fiber at Home and Abroad
　　Steam explosion method
　　Steam explosion method has recently been increasingly introduced in the pretreatment of plant fiber raw materials， which can effectively separate active fibers. Compared with microwave method， cooking method and traditional alkali method， the steam explosion method has the advantages of short term， energy saving， no pollution， high enzymatic hydrolysis efficiency and wide application range， and is a very promising pretreatment method. The principle of steam explosion techniques is mainly that the material suddenly drops from high temperature and high pressure to normal temperature and normal pressure in an instant， which leads to sudden vaporization of the moisture inside the raw material， producing a gas which suddenly expands， and the explosion occurs， resulting in an explosion effect. Steam explosion pretreatment can make the material structure spongelike， the material increase in volume and some structural tissues such as fiber bundles destroyed， and the inclusions are exposed， which is beneficial to the dissolution of the target product， improves the effect of enzymes on substrates， and improves the efficiency of enzymatic hydrolysis of raw materials[48-50]. Wang et al.[51]used the steam explosion technique to modify citrus pomace. When the pressure was 0.8 MPa and the time was 7 min， the content of SDF increased from 8.04%to 33.74%. Li et al.[52]applied CO2 blasting to treat wheat bran， and the content of soluble dietary fiber in the modified wheat bran increased from the initial 2.5 g/100 g to 9.97 g/100 g. 　　Combined treatment
　　In summary， in the methods of dietary fiber modification， whether for the physical methods， chemical methods or biological methods， each has its own advantages and disadvantages. The combination of multimethod treatment of dietary fiber can avoid the defects of single method on the one hand， and allows the multiple methods to cooperate with each other to more effectively improve the yield and quality of dietary fiber. He[53]used peanut shell as a raw material to prepare dietary fiber by extrusion pretreatment， chemical （acid） washing， amylase enzymatic hydrolysis， etc.， and the obtained peanut shell dietary fiber had an SDF content reaching 18.1% and a DF content reaching 80.7%， indicating that the availability of dietary fiber was greatly increased. The fermented soybean dietary fiber was further subjected to ultrahigh pressure homogenization treatment， and the SDF/TDF ratio was as high as 41.44%， which was 4.56 times of that before highpressure homogenization.
　　Prospects
　　With the development of society and peoples attention to their own health， dietary fiber is added to foods as a functional factor in many cases to enhance its nutritional value. Modification treatment can significantly increase the content of soluble components in dietary fiber and enhance the physicochemical properties of dietary fiber including water holding binding capacity， expansion force and water binding capacity， thereby allowing dietary fiber to better exert its physiological functions. Consequently， the application of dietary fiber in the food industry can be broadened， and it is endowed with broad prospects in development and utilization.
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