1. Changes in carbohydrates during extrusion
Carbohydrate is the main component of feed, usually 60 to 70% in feed, so it is the main factor affecting the characteristics of extruded feed. Carbohydrates can be classified into starch, fiber, hydrocolloid and sugar according to their molecular weight, structure and physical and chemical properties. Their changes and roles in extrusion process are different (Xu Xueming, 1995).
1) Starch extrusion can promote the cleavage of 1-4 glycosidic bonds in starch molecules to produce low molecular weight products such as glucose, maltose, maltotriose and maltodextrin, resulting in a decrease in the starch content of the extruded product. However, the main effect of extrusion on starch is to promote the intermolecular hydrogen bond cleavage and gelatinization. The effective gelatinization of starch makes the extrusion treatment not only improve the nutrition of the feed, but also facilitate the granulation of the feed, thereby improving the quality of feed processing.
2) Fibrous fibers include cellulose, hemicellulose, and lignin, which typically act as fillers in feed. Due to the different materials and process conditions for the fiber raw materials used for extrusion and extrusion, the literature on the change in the number of fibers during extrusion has been widely reported. Fornal et al.'s study on the extrusion of buckwheat and barley, Wang and collaborators' studies on wheat and wheat bran showed that the amount of fiber after extrusion decreased, while Bjorck (1984) and Ostergard (1989) respectively focused on whole wheat flour and whole wheat. The results of the extrusion study of wheat flour are exactly the opposite of the above; as for Silijestron (1986) and Schweizer (1986), it is believed that the total fiber content of whole wheat flour does not change during extrusion. However, the results of fiber quality change during extrusion process are consistent, which indicates that the amount of soluble dietary fiber is relatively increased after extrusion, and the general increase is about 3%. Wang and the collaborators respectively under different conditions. Results of fiber changes after squash wheat and wheat bran extrusion. This result is caused by high temperature, high pressure and high shear in the extrusion process, which causes valence bond cleavage, molecular cleavage and molecular polarity change. Since soluble dietary fiber has a special physiological effect on human health (Gordon, R. Huber, 1991; Cummings, JH, 1978.), it is undoubtedly a good method to develop dietary fiber by extrusion, but whether it is the same for animals. The effect of the whole intestine has not been reported.
3) Hydrocolloid colloid is mainly used in the production of aquatic feed. It usually has hydrocolloids such as gum arabic, pectin, agar, carrageenan and sodium alginate. After extrusion, the gelling ability will generally decrease. The hydrophilic properties during extrusion will also affect the conventional extrusion conditions, reduce the water evaporation rate and freezing rate of the extruded product, and improve the texture properties of the product. For a specific product, factors such as viscosity, gelation, emulsifying, hydration rate, dispersibility, mouthfeel, operating conditions, particle size and source of raw materials of the colloid are carefully considered when selecting a hydrophilic colloid.
4) Sugar sugar is hydrophilic, and will regulate the water activity of the material during extrusion, thus affecting starch gelatinization. The high temperature and high shear of the extrusion cause the sugar to decompose to produce a carbonyl compound, which causes a Maillard reaction with the protein, free amino acid or peptide in the material, which affects the color of the extruded feed. In addition, the addition of a certain amount of sugar during the extrusion process can effectively reduce the viscosity of the material, thereby increasing the expansion effect of the material at the exit of the die, which is helpful for controlling the ups and downs of the aquatic feed. Therefore, in addition to providing energy, the sugar in the extruded feed is mainly used as a flavoring agent, a sweetener, a texture regulator, a water activity and a product color regulator. The commonly used sugar is sucrose. , dextrin, fructose, corn syrup, molasses, xylose and sugar alcohol.
2. Protein changes during extrusion
The protein is re-distributed and homogenized by the high temperature, high pressure and strong mechanical shearing force in the extruder cavity. The molecular structure is stretched and recombined, and the secondary bonds such as intermolecular hydrogen bonds and disulfide bonds are partially broken. Lead to final denaturation of the protein. This degeneration makes it easier for proteases to enter the interior of the protein, thereby increasing digestibility. However, in terms of protein quality, different extrusion conditions have different effects, which mainly depend on the loss of effective lysine during extrusion. The general trend is that the lower the moisture content when the raw material moisture is lower than 15% and the extrusion temperature is higher than 180 °C, the higher the temperature, the greater the loss of lysine, the lower the biological potency of the protein. . The decrease in the effectiveness of lysine caused by extrusion is mainly due to the Maillard reaction of some reducing sugars or other carbonyl compounds in the feed with lysine ε-NH3, and the possibility of producing lysine alanyl is less. . Appropriate changes in extrusion process conditions, such as reducing the content of reducing sugars such as glucose and lactose in feed, and increasing the moisture content of raw materials, can effectively reduce the occurrence of Maillard reaction. K. Dahlin (1993) and other eight grains treated with corn, wheat, rye and sorghum under different conditions showed that the raw material moisture was 15%, the extrusion temperature was 150 ° C, and the rotation speed was 100 rpm. The biological potency of product proteins is significantly improved compared to untreated raw materials (Dahlin, K., 1993).
3. Changes in fat during extrusion
Extrusion will partially hydrolyze triglyceride to produce monoglycerides and free fatty acids. Therefore, from the simple treatment, the extrusion process will reduce the stability of the oil, but for the whole product, the extruded product is in the process of storage. The increase in free fatty acid content is significantly lower than that of unsqueezed samples, which is mainly due to the inactivation of factors that promote fat hydrolysis, such as lipohydrolase and lipoxygenase in the feed. The fat and its hydrolyzate can form a complex with the gelatinized starch during extrusion, so that the fat cannot be extracted by petroleum ether. The formation of such a complex makes it difficult for the fat to exude from the product to give the product a good appearance. This complex dissociates in the acidic digestive tract and therefore does not affect the digestibility of fat. The effect of fat on the texture, shape and palatability of the feed is great, but the overall presence of fat not only affects the quality of the final extruded product (mainly the degree of expansion), but may even affect the smooth progress of the entire extrusion process. For example, for the extrusion of defatted soybean powder, the fat content should not exceed 1%; in the production of bulk material in the feed industry, when the amount of grease added to the single-screw extruder is 0-12%, there is no effect on the extrusion effect. When the addition amount is 12-17%, the weight of the product increases by 16g/l for every 1% increase, and the effect is even worse when the addition amount continues to increase. When the amount exceeds 22%, the product loses the general extrusion characteristics. . Therefore, the extrusion should be based on raw materials with low oil content.
4. Changes in vitamins, minerals and flavors during extrusion.
Whether the vitamins are retained during processing depends largely on the processing conditions. During the extrusion process, heat-sensitive vitamins such as VB1, folic acid, Vc, VA, etc. are the most susceptible to damage, while other vitamins such as niacin, VH, VB12 are relatively stable. From the point of view of ease of production, the addition of vitamins prior to extrusion is superior to that after extrusion, but must be added in excess to overcome the effects of partial vitamin loss on animal nutrition during extrusion. It has been reported that the addition of vitamins prior to extrusion not only destroys the vitamins during the extrusion process, but also reduces the loss of vitamins during storage. Therefore, the vitamins of the extruded material may be more economical to add after extrusion. During extrusion, minerals are generally not destroyed, but the formation of new polymers with solidification properties may reduce the bioavailability of certain minerals. For example, phytic acid may be complexed with Zn, Mn, etc. Animal digested compounds. Due to the high temperature during extrusion, high moisture will decompose the flavor material, and the volatile flavor material will be mostly lost as it evaporates along with the water vapor at the die port. Therefore, the addition of the flavoring agent during the processing is added after extrusion. The fiber raw materials in the feed industry are mainly derived from corn, cakes, bran and dregs. During the extrusion process, the fiber mainly affects the degree of expansion of the extruded feed. The regularity is that the degree of expansion decreases with the increase of the fiber addition, but the difference in the purity of the fiber or fiber from different sources has a significant difference. The expansion ability of peas and soybean fiber is good. The addition of 30% in the starch-based feed has no significant effect on the puffing degree of the final product, and like oat bran and rice bran, because they contain higher Protein and fat, their ability to puff is very poor.
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