Change of feed nutrient composition during feed extrusion

1. Carbohydrate changes during extrusion

Carbohydrates are the main components of the feed and usually account for 60-70% of the feed. This is the main factor affecting the characteristics of extruded feeds. Carbohydrates can be divided into starch, fiber, hydrocolloid and sugar based on their abnormal molecular weight, structure and physicochemical properties. Their changes and effects during extrusion are different (Xu Xueming, 1995).

1) Starch squeezing can promote the breakage of 1-4 glycosidic linkages in starch molecules to produce low molecular weight products such as glucose, maltose, maltotriose and maltodextrin, resulting in reduced starch content after extrusion. However, the main effect of extrusion on starch is to promote intermolecular hydrogen bond cleavage and gelatinization. The effective gelatinization of starch makes the extrusion treatment not only improve feed nutrition, but also facilitate feed granulation, thereby improving feed processing quality.

2) Fiber fibers include cellulose, hemicellulose, and lignin, which often act as fillers in feed. Due to the different equipment and process conditions used for extrusion of fiber raw materials and extrusion, the literature reports on the changes in the number of fibers in the extrusion process are quite different. Fornal et al.'s study on the extrusion of buckwheat and barley, Wang and coworkers' research on wheat and wheat bran showed that the number of fibers after extrusion was reduced, and Bjorck (1984) and Ostergard (1989) were on whole wheat flour and whole foods respectively. The result of the extrusion study of wheat flour was just the opposite of the above; as for Silijestron (1986) and Schweizer (1986), the total fiber content of whole wheat flour did not change during extrusion. However, the results of the study on changes in fiber quality during the extrusion process are consistent, and all show that the amount of soluble dietary fiber after fiber extrusion is relatively increased. The general increase is about 3%. Wang and cooperators are aligned separately under different conditions. The results of fiber changes after grain wheat and wheat bran extrusion. The result is that high temperature, high pressure, and high shear in the extrusion process contribute to the breakage of intermolecular bonds, molecular cracking, and changes in molecular polarity. Since soluble dietary fiber has a special physiological effect on human health (Gordon, R. Huber, 1991; Cummings, JH, 1978.), the use of compression to develop dietary fiber is undoubtedly a good method, but it is the same for animals. It has not been reported that it has a bowel effect.

3) Hydrocolloid colloids are mainly used for the production of aquafeeds, usually gum arabic, pectin, agar, carrageenan, sodium alginate and other hydrocolloids, and their gelation ability will generally decline after extrusion. During the extrusion process, its hydrophilic properties will also affect the conventional extrusion conditions, reduce the moisture evaporation rate and freezing rate of the extruded product, and improve the texture properties of the product. For a specific product, the colloid viscosity, gelation, emulsification, hydration rate, dispersibility, mouthfeel, operating conditions, particle size, and raw material source must all be carefully considered when selecting a hydrocolloid.

4) Sugars are hydrophilic, which regulates the water activity of the material during the extrusion process, thus affecting starch gelatinization. Extrusion of high temperature, high shear sugar decomposition of carbonyl compounds, thereby the same material in the protein, free amino acids or peptides Maillard reaction, affecting the color of extruded feed. In addition, the addition of a certain amount of sugar in the extrusion process can effectively reduce the viscosity of the material, thereby increasing the bulking effect of the material at the exit of the die mouth, which is of certain help in controlling the floatation performance of the aquatic feed. Therefore, in addition to providing energy in extruded feeds, sugar is mainly used as a flavoring agent, sweetener, texture modifier, water activity and product color control agent. The commonly used sugar is sucrose. , dextrin, fructose, corn syrup, molasses, xylose, and sugar alcohols.

2. Protein changes during extrusion

Protein is subjected to high temperature, high pressure, and strong mechanical shear force in the extruder cavity to redistribute the surface charge and tend to be uniform. The molecular structure expands and recombines, and intermolecular hydrogen bonds, disulfide bonds and other secondary bonds break. Causes the protein to eventually denature. This denaturation makes it easier for proteases to enter the interior of the protein, thereby increasing the digestibility. However, in terms of protein quality, different extrusion conditions have different effects on it, which mainly depends on the loss of effective lysine during the extrusion process. The general trend is that when the moisture content of raw materials is below 15% and the extrusion temperature is higher than 180?°C, the lower the moisture content is, the higher the temperature is, and the higher the lysine loss, the lower the biological potency of the protein. . The decrease in the effectiveness of lysine caused by extrusion was mainly attributed to the Maillard reaction of some reducing sugars or other carbonyl compounds in the feed to lysine ?μ-NH3, and the possibility of producing lysine alanine . Appropriate changes in extrusion process conditions, such as reducing glucose, lactose and other reducing sugars in feed, and increasing the moisture content of raw materials can effectively reduce the occurrence of Maillard reaction. K. Dahlin (1993) treated the eight grains of corn, wheat, rye, and sorghum under different conditions and showed that they were extruded under conditions of 15% moisture, 150?°C extrusion temperature, and 100rpm rotation speed. At pressure, the biological titer of the product protein was significantly increased compared to the untreated material (Dahlin, K., 1993).

3. Changes in fat during extrusion

Extrusion will partially hydrolyze triglycerides to produce monoglycerides and free fatty acids, so from the standpoint of handling, the extrusion process will reduce the stability of the oil, but for the entire product, the extrusion product is stored during storage. The increase in free fatty acid content was significantly lower than that of unsqueezed samples, which was mainly attributed to the inactivation of factors such as fat hydrolyzing enzymes and lipoxygenases in the feed that promoted fat hydrolysis. Fats and their hydrolysates can form complexes with gelatinized starch during the extrusion process so that fats cannot be extracted with petroleum ether. The formation of this complex makes it difficult for the fat to seep out of the product and give the product a good appearance. This complex can be dissociated in the acidic digestive tract and therefore does not affect the digestibility of fat. Fat has a greater effect on the texture, shaping, palatability, etc. of feed, but overall, the presence of fat not only affects the quality of the final extruded product (mainly the degree of puffing), but may even affect the smooth progress of the entire extrusion process. For example, the fat content of defatted soy flour should not exceed 1%; in the production of extruded material in the feed industry, when the oil content of the single screw extruder is 0-12%, it has no effect on the extrusion effect. When the added amount is 12-17%, the bulk density of the added product increases by 16 g/l for every 1% of the added amount, and if the added amount continues to increase, the effect is even worse. When the added amount exceeds 22%, the product loses the general extrusion characteristics. . Therefore, the extrusion should be performed with a low oil content material.

4. Changes in vitamins, minerals and flavors during extrusion

Whether vitamins can be retained during processing depends largely on processing conditions. During the extrusion process, heat-sensitive vitamins such as VB1, folic acid, Vc, VA, etc. are the most vulnerable to destruction, while other vitamins such as niacin, VH, and VB12 are relatively stable. From the perspective of production convenience, the addition of vitamins prior to extrusion is preferable to the addition after extrusion, but it must be added in excess to overcome the effects of partial loss of vitamins during the extrusion process on animal nutrition. There are reports that adding vitamins before squeezing not only damages the vitamins during the extrusion process, but also increases the loss of vitamins during storage. Therefore, the vitamins of the extruded material may be added more economically after extrusion. During the extrusion process, minerals are generally not destroyed, but the formation of new polymers with solidification properties may reduce the biological potency of certain minerals. For example, phytic acid may be complexed with Zn, Mn, etc. Animal digested compounds. Due to the high temperature and high moisture content at the time of extrusion, flavor substances are decomposed, and the volatile flavor substances are mostly evaporated at the mouth of the die and evaporate together with steam. Therefore, the addition of the flavor during the processing is added after extrusion. Fiber raw materials in the feed industry are mainly derived from corn, rice cake, bran and bad residue. In the extrusion process, the fibers mainly affect the degree of puffing of the extruded feed. The general rule is that the degree of puffing decreases with the increase of the amount of fiber added, but different sources of fiber or fiber purity have different effects on the puffing degree. The pea and soy fibres have better puffing ability. Their addition of 30% in the feed based on starch has no significant effect on the degree of puffing of the final product, like oat bran and rice bran, due to their high content. Proteins and fats have poor bulking power.

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