Application of puffing technology in the development of protein feed raw materials

       Protein feed raw materials is expanded in a certain temperature, pressure, under the action of water and time, make the raw material under high temperature, pressure, mixing, shearing, pasting, curing, sterilization effect, high temperature and high pressure of protein feed instantaneous relief during extrusion die, the material volume enlargement, swelling and a lot of air, water molecules into the material, the formation of porous structure of bulk material, size several times to ten times. In this process, the structure and physicochemical properties of each component in the protein feed raw materials changed, such as protein denaturation, starch gelatinization, fiber, anti-nutrient factors and so on were degraded to different degrees, and some enzymes and toxic substances were destroyed.

　　1 Expansion and composition change of protein feed

　　1.1 Puffing of protein feed

　　In terms of water content of raw materials, swelling can be divided into dry extrusion method and wet extrusion method. Dry extrusion means that the raw material is expanded without adding water, and only depends on the friction heat generated between the material and the outer cylinder wall and screw of the extruder. The operation is simple and the equipment cost is low, but the extrusion temperature is not easy to control, the nutrition is destroyed greatly, the power is consumed greatly, the equipment is easy to wear and the product quality is unstable. Wet extrusion refers to the extrusion process in which water or steam is added and heated externally to increase the flow of material. The temperature of wet extrusion is lower than that of dry extrusion and easier to control.

　　According to the number of screw extruders can be divided into single screw and double screw extruders, protein feed raw materials generally used single screw extruder.

　　1.2 Effect of extrusion process on main components of protein feed materials

　　1.2.1 Protein

　　Protein content in protein feed material is 20% ~ 80%. Protein molecules in high energy field, its three-dimensional structure is broken, after a globular protein molecules rearrange combination, under the effect of die shear and decompression, the protein molecules into linear, large clumps of protein split into smaller linear protein, make protein peptides inside and outside of the hydrogen bond, ionic bonds and the disulfide bond distribution, tissue degeneration and disulfide chain rupture, cystine in the process of the damage.

　　At higher cylinder temperature and lower feed moisture, lysine free terminal amino group reacts with other amino acids, and may also react with free sugars, reducing nutritional value. Therefore, the process parameters and methods must be optimized in the process of puffing, so that the product has high digestibility and reduce the damage of amino acids.

　　1.2.2 Starch

　　When the protein feed is expanded, the starch will be gelatinized, and the starch will be degraded during gelatinization, which improves the digestibility of the starch. Starch gelatinization is mainly characterized by starch mixed with excess water. Under the action of shearing and extrusion, the temperature rises, and water infiltration also increases, and a large amount of water is absorbed, making starch granules broken and gelatinized. The gelatinization was characterized by the change of hydrogen bonds in starch molecules, and the water holding capacity of gelatinized starch increased significantly. The expansion makes the starch chain bare and accelerates the enzymatic hydrolysis.

　　1.2.3 Cellulose

　　During the extrusion process, soluble dietary fiber (SDF) content of protein feed raw materials increased, while total fiber content decreased after extrusion (Figure 1), and the composition of soluble dietary fiber such as xylose, arabinose, mannoose and uronic acid increased.

　　1.2.4 Vitamin

　　The vitamins will be destroyed when the protein feed raw material is extruded, and the stability of different kinds of vitamins is very different during extruding.

　　In fat-soluble vitamins, VD and VK are fairly stable, VA and VE and their compounds, carotenoids and tocopherol monomers are unstable under the action of oxygen and heat, and higher cylinder temperature (200℃) can reduce -carotene by more than 50%. Oxidation promotes pigment loss, which can be reduced by adding antioxidants. Among water-soluble vitamins, VB1 is very sensitive to heat treatment. Killeit(1994) proved that the loss of VB1 during swelling was 5% ~ 100%. Andersson and Hedlund(1990) also noted that the loss of VB1 was higher during dry extrusion, while riboflavin (VB2) and niacin were not affected. VC is also sensitive to heat and oxidation.