The protein feed material is puffed under the action of certain temperature, pressure, moisture and time, so that the raw material is subjected to high temperature, pressurization, mixing, shearing, gelatinization, ripening, sterilization, etc. The high temperature and high pressure protein feed is in the extrusion die hole. When the instantaneous decompression, the volume of the material expands, a large amount of air and water molecules expand rapidly and enter the material, so that a porous and bulky structure is formed in the material, and the volume is increased several times to ten times. In this process, the structure and physical and chemical properties of various components in the protein feed ingredients have changed, such as protein denaturation, starch gelatinization, fiber, anti-nutritional factors, etc., which have been degraded to varying degrees, and some enzymes and toxic substances have been destroyed.
1 protein feed expansion and composition changes
1.1 Puffing of protein feeds
In terms of the moisture content of the raw materials, the expansion is classified into a dry extrusion method and a wet extrusion method. The dry extrusion method means that no water is added when the raw material is puffed, and the friction heat generation between the material and the outer cylinder wall of the extruder and the screw is simple, the operation is simple, the equipment cost is low, but the extrusion temperature is not easy to control, the nutrient damage is large, and the power is large. The consumption is large, the equipment is easy to wear, and the product quality is unstable. Wet extrusion refers to the addition of moisture or water vapor during the extrusion process, with additional heating to increase material flow. The wet extrusion temperature is lower than the dry extrusion and is easier to control.
The extruder can be divided into single-screw and twin-screw extruder according to the number of screws, and the single-screw extruder is generally used for the processing of protein feed materials.
1.2 The effect of extrusion process on the main components of protein feed ingredients
1.2.1 Protein
The protein content of the protein feed material is 20% to 80%. In the high energy field, the original three-dimensional structure of the protein molecule is destroyed, and the unfolded globular protein molecules are rearranged and combined. Under the action of die pore shearing and decompression, the protein molecules are linearly ejected, and the large lumps of protein are split. As a small linear protein, the hydrogen bonds, disulfide bonds and ionic bonds inside and outside the protein peptide chain are rearranged, and tissue denaturation and disulfide chain breakage occur, in which cystine is destroyed.
At higher barrel temperatures and lower feed moisture, the free terminal amino group of lysine reacts with other amino acids and may also react with free sugars, reducing nutritive value. Therefore, process parameters and methods must be preferred during the expansion process to allow the product to have both high digestibility and reduced amino acid damage.
1.2.2 Starch
When the protein feed is puffed, the starch will undergo a gelatinization reaction, and the starch will be degraded during gelatinization, which will increase the starch digestibility. The main feature of starch gelatinization is that starch is mixed with excess water. Under the action of shear extrusion, the temperature rises and the water penetration increases, and a large amount of water is absorbed, causing the starch particles to rupture and gelatinize. The gelatinization effect is manifested by a change in the hydrogen bond in the starch molecule, and the water retention of the gelatinized starch is remarkably improved. Puffing exposes the starch chain and accelerates enzymatic hydrolysis.
1.2.3 Cellulose
In the process of extrusion, the protein feed ingredients can increase the content of soluble dietary fiber (SDF), and the total fiber content is also reduced after extrusion (Fig. 1), increasing xylose, arabinose, mannose and soluble dietary fiber. Composition of uronic acid and the like.
1.2.4 Vitamin
When the protein feed ingredients are extruded, the vitamins are destroyed, and the stability of different types of vitamins during puffing varies greatly.
Among the fat-soluble vitamins, VD and VK are quite stable, VA and VE and their compounds, carotenoids and tocopherol monomers are unstable under the action of oxygen and heat, and the higher barrel temperature (200 ° C) can make The β-carotene is reduced by more than 50%. Oxidation promotes pigment loss, and the addition of antioxidants reduces pigment loss. Among the water-soluble vitamins, VB1 is very sensitive to heat treatment. Killeit (1994) test proved that VB1 lost 5% to 100% during puffing. Andersson and Hedlund (1990) also noted that VB1 loss was higher during dry extrusion, while riboflavin (VB2) and niacin were unaffected. VC is also a raw material that is sensitive to heat and oxidation.
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