Extrusion technology is widely used in modern feed processing. After processing feed through extrusion, the starch in the feed is gelatinized and degraded, proteins undergo denaturation, antinutritional factors are reduced, and palatability is enhanced, among other advantages. However, there are also disadvantages such as the formation of indigestible substances, destruction of vitamins, and increased costs.
Extrusion technology involves feeding materials containing a certain amount of moisture into an extrusion machine. Under the action of screws and spirals, the material moves forward axially, experiencing mechanical friction between the material and the screw, the material and the barrel, as well as internal friction. This results in intense mixing, shearing, and refining of the material, followed by increasing pressure and temperature inside the machine. Under high temperature, high pressure, and high shear force, the components of the material undergo complex physical and chemical changes. Finally, the paste-like material is extruded through the die hole, and the pressure difference causes the material to expand, forming a loose, porous, and crisp extruded product.
Feed materials processed by extrusion have a unique aroma and fluffy texture, making them highly palatable. The degree of gelatinization is high, which is highly effective in attracting animals to eat. Additionally, the long-chain structures of some proteins and fats are broken down into shorter chains, making them easier for animals to digest and absorb.
2.1 Starch Gelatinization and Degradation
After extrusion, starch undergoes two main changes:
1.Gelatinization: The extrusion process breaks the dense crystalline structure of the starch molecules, causing the crystalline structure to absorb water and break the hydrogen bonds. The starch granules break apart and become a viscous molten substance. Upon exiting the extruder, a sudden drop in pressure causes the steam to quickly dissipate, resulting in the breakdown of the starch and the formation of porous extruded feed.
2.Degradation: The average molecular weight of starch is significantly reduced, and through hydrolysis, smaller molecules like maltodextrin are produced. Gelatinized starch has stronger water absorption and bonding properties compared to regular starch, reducing the need for excessive starch use in production and providing more opportunities to utilize other raw materials. Additionally, gelatinized starch tightly binds with protein, forming undegradable proteins in ruminant stomachs, known as bypass protein, improving protein utilization in ruminants.
2.2 Protein Denaturation
Protein-containing materials undergo denaturation under the combined action of high temperature, high pressure, and high shear in the extruder. The tertiary and quaternary structures of proteins are destroyed, the protein molecules unfold and reorganize, and the structure becomes more uniform. Hydrogen bonds, disulfide bonds, and other molecular interactions are partially broken, leading to protein denaturation. The degree of protein denaturation is closely related to the extrusion parameters. Extrusion also deactivates many antinutritional factors, such as urease, trypsin inhibitors, and lectins in soybeans, glucosinolates in canola meal, and gossypol in cottonseed.
Feathers are considered a high-quality protein source, with a protein content of about 75%–90%. However, the protein in feathers consists mainly of polypeptide chains arranged in a helical structure, which is stabilized by extensive cross-linking, disulfide bonds, and hydrogen bonds. This makes it difficult for animals to digest. Unprocessed feather meal has low digestibility, around 7%. After extrusion, the keratin in the feathers is denatured, breaking the spatial structure and making it digestible, with digestibility increasing to over 70%.
2.3 Fat Denaturation
Extrusion processing breaks the cell wall structure of oilseeds, releasing oils, and improving oil utilization. It also allows fats to combine with starch or protein to form complex products (lipoproteins or lipopolysaccharides), reducing free fatty acids and deactivating esterases. This inhibits the degradation of oils, reduces acidification during storage and transportation, and extends the shelf life of the feed.
2.4 Improved Palatability and Digestibility
Extruded feed particles are small, crisp, and have a roasted flavor, increasing palatability. The loose, disordered structure of the extruded feed provides a larger surface area for enzymes to act upon, enhancing the contact between starch chains, peptide chains, and digestive enzymes, thus improving digestibility and absorption.
2.5 Increased Fiber Solubility
Extrusion can significantly reduce the crude fiber content in feed. During the extrusion process, high temperature and pressure cause the lignin in the cell walls to melt, breaking hydrogen bonds and decomposing high molecular weight substances into lower molecular weight materials. The originally tight structure becomes fluffy, releasing some digestible substances, thus improving feed utilization.
2.6 Improved Feed Storage and Shelf Life
Under high temperature and pressure, extrusion kills molds, bacteria, and fungi in raw materials, improving the sanitary quality of the feed. This effectively reduces the occurrence of diseases like diarrhea, gastroenteritis, and dysentery in animals.
During the extrusion process, reducing sugars and free amino acids in the feed undergo Maillard reactions, which decrease the content of reducing sugars and free amino acids, weakening their effectiveness. The high temperature, pressure, and moisture evaporation also lead to the loss of vitamins, particularly water-soluble vitamins, which can be reduced by up to 50%. Therefore, additional vitamins need to be added to the feed to compensate for the losses caused by extrusion.
