How does an extruder ensure sufficient gelatinization and structural formation in low-starch feed formulations?
Publish Time: 2025-09-04
As the aquaculture industry shifts toward high-protein, low-starch, and environmentally friendly feeds, traditional formulations that rely on starch as a binder and bulking agent are no longer able to meet demand. In particular, starch content in feeds for carnivorous fish like California bass, mandarin fish, and grouper is strictly controlled below 10% to minimize liver metabolic burden and water pollution. However, this reduction in starch content means the material lacks sufficient thermoplasticity and viscoelasticity during extrusion, easily leading to problems such as inadequate gelatinization, loose structure, and poor water resistance. Leveraging its powerful shearing, mixing, and energy input capabilities, the extruder, through systematic process control, has successfully overcome the molding challenges associated with low-starch formulas, achieving efficient gelatinization and stable structural formation.
1. High-intensity mechanical energy input replaces thermal gelatinization
In traditional high-starch feeds, gelatinization primarily relies on high temperature and moisture, causing starch granules to absorb water, swell, and rupture, forming a gel network. In low-starch formulas, the extruder utilizes high speed, a special screw assembly, and a segmented compression design to convert mechanical energy into shear heat, directly acting on the protein and a small amount of starch molecules. The intense shear force disrupts the secondary structure of the protein, causing it to denature and unfold, exposing hydrophobic groups and active sites. This in turn forms a three-dimensional network through intermolecular crosslinking. This "shear-induced protein gelation" mechanism, even in extremely low starch content conditions, serves as the primary support for the feed structure, replacing the function of traditional starch gels.
2. Precise Temperature Zone Control and Moisture Management
While mechanical shear is the core driving force, the coordinated regulation of temperature and moisture remains crucial. The twin-screw extruder utilizes a multi-stage heating and cooling system, enabling precise temperature profiles to be set in different barrel zones. A low temperature is maintained in the feeding section to prevent premature gelatinization and clogging. In the compression and homogenization sections, the temperature is gradually raised to 120-140°C, combined with the injection of an appropriate amount of water to promote complete protein denaturation and melting. Water not only serves as a heat transfer medium but also reduces material viscosity, enhances fluidity, and ensures uniform heating under high shear. For heat-sensitive ingredients (such as vitamins and enzymes), heat damage can be avoided by cooling them in the post-cooking section or by side feeding.
3. Screw Configuration Optimization: Achieving Synergistic Functionality in Each Segment
The screw assembly of a twin-screw extruder is central to process control. For low-starch formulations, a segmented configuration consisting of "conveying-compression-shearing-homogenizing-die" is typically employed. The front section primarily features a high-lead conveying element to ensure stable feeding. The middle section incorporates reverse-direction flights and kneading discs, creating a closed chamber that extends material residence time and enhances compression and shearing. The rear section utilizes a small-gap kneading and homogenizing ring to further refine the material and improve uniformity. This gradient energy input avoids localized overheating or insufficient shearing, ensuring sufficient protein denaturation and uniform distribution, laying the foundation for subsequent puffing.
4. Mold Design and Puffing Control
The final step in forming lies in the mold. Low-starch materials have low melt strength, requiring high die pressure to maintain melt density. The thick die plate and short, flat die hole design prolong the melt's flow time within the die, enhancing orientation and compaction. The high-speed rotation of the cutter ensures smooth pellet cuts and minimizes pulverization. After extrusion, the material undergoes instant flash dehydration at atmospheric pressure, forming porous yet dense pellets. By adjusting the die hole shape and cutter speed, it is possible to produce buoyant, slow-sinking, or sinking feeds to meet diverse aquaculture needs.
5. Synergistic Effect of Functional Additives
In low-starch systems, functional binders or structural proteins are often added to aid network formation. The intense mixing action of the twin-screw extruder ensures uniform dispersion of these additives, fully maximizing their crosslinking and reinforcing effects, further enhancing the pellets' water resistance and mechanical strength.
Through high-intensity shearing, precise temperature and humidity control, optimized screw geometry, and die design, combined with functional excipients, the extruder successfully achieves full gelatinization and stable formation of low-starch formula feeds, providing key technical support for the production of high-nutrition, low-pollution, and environmentally friendly aquatic feeds.
