by Ke Tianhong, Process Engineer
The extrusion process is a primary driver of aquafeed quality. In practice, pellet buoyancy, extruder operating stability and functional ingredient retention are tightly coupled: a change intended to improve one often shifts the others. When this interdependence is not managed systematically, feed mills and farms may experience feed losses, higher conversion costs and suboptimal animal performance.
This article summarises the process logic and key control points behind these three elements, providing a practical framework for technologists to design and run a robust extruded aquafeed process.
Buoyancy: The first ‘fit-for-feeding’ requirement
Buoyancy describes whether pellets remain floating, slow-sinking or sinking after water entry. Technically, it is governed by bulk density and internal pore structure, which are primarily created at the die through the combined effects of moisture, thermal energy and mechanical energy. Because feeding habits differ by species and life stage, buoyancy mismatch leads directly to feed waste, water quality deterioration and lower feeding efficiency that translates into inconsistent growth.
Three key extrusion levers affect buoyancy.
- Moisture level at the extrusion inlet
As a general starting point, higher moisture tends to reduce bulk density and increase expansion, supporting buoyancy. Typical targets depend strongly on formulation, preconditioning intensity and equipment configuration, but a common pattern emerges – floating feed is often associated with lower inlet moisture and higher expansion potential, with many plants operating in the low-to-mid teens depending on system design. Slow-sinking feed occupies an intermediate position, while sinking feed generally requires higher density and lower expansion, frequently supported by higher moisture and lower specific mechanical energy.
- Die temperature and screw speed
Higher die temperature and appropriate screw speed typically improve starch melt and expansion, increasing porosity and buoyancy. Many plants use higher thermal input for floating products and lower thermal input for sinking products, with ranges such as approximately 120–150°C versus 100–110°C serving as reference points depending on recipe, die configuration and line capacity.
- Formulation: Starch vs protein/fibre/lipid
Starch contributes to expansion and cell structure formation, while protein, fibre and lipid generally reduce expansion potential and promote sinking behaviour. For high-density sinking products, technologists often constrain starch and manage lipid strategically – either in-barrel or post-extrusion – while ensuring pellet integrity and water stability.
Even when extrusion settings are correct, buoyancy consistency can be undermined by post-extrusion drying and cooling. Uneven drying creates mixed pellet density, resulting in some pellets floating while others sink. As a common industry practice, finished moisture is controlled within a defined window – often around nine to 12 percent, depending on product and storage requirements – to stabilise both buoyancy performance and shelf life.
Extruder performance: The backbone of quality consistency
Extruder performance means more than machine uptime; it represents repeatable control of cooking, expansion and shaping under changing raw materials and ambient conditions. Operational stability is primarily determined by the coordinated management of temperature profile, pressure and restriction, screw speed and torque load, and raw material variability alongside preconditioning effectiveness.
During extrusion, the screw conveys and works the mass, generating heat via steam input and mechanical shear. Starch gelatinisation and protein structuring occur under this thermo-mechanical history, after which the product experiences a rapid pressure drops at the die, forming the porous structure that defines pellet quality.
Temperature management through barrel zoning deserves particular attention. A zoned temperature gradient supports progressive cooking and reduces the risks of localised undercooking or overcooking. A representative concept – though plant settings vary – is a rising profile from lower temperatures in early zones toward higher temperatures near the die to stabilise viscosity and expansion. What matters most is stable heat transfer, avoiding ‘hot spots,’ and preventing excessive melt viscosity swings that destabilise pressure and pellet shape.
Pressure management is equally critical, governed by die open area, hole geometry and land length, screw elements and restriction, and formulation alongside inlet moisture. Higher restriction combined with lower open area increases pressure and expansion potential; a configuration often used for floating products. Conversely, lower restriction with higher open area reduces pressure and expansion, supporting higher-density sinking products. Many plants operate within a controlled internal pressure band to stabilise expansion behaviour, though the appropriate band depends on equipment size, die design and product type.
Screw speed plays a multifaceted role, influencing specific mechanical energy and shear history, residence time distribution, throughput and torque load, and expansion behaviour alongside pellet texture. Too high a speed can raise specific mechanical energy excessively, causing over-expansion, brittle pellets and higher energy costs. Too low a speed can lead to insufficient cooking or expansion, resulting in high density and unstable pellet formation. For some sinking formulations, technologists often operate within a moderate speed range such as 250 to 350 rpm on certain machine sizes, but the correct window remains line- and recipe-specific.
Functional ingredient stability: Protecting nutritional value
Functional ingredients – including vitamins, enzymes, probiotics, amino acids and other bioactives – are inherently sensitive to heat, pressure, moisture and oxygen. Extrusion’s high-temperature and high-shear conditions can reduce activity, meaning pellets may meet physical specifications while failing nutritional objectives. The main drivers of loss are peak temperature, residence time at damaging conditions, moisture and mechanical energy history, and oxidation during drying and storage. Certain vitamin forms, for example, show accelerated degradation at elevated temperatures, while many probiotic strains cannot survive severe thermo-mechanical conditions.
Two practical mitigation strategies are available to formulators and process engineers.
- Low-temperature, short-time process design
This approach optimises the process to achieve required cooking and pellet properties with controlled thermal input, minimised unnecessary residence time, stable preconditioning and appropriate die restriction alongside screw configuration to avoid over-processing.
- Post-extrusion application (coating or spraying)
Heat-sensitive ingredients can be applied after extrusion and cooling using spraying systems for liquids or slurries, vacuum coating for deeper penetration and improved retention, and protective carrier or film-forming approaches where appropriate. In addition to preserving activity, well-designed coating can reduce leaching and improve oxidative stability – critical considerations for aquatic applications.
Managing the synergy: One system, not three topics
Buoyancy, extruder stability and functional ingredient retention do not operate in isolation; they form a single coupled system. Raising temperature to improve expansion may increase buoyancy but reduce bioactive retention and risk over-expansion. Reducing screw speed to lower specific mechanical energy may improve ingredient retention but can reduce cooking, destabilise pressure and cause inconsistent buoyancy. Pushing density for sinking feed can reduce expansion but may raise mechanical load or affect pellet integrity unless restriction, moisture and formulation are carefully balanced.
A practical workflow can help technologists navigate these trade-offs.
- Define target feeding behaviour (floating/slow-sinking/sinking) based on species and farming practice.
- Translate this into density and porosity targets, then select formulation strategy (starch/protein/fibre/lipid), die concept (restriction/open area/land length) pre-conditioning intensity and inlet moisture.
- Set extrusion conditions (temperature profile, screw speed/torque window) to achieve stable pressure and pellet structure.
- Protect sensitive nutrients via process minimisation (avoid over-processing) and post-extrusion application where required.
- Validate with in-water behaviour testing, durability/water stability and nutrient retention checks.
A concept illustration for slow-sinking shrimp feed helps ground these principles in practice. For shrimp, slow-sinking behaviour with good water stability is often required. A typical approach may include moderate starch level – approximately 15 percent, adjusted by formulation and binders – combined with a controlled die temperature around a lower expansion regime such as 110°C as a reference, depending on system configuration. Post-extrusion coating for heat-sensitive vitamins and probiotics then improves retention and functional delivery.
Conclusion
High-quality extruded aquafeed production is fundamentally a balancing act. Buoyancy must match feeding behaviour, the extruder must run in a stable and repeatable operating window, and functional ingredients must retain their intended activity. Mills that manage these factors as an integrated system – rather than as isolated adjustments – are better positioned to deliver consistent performance, lower waste and stronger cost-effectiveness.
Note: The parameter examples and ranges referenced in this article are intended as industry and engineering reference points. Final settings should be validated against specific formulations, equipment configurations and product requirements. Technical insights and illustrative data are based on development experience from Bühler (Changzhou) R&D and Application Center.
