1. Introduction
By preconditioning the mixed feed formulation, the conditioning process can significantly improve the physical quality and production capacity of subsequent pelleting processes (including die pelleting and extrusion pelleting). This process mainly uses steam to heat the material, and—depending on the formulation requirements—simultaneously adds liquid additives such as water, meat/fish slurry, and binders. Within a short residence time (typically from several tens of seconds to a few minutes), the material is fully moistened and softened.
Heating is primarily achieved through direct steam injection, although jacket heating is also used in some applications. Under the combined action of heat and moisture, various components in the material gradually undergo physical and biochemical changes, including starch gelatinization and protein denaturation, thereby improving overall processing performance.
The conditioning process plays a critical role in downstream pelleting, with the following key benefits:
· By introducing a large amount of thermal energy, it enables preheating, hydration, and partial cooking of raw materials, ensuring smoother downstream pelletizing;
· Reduces shear stress in the melting section of subsequent screw extrusion, minimizing nutrient loss caused by degradation of macromolecules and improving final product quality;
· Increases throughput of pellet mills and extruders, enhancing overall production efficiency;
· Reduces wear on die plates, screws, and barrels, extending equipment service life and lowering maintenance costs.
2. Principle of the Conditioning Process
During conditioning, the formulated mixture is continuously blended with steam, water, or other liquid additives under mechanical agitation. Driven by specially designed paddles, the material moves from one end of the conditioner to the other. In this process, dry particles are mixed with water, fully absorb moisture, and become thoroughly wetted. Meanwhile, under steam heating, starch in the raw materials begins to gelatinize and proteins start to denature (as illustrated in the diagram).
The key factors influencing particle transformation during this process include moisture content, temperature, and time. To achieve effective heating or thermal cooking in conditioning, the equipment must maintain an appropriate mixing speed while introducing a controlled amount of water and steam, ensuring sufficient residence time for complete wetting and heating.
Other equipment variables—such as paddle number, angle, and rotational speed—are essentially geometric design parameters. These structural configurations are intended to ensure uniform and sufficient mixing of materials with water and other additives.
From a feed processing perspective, combining conditioning with pelleting or extrusion significantly improves process flexibility. Their characteristics are as follows:
Ring-die pelleting process
Before entering the pellet mill, conditioned material undergoes preconditioning, which significantly expands process adjustability. Pelleting relies on mechanical compression forming; therefore, additional water or oil cannot be introduced during forming, as it would negatively affect pellet quality.
Extrusion pelleting process
Preconditioning before extrusion also improves operating conditions of the extruder.
Although extruder barrels may have injection ports for direct addition of water or steam, the contact time between injected media and raw material is very short, making it difficult for moisture to penetrate into the core of solid particles.
Extending the extruder barrel length can increase residence time and moisture penetration, but it significantly raises equipment cost and energy consumption, making it economically inefficient.
Therefore, preconditioning via a conditioner provides pre-cooking and pre-hydration at lower cost while creating greater flexibility for downstream extrusion operations.
The amount of water added during conditioning is mainly determined by the process requirements of downstream equipment (pellet mill or extruder), and is also constrained by feed formulation and final product physical/chemical specifications.
From downstream process standards:
· For pelleting: post-conditioning moisture content is typically controlled at 16%–18%
· For extrusion: post-conditioning moisture content is typically 23%–30%
In addition, different raw material formulations have different physical properties, requiring adjustment of water addition accordingly. In industrial practice, operators may further fine-tune water addition based on final pellet quality requirements.
Saturated steam is typically used as the conditioning medium. It plays a dual role:
First, it heats the mixture and initiates starch gelatinization; second, it condenses upon contact with cooler materials, forming condensate water that provides both moisture and heat. Together with added process water, this ensures thorough hydration and continuously promotes starch gelatinization.
The effective conditioning time (residence time in the conditioner) is mainly determined by paddle speed and production throughput, and can reach up to several minutes.
3. Common Conditioning Equipment
Conditioners used in feed production are generally classified into three types: single-shaft conditioners, twin-shaft conditioners, and differential diameter conditioners.
1) Single-shaft Conditioner
The single-shaft conditioner is equipped with one main shaft fitted with helical paddles. During operation, the shaft simultaneously mixes and conveys material from the inlet to the outlet. The shaft can operate at fixed or variable speed, and typical filling rate is 5%–15%.
It features a simple structure, stable “first-in-first-out” material flow, easy cleaning and maintenance. However, its limitations include relatively weak mixing performance, suitable only for moderate water and steam addition. It is not suitable for high-viscosity materials such as meat slurry or high-oil formulations and is prone to blockage. Residence time is also shorter compared with twin-shaft systems.
2) Twin-shaft Conditioner
The twin-shaft conditioner consists of two parallel shafts equipped with paddles. The shafts have identical diameter and speed and rotate in opposite synchronized directions. Different paddle designs can be installed as required.
It provides significantly better mixing performance than single-shaft systems, with typical filling rates of 15%–25%.
3) Differential Diameter Conditioner
The differential-speed conditioner is also a twin-shaft design, but the two shafts operate at different diameters and speeds. The auxiliary shaft speed can reach up to 5 times the linear speed of the main shaft, and paddle angles can be adjusted as needed.
This is the most widely used and highest-performance conditioner in the feed industry. The high-speed shaft generates strong turbulent mixing for excellent homogeneity, while the low-speed shaft controls conveying speed and ensures sufficient residence time. Combined operation ensures both mixing efficiency and processing duration. Filling rate can reach 25%–40%.
Compared with single-shaft systems, twin-shaft and differential-speed conditioners provide strong internal convection and superior mixing, enabling processing of high-moisture and high-slurry formulations.
However, they also have drawbacks: difficult to achieve strict first-in-first-out flow, limited adjustment via paddle angle, higher manufacturing cost, and more complex cleaning and maintenance.
4. Mixing and Flow Characteristics inside the Conditioner
After materials are mixed with water, steam, and additives and held for a certain period, they must be discharged efficiently. This is achieved by adjusting paddle angles on the main shaft, which control material flow direction, mixing intensity, and residence time.
0° / 15° paddles
· Mild axial conveying force
· Best mixing uniformity due to strong lateral agitation
· Excellent wall-cleaning performance and minimal residue
· Suitable for high mixing quality requirements with lower conveying demand
45° paddles
· Maximum axial conveying efficiency
· Moderate mixing performance
· Best choice for high-throughput production lines
75° paddles
· Weak conveying capacity
· Poor mixing and low flow uniformity
· Used only for special slow-retention applications
90° vertical paddles
· Minimal conveying capability
· Severe material buildup risk
· Poor cleaning and mixing performance
· Typically used only as blocking or flow-control elements
Overall trend: smaller angles improve mixing and self-cleaning; ~45° maximizes conveying; angles near 90° reduce all performance aspects.
The optimal paddle configuration should allow material to remain concentrated in the middle section of the conditioner. This can be achieved by combining backward, neutral, and forward paddles. After backward paddles, forward paddles should be installed near the discharge section to facilitate material discharge.
It is important to note that excessive backward paddles may cause overfilling and overload the drive motor. The ideal configuration should balance maximum filling, sufficient residence time, and optimal mixing performance, ensuring continuous effective contact between particles and injected steam.
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