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Extrusion Process

Raw material receiving and cleaning

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The designed capacity for raw material intake in an aquafeed mill shall be 2~3 times of its production capacity. For materials of different properties, special cleaning operation should be carried out where it is received.

Milling

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As the pretreatment of in aquafeed production, milling is designed to diminish the difference in and the variation of material particle size, to improve the working performance and working efficiency of the pulverizer thereof, and to ensure constant quality of finished products.

Mixing

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In such processing section, the major ingredients, i.e. the ingredients of larger proportion in aquafeed formulation, are batched by an electronically controlled proportioning system, and then sent to the mixer for mixing.

Preconditioning

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Preconditioning is used to properly condition the material before the follow-up extruder cooking stage so as to supplies the extruder with a uniformly mixed and hydrated material by mechanical stirring and adding saturated steam. Under the turning and blending efforts of conditioner paddles, the moisture content of mash is uniform and the temperature is steady eventually.

Extrusion

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Extrusion process is mainly executed by an extruder that is basically composed of a stationary barrel housing in which such elements as segmented screws and shearlocks rotates, fulfilling extrusion operation together with a disc and knife assembly. There are two kinds of extruders that might be used in aquafeed production: the single-screw extruder and the twin- screw extruder.

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Single-screw extruder: As its name implies, a single-screw extruder has only one extruding screw and is of simple structure, so its price is relatively lower. Since it has been applied in aquafeed manufacturing for over 40 years, the single-screw extruding technology becomes mature, and the working performance is relative stable and reliable.
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Traditionally, single-screw extruders are widely used for producing the feeds for low protein adult fishes such as tilapia, catfish fish, grass carp, etc. Besides, for the cultivation of different fish species in lake, the grinding fineness of feed only requires that 95% of ground particles should pass through the screen of 30~50 mesh, which can be achieved by a common grinding process without ultra-fine grinding. In such case, the advantages of a single-screw extruder are obvious and superb, because of its low equipment cost and high production efficiency.

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Twin-screw extruder: The twin- screw extruder is characterised by its wide adaptability, sliding conveying in the barrel and self-cleaning function, etc. However, it is relatively expensive due to the high primary investment and operation and maintenance costs. Therefore, the twin-screw extruder is usually applied in the production of petfood and feed for high value-added aquatic animals such as eels, soft-shelled turtle and young fish. For other special aquafeed production like the production of micro aquafeed (pellet size: Φ0.8~1.5mm), oil-rich aquafeed and the aquafeeds with changeable formulas, the twin screw extruder is of genuine differentiation.

Drying

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Once the product leaves the extrusion system, a drying step is required to remove the excess water and return the product back to a shelf-stable moisture condition. It is recommended to apply a crossflow dryer for drying wet extrudates, in which moisture and heat of product is exchanged with a perpendicular airflow.

Coating and cooling

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If necessary, liquid fats, flavours, and vitamins can be added externally to extruded feeds after drying. Post liquid application by coating will not only avoid the risk of damaging heat sensitive ingredients, but also improve the palatability and reduce powder generation of the finished product. After coating, extruded aquafeed is cooled as required.

Bagging

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When bagging aquafeed, the amount of air entrained in the bag or that penetrating into the bag should be controlled to a minimum, so as to prevent feed from oxidation and deterioration. Therefore, it is recommended to close the bag through sewing plus heat sealing (plastic film lining), which can prolong the shelf life of finished product by 50-100%.