Extrusion processing technology has become of major importance in the production of modern feeds used in intensive aquaculture. In recent years there has been a constant growth in the application of extruded diets for aquatic feeding. Aquatic feed by extrusion have superior water stability, better floating property and a higher energy than pelleted diets, which contributes to the increase in fish growth and improvement in feed conversion. Therefore compared with the conventional pelleting process, extrusion technology is the most efficient way to improve the performance of aquatic feed.
Extrusion and Expansion Process
Extrusion, including dry extrusion and wet extrusion, is the process where the raw material is pushed forward in the barrel by means of screws and pressed through the die at the end of barrel. The general extrusion process involves a high temperature over a short time. Temperature higher than 100℃ is needed in order to achieve expansion of the feed as it leaves the die. The high temperature in dry extrusion is acquired through dissipation of mechanical energy from heated surfaces such as barrel and screw surface, or generated by shear forces between wall and material and screw and material. For wet extrusion, the temperature is achieved through preconditioning and steam injection. At the same time the material also undergoes relatively high pressure. The pressure difference in the interior of extruder and the external environment will cause the extrusion of the aquatic feed.
Basically, the operating principle of expansion is similar to that of extrusion, and their processes are both based on the same principle. However, expansion generates less shear, pressure and temperature in the barrel compared with extrusion, which leads to difference in the shaping method of final pellets and intensity of treatment.
Extruder and Expander
Extruder and expander are machines that mainly apply extrusion and expansion technology in feed pellet production. An extruder is a bio-reactor providing the necessary pressure to force feed mash in the barrel through a restrictive die. Through changes in temperature, pressure, and shear within the barrel, the raw material is forcibly homogenized before it assumes the cross-sectional shape of the die opening. Extrusion equipment is mainly composed of power transmission device, feeding device, pre-conditioner, extruder barrel and discharging cutting device, etc. The barrel of extruder is composed of barrel heads, screw auger, flow restrictors (called shearlocks) and a cross-sectional die.
Expanders are similar to extruders, but they have difference in treatment effects and applications. Expanders are commonly used as mechanical conditioners for treating materials which are difficult to pellet, to increase the digestibility of cellulose and protein components in order to perform feed hygienistion. Extruders consist of barrels with one or two screws to transport materials and force them through a die. Extrusion can be considered as a high shear treatment. The shear action during expander treatment is much less. Compared to an extruder, the simplicity of the expander allows an effective treatment of relatively large quantity of feed at a low cost. Extrusion machine has received the dominating role production of fish feed. For extruder, there are different types available in the market, such as single screw extruders, twin screw extruders, wet extruders, dry extruders and etc..
Single Screw Extruder and Twin Screw Extruder
As the name suggests, single screw extruder contains one screw, and twin screw extruder is with two screw of equal length placed inside the same bareel. Twin screw extruder develops on the basis of single screw extrusion machine, which is more complicated than single screw extruder. Twin screw extruder owns much priority in quality control and processing flexibility, which can deal with sticky, multiple oil or wet raw materials. The screw design of twin-screw extruders can dramatically affect operating efficiency and machine capability. Notice that throughput and screw speed of twin screw extruder are not interdependent.
In contrast to twin screw extruder, single screw extruder is simple, using rotating belt way with a constant unadjustable spindle speed. It can get good blending through high speed operation in short time. The barrel of a single screw extruder can be divided into three separate zones: feeding zone, melting zone and molding zone. These steps occur simultaneously and are independent. In the feeding zone, feed material is simply received form the conditioner and transported forward in the barrel, go through solid conveying and melting process. Making feedstocks from a powder to an elastic dough. Then it will be formed into pellets by flat die shaping. Nevertheless, the conveying capacity of single extruder is poor under high pressure, compared to the same power twin-screw extruder, its production is lower.
In general, single screw extruders are more economical to operate than twin-screw extruders, while twin screw extruders are more advanced, more productive,and more extensive use of performance than single screw extruders. Choose the right one depend on the production demand.
Dry Type and Wet Type Extruder
Dry type extruder is the type of machine that does not require a steam boiler for heat or steam injection or jacket heating, and all product heating is accomplished by mechanical friction force. Without utilizing preconditioning, dry extrusion employs lower moisture levels. Therefore it is suitable for processing low moisture and high fat feed material.
While wet type extruder is a piece of more complex equipment with precision metering systems for steam or water injection into the conditioner and/or the extruder barrel. Preconditioning is a very important step in wet extrusion process for chemical or physical reactions to take place. The heart of the extruder machine working is: preconditioning, cooking and die shaping. High temperature and high pressure process can kill the salmonellosis and bacterial infections and also makes the pellets easily to digest.
Advantages of Extruded Aqua-feed Pellet
The capability of an extruder enhances the feed manufacturer’s flexibility to produce fish feed pellets for shrimp, crab, eel, catfish, carp, frogs and etc..
2.High starch gelatinization
Feed material goes through the processing of HTST, starch gelatinization degree increase can reach 80-99% for aquatic animals easily digest.
3.Better digestion and absorption
Pellets can breakdown quickly in the stomach of fish loses nutrients (during regurgitation), which contributes to better absorb nutrients.
4.Improved water stability
The floating pellets are more resistant to disintegration in water and a floating feed allows the fish farmer to observe the condition of the fish and the amount of feed consumed.
5. Water absorption
Extrusion moisture content during processing can be controlled over a wide range, which helps to increase the water stability of the final feed. These pellets absorb more water, retains shapes for longer time and results in reduced losses of nutrients. It benefits pellet technical qualities and fish health.
6.Convenient for density control
It is entirely feasible for an extruder to produce floating feed, sinking feed and slow sinking feed by density control; that is more conducive to different fish species.
7.Fine mechanical resistance
Feed made using extrusion technology is more resistant to mechanical durability and produces fewer fines in the finished feed during transportation. It can reduce feed waste, water and air pollution.
8.Good to aquatic organism health
Extrusion cooking provides hygienic feed processing and destroys the pathogens & other most viruses to avoid animal illness.
From the above, it is obvious that extruders are very versatile and can make feeds with many different characteristics, which can greatly benefit aquatic feed production! There are various types of extruders available in the market such as: dry type extruder, wet extruder, single screw extruder, twin screw extruder and etc.. Different types of extruders own its features, which contribute to extruded pellets with different performance. Therefore before choosing an extruder for your aqua-feed production, you should clearly know what type of you need for your business.