Extrusion processing technology is highly important in production of feeds used for intensive aquaculture. In recent times, there has been a constant growth in the application of extruded diets for aquatic feeding as it results in better water stability, better floating property and a higher energy than pelleted diets and thus contributing to increased fish growth and improved feed conversion ratio (FCR). Therefore, compared with the conventional pelleting, extrusion technology is the most efficient way to improve the performance of aquatic feed.
Extrusion (dry or 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 between internal extruder and 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: It 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. The 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 (shear locks) and a cross-sectional die.
It is 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 achieve feed hygienistion. Extruders consist of barrels with one or two screws to transport materials and force them through a die. It 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.
Different types of Extruder
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 equipment, 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 oils or wet raw materials. The screw design of twin-screw extruders can dramatically affect operating efficiency and overall equipment capability. It is to be noted 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 fixed 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 following three zones:
a) feeding zone
b) melting zone
c) molding zone
In the feeding zone, feed material is simply received from 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. Selection of extruder depends upon the production demand.
Dry Type and Wet Type Extruder
Dry type extruder 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.
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. The extruder machine working is: preconditioning, cooking and die shaping. High temperature and high pressure process can kill salmonella and other bacteria, also makes the pellets with improved digestibility.
Benefits of Extruded Aqua feed
1. Wide adaptability: The capability of an extruder enhances the feed manufacturer’s flexibility to produce fish feed pellets for shrimp, crab, eel, carp etc.
2. High starch gelatinization: Feed material goes through HTST processing thus starch gelatinization degree may increase up to 80-99% making it more digestible.
3. Better digestion and absorption: Quick breakdown of pellets in the fish makes nutrients available for better absorption.
4. Improved water stability: The floating pellets are more resistant to disintegration in water and floating feed allows the fish farmer to observe 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 shape for longer time thus resulting in reduced losses of nutrients.
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 wastage, water and air pollution.
8. Good to aquatic organism health: Extrusion cooking provides hygienic feed processing and destroys pathogens & other viruses to avoid animal illness.