The aquatic feed manufacturing industry is widely recognized as one of the fastest expanding agricultural industries in the world and it is the wide application of aquatic feed extruder that contributes a great share for the huge step forward. Extrusion technology is the most efficient way of turning various fish meal, fish oil, soybean meal, cereals, some slaughterhouse by-products into palatable feed for aquaculture organisms.
1. Aquatic Feed Extruder Basic Structure
From an engineering point of view, an extruder is simply a pump providing the necessary pressure so as to force the process feed mash through a restrictive die. It is a high-temperature short-time bio-reactor that transforms a variety of raw ingredients into feed pellets. Extrusion equipment is mainly composed of power transmission device, feeding device, pre-conditioner extruder barrel and discharging cutting device, etc. The barrel of the extruder is comprised of barrel heads, screws, flow restrictors (called shearlocks) and a die.
1.1 Wet Type Extruder and Dry Type Extruder
According to whether there is pre-conditioning of feed ingredients with steam or moisture before entering the extruder barrel, extrusion devices falls into two categories: dry extruder and wet extruder. During dry extursion process, all the heat produced is generated by the friction and mechanical energy. It’s suitable for processing low moisture and high fat feed material like full-fat soybean expansion. While wet cooking extruder is a piece of more complex equipment with precision metering systems for steam or water injection into either 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.
1.2 Single Screw Aquatic Feed Extruder VS Twin Screw Extruder
Generally in accordance with the number of screw in extruder barrel, extrusion device is divided into single screw extruder and twin screw extruder. In contrast to twin screw extruder, recently widespread used single screw extruder possesses the advantages of less investment and simple operation. Currently, single-screw extruders produce over 90% of expanded pet foods, which represent the greatest volume of extruded products on the market. It has simple structure and constant spindle speed generally by belt drive. The barrel of a single screw extruder can be divided into three separate zones:feeding zone,melting zone and molding zone. In the feeding zone, feed mash is simply received from the conditioner and transported forward in the barrel, go through solid conveying and melting process, making feed stocks from a powder to an elastic dough. Then it will be formed into pellets by flat plate shaping.
1.3 The Advantages of Aquatic Feed Extruder in Production
☞Wide adaptability: the capability of an extruder enhances the feed manufacturer’s flexibility to produce fish feed pellets for shrimp, crab, eel, catfish, carp, frogs, etc. as well as process full-fat soybean, dogs and cats feed, pet food, ornamental animal feed and others.
☞High starch gelatinization: feed material goes through the processing of HTST, starch gelatinization degree increase can reach 80-99% for Aquatic animals easily digest.
☞Improved water stability: aquatic feed water stability is an important indicator. Our extruded aquatic feed pellets can soak in the water for more than 12 hours without decomposition for fish bite.
☞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.
☞Fine mechanical properties: puffed fodder has good mechanical performance and less dust, unbreakable. It can reduce feed waste, water and air pollution.
☞Good to aquatic organism health: extrusion cooking provides hygienic feed processing and destroys the pathogens & other most viruses to avoid animal illness.
2. Extrusion Technology Overview
Extrusion cooking is a process involving feed ingredients plasticizing and cooking in the extruder barrel by the combination force of pressure, heat and friction. Feed materials actually go through the cooking process under the condition of high temperature, high humidity and high pressure; thus their physical and chemical properties are changed. When feed material is squeezed out from the die hole, moisture in the feed immediately changes from liquid into gas form owing to pressure sharply drop to 100 kpa, so that the particle is expanded and forms the so-called puffed feed. Extrusion technology is the ideal choice to process aqua feed because of the benefits it offers to fish farmers. Extruded feed has also become much popularity due to its advantages such as high starch gelatinization degree, easily digestible proteins, high feed utilization rate and good floatability,etc.
2.1 Characteristics Analysis on Floating Feed Pellets and Sinking Feed Pellets
Commercial fish diets are manufactured in either floating form or sinking form. Floating,sinking, and slow sinking aquatic feed can be produced by adjusting feed formulation and related parameters of expander.
–Floating feed pellets can float on the surface of the water for a long time; its feeding method is conducive to raising management, saving labor and reducing water pollution; especially make it convenient to observe fish ingestion condition and adjust feeding amount. Generally, fish farming adopting puffed feed can save 5%-10% aquatic feed compare to mash and pressure-pelleted feed.
–Puffed sinking pellets absorb water and become soft easily. It is endowed with the properties of high ripening degree and water stability; thus is suitable to crabs and other benthic animals; Twin-screw wet extruder is ideal for sinking aquatic feed production. Pellets expanding degree is closely related to modulation conditions, operation process and feedstocks moisture content.
2.2 Option of Floating Feed or Sinking Feed
For fish farmers, how to choose the suitable fish pellets is crucial. In fact, the options of loating or sinking pellets mainly depends on fish species that you raise. If you have bottom feeders like shrimp pleco and catfish, it would make perfect sense to give them sinking pellets. While,catfish, carp, tilapia top as well as middle dwelling fish like floating feed. Besides, salmon, trout, sea bass/sea bream, and cod like to eat the slow-sinking feed pellets.
Conclusion: Extrusion technology has more technical and economical advantages in the production of pelleted feed owing to its flexibility and reliability production. In general, single screw extruders provide adequate operating ranges and are more economical to operate; Whereas twin screw extruder with high good material adaptability enables fish farmers to adopt wider ranges of ingredients. Pre-conditioning in wet cooking extrusion can enhance pellet quality greatly. There is no doubt that extruder and expander has a broad prospect in the aquaculture feed industry development and will result in usage of new extrusion technology