With the further development of fish farming industry, extruded fish feeds have obtained more and more wide applications in virtue of unique advantages including high nutrient, good palatability, strong digestibility and so on. As a new technology to produce feed, extrusion processing means that in the environment of high temperature(110-200°C), high pressure(25-100kg/cm2) and high water content(10%-20%, even 30%), fish feed stuffs mixture, through heating tempering, deep mixing and particles formation, would puff and afterwards shape into extruded feed when out of fish feed extruder machine.
On the basis of density variation and puffing degree, extruded fish feed can be segmented into floating fish feed and sinking fish feed. Currently, the floating feed is the most widely used extruded feed in fish farming, and it has relatively mature technology. That is, floating extruded feed pellets are both produced through modulator for heating and modulation, via continuous mixing process to fish feed raw materials after entering extruder machine, heating up, pressurizing, ripening, extruding and then puffing and granulating. Compared with floating fish feed, sinking pellets has different requirement on fish feed manufacturing technique because of the diverse request for specific gravity, even if when they use the same fish feed machine. Briefly, fish feed extrusion process requires a hammer mill to grind raw materials, mixer to blend raw materials including vitamin-mineral premix, the wet type fish feed extruder with steam preconditioner (yes, steam is equired for quality pellets) and cutter head to shape the pellets, and finally a dryer-cooler to ensure pellet shelf-life and stability.
There are many factors affecting the expansion situation of feed stuffs, but expansion is mainly closely related to tempering condition, operating conditions of extruder machine, property and shape of raw materials, which means changing the extruding conditions can control the floating and sinking status of extruded fish feed.
In addition to those benthonic animals, most aquaculture species can ingest floating extruded fish feed very well, not only for the famous-special fish species (like aquarium fish), but also for conventional cultivation (like grass crap, crucian, catfish, tilapia and so on). What’s more, in the aspect of breeding method, floating fish feed has more extensive applicability, that is to say, for pond fish farming, culturing fish in rice field, running water fish culture, cage culture, and especially for fish farm with lower breeding density, extruded floating feed has much more superiority than other fish feed.