Fish Nutrition & Feed Technology

Unit 10 - Methods of feed formulation and manufacturing

10.6.1.Extruder

• The extruder can be defined as a bioreactor where the raw materials are subjected to high temperature and pressure over a relatively short time.
• Feed ingredients are propelled along the barrel of the extruder by one or two screws.
• The mechanical screw action and the friction it creates, blends, shears, and cooks the material. (Held at 120-175ºC for about 30 seconds).
• At the end of the barrel, the mixture is forced through a die at high pressure; because of the difference the between pressure inside and outside of the barrel, the material expands.
• This sudden drop in pressure causes the water in liquid phase to change to the gaseous state producing what is known as expansion.
• As the steam escapes small voids are created which determine the cellular structure of the extrudate.
• The more cells the final product has, the less dense it becomes, and hence becomes a floating feed.
• If the cellular structure is more compact the final product is denser; therefore, heavier and can be considered a sinking feed.
• Floating feeds have a density that is usually below 450 g/l, while sinking feeds have densities above 550 g/l.