screw extruders

Twin screw extruders were first used in the 1950s for manufacturing of thermoplastics. Only since the 1970s have these machines been used for feed and food products. Twin screw extruders (TSE) can be classified by their mechanical way of working into co-rotating (both screws turn same direction) and counter rotating (one screw turns right and the other turns left) and the screw configuration into intermeshing self wiping and non-intermeshing. A TSE compared to a single screw extruder has a much wider operating range in terms of use of moisture, internal fat, mechanical energy input and product size.

 

Extruded aquatic feeds have several qualities. Looking at the physical properties, with TSE the shape of the final pellet can be adapted to the animal’s mouth and calibration of shape and size is easy. A TSE can influence the texture of the pellet varying in hardness, brittleness, cohesiveness, etceteras. Density can also be influenced, resulting in sinking and floating properties adjusted to the fish’s eating behaviour. Finally storage and handling abilities can be influenced regarding fines content and moisture content amongst others.

 

The biological properties of aquatic feeds are much easier to match with the animal’s needs when a TSE is used. Recipes can be adapted to physiological needs and depending on species and age. Availability of nutrients can be improved when using TSE and extrusion has a significant effect on protein. This protein can originate from plant sources such as soybeans, legumes, glutens and cereal grains. These have good functional properties, relatively low cost, but amino acid profiles may sometimes be lacking some essentials. Furthermore glutens and cereal grains are excellent binders and expand well. Proteins from animal sources such as meat, fish, blood and gelatine have poor functional properties unless they are fresh or spray dried. Costs are usually higher compared to plant proteins, but amino acid profiles are good. Proteins cannot be processed at a too high temperature (>150°C) since this will damage the protein and make it indigestible for the animal.