Soybean meal improves the physical quality of extruded fish feed

The study was designed to investigate effects of processing of soybean meal on physical quality of extruded fish feed. Three diets were produced; one fish meal (FM) based control diet, and two diets with 40% amino acids from FM replaced by defatted, toasted soybean meal (SBM) or defatted, untoasted soybean meal (white flakes, WF). The diets were extruded under steady state standardized conditions. The WF diet was also extruded with an alternative screw-configuration and reduced screw revolution (RPM) generating a specific mechanical energy (SME) during the production similar to the FM and SBM diets. The WF feed from the first and second production is thus denoted as the WF1 diet and the WF2 diet, respectively.Partial replacement of FM with either SBM or WF improved the physical quality of feed in terms of breaking force and durability, bulk density was elevated and radial expansion reduced. Inclusion of WF increased the motor load, die pressure and SME of the WF1 diet compared to the FM and SBM diets, resulting in significantly longer pellets and higher bulk density. Extrusion of the WF2 diet with a less shearing screw configuration in combination with reduced screw rotation gave SME comparable to the energy consumption used during the production of the FM and SBM diets. The new processing conditions caused a significant reduction in length and diameter, and hence increased bulk density. Breaking force corrected for the diameter of the pellet was significantly higher for the WF2 diet produced with lower shear and screw rotation compared to the other three feeds produced under the same extruder conditions. The overall conclusion from the experiment was that durability and breaking force of extruded fish feed was significantly improved by inclusion of soy, regardless of previous heat treatment of this feed ingredient.