Effects of online pretreatment of plant ingredients on processing responses and physical properties in extruded fish feed

A feed processing study, using a 4×2×2 design, was conducted to evaluate the influence of plant ingredient (soybean meal, rapeseed meal, field peas and faba beans), pretreatment of the mash (water addition of 400 g/kg total moisture to the plant meal fraction of the mash for 45 min at 45 °C) and conditions applied during the extrusion process (screw speed of 325 or 225 rpm) on quality of fish feeds. Pretreatment of the plant ingredients increased the extrusion temperature (P<0.001) and the steam pressure in the barrel (P<0.05) and reduced the specific mechanical energy (P<0.05), resulting in a decrease (P<0.01) in hardness in the diets subjected to the pretreatment. Pretreatment of the feed also resulted in a decrease in bulk density (P<0.01) that was lowest for the rapeseed meal diet (P<0.01). There was an effect of plant ingredient on temperature in the extruder (P<0.05), but only in the third section, while no effect was observed on pressure. Diets with field peas and faba beans resulted in increased (P<0.05) pellet hardness. Increased screw speed reduced both pellet hardness (P<0.05) and bulk density (P<0.001). In conclusion, the novel pretreatment method can be implemented online in an extrusion line, if desired, for nutritional purposes. However for regular ingredients it does not affect physical quality.