Effect of rapeseed press cake and peel on the extruder response and physical pellet quality in extruded fish feed

Alternative protein sources are required to meet the demand for fishmeal-free feed in the aquaculture industry. We assessed rapeseed press cake (RPC) as fishmeal replacement, including the effect of rapeseed peel (RP), by testing its effect on extruder response and the physical quality of feed pellets. Eight rainbow trout (Oncorhynchus mykiss W.) feed formulations were extruded containing 0, 8, 16, 24 or 32 g/100 g RPC or 4, 11 or 14 g/100 g RP. We varied the extrusion temperature (90-110 °C) and screw speed (200-400 rpm) and monitored the pellet quality (expansion indices, bulk density, sinking velocity, specific hardness, water stability and durability) and extrusion process parameters (pressure at the die, SME, product temperature and torque). The pellet quality was affected by the feed formulation. Formulations with high RPC/RP contents increased the extrusion die pressure and caused greater longitudinal expansion, but simultaneously reduced the sectional expansion, bulk density, sinking velocity and specific hardness compared to the reference feed. Although RPC or RP reduced the pellet quality, low concentrations were compatible with functional pellets and can be included in fish feed as an alternative protein source.