Assessment of the effects of fish meal, wheat gluten, soy protein concentrate and feed moisture on extruder system parameters and the technical quality of fish feed

Evaluation of feed ingredient functionality plays a vital role in modern fish feed manufacturing practice. The aim of this study was to examine the extrusion behaviour of blends containing alternative protein sources from plant origin to fish meal (FM), such as wheat gluten (WG) and soy protein concentrate (SPC), and the consequences for the physical attributes of the resulting feed extrudates. A mixture design was applied, varying the levels of protein sources included in the formulation from 50 to 450 g kg−1. Each diet was produced with added feed moisture content of 20, 26 and 32 g/100 g (wet basis). The partial least squares regression models were fitted and their performance was evaluated on the basis of R2 and the root mean squared error of cross-validation over the complete data set. A higher inclusion level of FM in the diet decreased the values of the extruder system parameters, such as torque, pressure at the die and melt temperature. In contrast, inclusion of SPC significantly increased the values of these extruder-related parameters. The viscoelastic properties of WG gave higher radial expansion; FM showed the opposite effect. The results show that the feed moisture was the dominant factor for extrudate density and oil absorption capacity. Products with higher breaking strength were observed with increasing levels of WG and SPC. Combining the product requirements for both extrudate density and hardness showed that the largest optimal compositional range is available at low feed moisture content. However, maximum FM replacement is possible at high feed moisture content.