Optimization of the process parameters for slow-sinking extruded aquatic feed

Abstract: A fish feed extruder has been widely used to process a variety of raw ingredients into the pellets for the floating or sinking aquatic feed. The extrusion generally requires higher levels of moisture, heat, and pressure than pelleting. The purpose of this experiment was to clarify the effects of processing parameters on some quality indicators, including the bulk density, the expansion ratio, the energy consumption, the sinking speed, and the sinking ratio. The optimal processing parameters were then obtained for the slow-sinking aquatic feed during extrusion. A central composite experimental design was used to determine three factors: the moisture content of mash feed after conditioning, the screw speed of the extruder, and the die temperature. A total of 20 groups of processing experiments were carried out, where each factor was set at five levels. The processing parameters of slow-sinking extruded aquatic feed were optimized, where the hole area per ton was fixed at 450 mm2/(t·h), the moisture content was ranged from 23% to 27%, the die temperature was from 100 ℃ to 140 ℃, and the screw speed of the extruder from 180 r/min to 300 r/min. The experiment results showed that the bulk and unit densities of the extruded feed pellets decreased, whereas, the expansion ratio increased significantly, with the increase of moisture content of the mash feed after conditioning. The difference in sinking speed increased at various die temperatures under the same screw speed. Specifically, the bulk density and the sinking speed decreased, but the expansion ratio increased, while the 10 min sinking ratio first increased and then decreased, with the increase of die temperature. The energy consumption increased significantly, as the screw speed increased. The initial sinking ratio increased at the moisture content between 23%-25%. Once the moisture content was higher than 25%, the 10 min sinking ratio decreased first and then increased. It infers that the interaction between the moisture content and screw speed posed a negative correlation with the initial sinking ratio, the same as the interaction between the moisture content and die temperature with the sinking ratio within 10 min. Therefore, the optimal process parameters were achieved for the ideal quality of slow-sinking extruded aquatic feed, where the moisture content of material after conditioning was 27%, the die temperature was 110.68 ℃, and the screw speed was 265 r/min. In this case, the bulk density was 533.17 g/L, the sinking speed of extruded aquatic feed was 7.10 cm/s, the sinking ratio was 70.33% at 0 min and 99% at 10 min. The findings can provide a strong reference for the production of slow-sinking extruded aquatic feeds.