Effects of Clostridium autoethanogenum protein inclusion levels and processing parameters on the physical properties of low-starch extruded floating feed

Clostridium autoethanogenum protein (CAP, RichMore®, China) is a safe and effective alternative protein source. However, the physicochemical properties of CAP and the effects of CAP inclusion on the physical properties of extruded fish feed are still unclear. The extrusion trials were performed to investigate the effects of graded CAP substitution levels (replacing 0%, 25%, 50%, 75% and 100% of fishmeal in the diet), preconditioning moisture content (28% and 32%) and screw speed (200 rpm and 300 rpm) on the physical qualities of low-starch extruded floating feed. The results showed that CAP had high crude protein content (832 g/kg), low crude lipid content (19.0 g/kg) and low ash content (35.0 g/kg). The water-holding capacity, foaming capacity and foaming stability of CAP were significantly higher than those of low-temperature steam dried fish meal (LTFM) (P < 0.05). The floatability of all extruded feeds reached 100%. As the replacement level of CAP increased, the peak viscosity of the unprocessed diets increased significantly (P < 0.05), whereas the pasting temperature decreased significantly (P < 0.05). Specific mechanical energy significantly increased with the increase in CAP substitution level (P < 0.05) and decreased with the increase in screw speed and moisture content (P < 0.05). Dietary CAP inclusion significantly reduced the bulk density, water solubility index, water solubility and oil leakage (P < 0.05), significantly increased the expansion ratio, hardness, water absorption index and oil absorption (P < 0.05), and improved the microstructure of the feed. Moisture content and screw speed also had significant effects on the physical qualities of the extruded feed, and the recommended process parameters for the test diets are 28% moisture content and 300 rpm screw speed. Overall, CAP is a promising protein substitute in aquaculture, which can be used to manufacture high-quality low-starch extruded floating feed.