Effects of Internal Fat Content and Screw Configuration on Extrusion Traits and Kibble Characteristics of High-Protein, Low-Starch Fish Feed

In aquaculture, feed production influences nutrition, performance, water quality, and overall profitability. This study evaluated the effects of three levels of internal fat (IF), resulting from the inclusion of 0%, 2%, or 4% fat in the preconditioner during extrusion, and their interaction with two extruder screw configurations: medium-shear (MS) and high-shear (HS), on kibble physical quality and extrusion parameters. Increasing IF resulted in a quadratic increase in amylose–lipid complexation under the HS configuration (p = 0.030; r2 = 0.9) and a linear reduction (p < 0.001) in specific mechanical energy (SME) with a strong negative Pearson correlation (r −0.9; p = 0.009) in both configurations. Fat inclusion also reduced mass temperature and die pressure (p < 0.05), leading to lower starch gelatinization degree (p < 0.05) from 87.9 ± 0.6% to 83.4 ± 0.3% in MS configuration and 95.6 ± 0.7 to 86.3 ± 0.8% in HS configuration, increased bulk and piece density (p < 0.001), and reduced radial expansion (p < 0.001). These changes decreased floatability (p < 0.05) and water stability, increasing mushiness (p < 0.01). Increased shear partially improved SME transfer, starch cooking, expansion, floatability, and mushiness; however, the negative effects of 4% IF could not be fully mitigated. Overall, higher IF compromised kibble structure, starch gelatinization, and floatability, while screw configuration resulted in only a limited compensatory effect.