Impact of extrusion moisture and temperature on digestibility of barramundi (Lates calcarifer) diets containing bacterial protein meal

Bacterial protein meals (BPMs) are emerging as promising alternatives to conventional protein sources in aquafeeds. However, clear limitations have also been evidenced regarding the high inclusion rate of BPMs in aquafeed. This study investigates how feed extrusion settings can improve digestibility of diets containing a BPM derived from bacteria cultivated on brewery by-products in barramundi ( Lates calcarifer ). Over 36 days, 2640 barramundi (85.9 ± 1.6 g) were reared in 24 tanks (1000 L) and fed 8 experimental diets. These diets were extruded at either 100 °C or 140 °C, with barrel moisture levels of 33 % or 40 %, and included either 0 % (CTL diet) or 30 % BPM (BPM diet), replacing 50 % of the soy-based ingredients. Fish performance, diet apparent digestibility coefficients (ADCs) for macronutrients, 42 fatty acids and 17 amino acids, digestive enzyme activity, and gut transcriptomics were assessed. BPM inclusion did not impact feed intake but reduced growth, lowered the ADCs, and increased feed conversion ratio. However, the extrusion settings specifically improved BPM diet digestibility (P < 0.05) without affecting CTL diet. Extruding the BPM diet at 140 °C with a 33 % moisture rate showed the greatest benefits for energy digestibility. The activity of digestive trypsin, chymotrypsin, and leucine aminopeptidase, remained unaffected by BPM incorporation (P < 0.05), suggesting no anti-nutritional factors in BPM. The lower digestibility of the BPM diet likely stems from reduced nutrient absorption, supported by transcriptomic analyses. These revealed 58 upregulated and 146 downregulated genes (P < 0.1) in the midguts of BPM diet-fed fish, indicating changes in lipid and amino acid metabolism, oxidative phosphorylation, and chemokine signaling. By controlling extrusion temperature and moisture, it was possible to enhance the nutrient digestibility of a diet containing 30 % BPM. The study also revealed the impact of interactions between extrusion parameters and feed formulation on nutrient digestibility. • Replacing 50 % of soy protein with BPM reduced growth and digestibility in barramundi. • Feed extrusion at 140 °C with 33 % moisture specifically improved BPM diet digestibility. • Gut protease activity remained unaffected by BPM incorporation. • Gene expression changes relating to gut metabolism were seen in fish fed BPM. • Extrusion parameters and ingredients had visible effects on digestibility.