Impact of processing technologies on insect meal digestibility in rainbow trout (Oncorhynchus mykiss) and European sea bass (Dicentrarchus labrax)

The objective of this study was to compare the apparent digestibility coefficients (ADCs) of nutrients and energy of three differently processed black soldier fly (BSF) meals in rainbow trout and European sea bass. The processing techniques included defatting with heat treatment followed by tricanter centrifugation, with or without an enzyme hydrolysis step, as well as microwave drying. In the experimental design, each processed BSF meal was mixed with a reference diet at a 30% inclusion level, using celite as an inert digestibility marker and fed to triplicate groups of rainbow trout and European sea bass. For the European sea bass trial, an additional diet with full fat oven-dried BSF meal was also evaluated. In rainbow trout, all the processed insect meals exhibited high digestibility, with no significant differences among the BSF meals. All the amino acids were highly digestible, with ADCs ranging from 84.8% to 93.6% for the essential amino acids and from 78% to 93.2% for the non-essential ones without significant differences between treatments (P > 0.05). In European sea bass, while fat and energy digestibility was similar, protein digestibility was significantly higher in all the processed BSF meals when compared to the oven-dried meal (P = 0.004). Between the two full fat insect meals, microwave drying significantly improved dry matter and protein digestibility of BSF meal when compared to oven-drying (P = 0.020 and P = 0.004, respectively). No differences were observed in the digestibility of the two defatted insect meals (P > 0.05), thus suggesting that enzymatic hydrolysis did not affect their digestibility in either fish species. This study suggests that processing techniques such as defatting and microwave drying can enhance the nutritional quality and digestibility of BSF meals and offers insights for optimizing insect-based feeds in aquaculture.