Low feed dose-to-larval density attenuates pathogen inactivation in black soldier fly larvae bioconversion

Black soldier fly, Hermetia illucens larvae (BSFL) convert biowaste substrates into high-value products and can reduce notorious food-borne pathogens during the bioconversion process. Substrate properties, feed dose, and larval density are key factors that influence process efficiency, yet their effect on pathogen reduction is unclear. We addressed this gap in laboratory-scale experiments using BSFL reared at different larval densities (0, 1, 3 and 6 larvae cm^-2) with a corresponding feed dose of 0.6, 0.2 and 0.1 g VS larva^-1, respectively. This resulted in larval density and area adjusted feed dose, or feed dose-to-larval density, of 2.6, 0.9 and 0.4 mg VS larva^-1 cm^-2. BSFL were fed a carbohydrate-rich or protein-rich substrate supplied in three feeding events. Pathogen inactivation was assessed in substrates inoculated with a cocktail of bacterial pathogens (Salmonella spp. and Escherichia coli) either on the first feeding event (single inoculation), or on all three feeding events (multiple inoculations). Low feed dose-to-larval density enhanced bioconversion efficiency, larval yield and substrate reduction, but reduced larval survival, final larval weight and frass yield. Bioconversion efficiency and substrate reduction were higher in the carbohydrate-rich than the protein-rich substrate. Conversely, pathogen reduction (Δlog₁₀Red) was higher in the protein-rich than carbohydrate-rich substrate. Unexpectedly, Δlog₁₀ Red declined with decreasing feed dose-to-larval density, with multiple substrate inoculations causing greater decline than single substrate inoculation. Collectively, our findings suggest that process efficiency and pathogen inactivation are inextricably linked: low feed dose-to-larval density enhances process efficiency but could compromise microbiological hygiene of the process and products. This trade-off underscores a need for integrated optimization of process efficiency and hygiene in BSFL bioconversion.