In vitro digestibility and fermentability of selected insects for dog foods

Insects are considered as a sustainable protein source for future pet foods. Here we aimed to evaluate the protein quality of larvae of the black soldier fly (Hermetia illucens, BSF), housefly (Musca domestica, HF) and yellow mealworm (Tenebrio molitor, YMW) and to evaluate the fermentation characteristics of their indigestible fractions. Clean freeze-dried larvae were subjected to in vitro simulated canine gastric and small intestinal digestion. Undigested insect residues, shrimp chitin and fructooligosaccharides (positive control, FOS) were incubated for 48h with inoculum with fresh feces from three dogs simulating large intestinal fermentation. The AA profiles differed among the larvae with proteins from BSF and YMW larvae containing more Val and less Met and Lys than HF larvae. The in vitro N digestibility of the HF (93.3%) and YMW (92.5%) was higher than BSF larvae (87.7%). The BSF larvae also had lower in vitro digestibility values for essential AA (92.4%) and non-essential AA (90.5%) compared to the larvae of the HF (96.6 and 96.5%) and YMW (96.9 and 95.3%). Gas production for FOS increased rapidly during the first 6h. Low and similar amounts of gas were found for HF larvae and chitin whereas gas production slowly increased over 30h and was slightly higher at 48h for BSF than for chitin. Gas production for YMW increased considerably between 6 and 20h. At 48h, gas produced for undigested residues was comparable to shrimp chitin and lower than FOS (P<0.001). Incubation with insect residues resulted in more N-acetylglucosamine than with shrimp chitin (P<0.05), suggesting higher microbial degradation of insect chitin. Fecal microbiota from one dog appeared to be better able to ferment the undigested residue of YMW as gas production increased considerably between 6 and 20h of incubation and was higher than for the microbiota from the other two dogs. The associated metabolite profile indicated that acetate, propionate and butyrate were the main fermentation products. Furthermore, formate was produced in relatively high amounts. It is concluded that the protein quality, based on the amino acid profile and digestibility, of selected insect larvae was high with the undigested insect fractions being at least partly fermented by the dog fecal microbiota. As the microbiota from one dog was better capable of fermenting the undigested residue of YMW larvae, it is of interest to further study the selective growth of intestinal microbiota in dogs fed insect-based food.