Preliminary in vitro model of the black soldier fly (Hermetia illucens) larvae midgut

In vitro digestion models have been widely applied for farmed animals to predict feed quality and digestibility. Simulation of digestion could have similar applications for the emerging insect farming industry. Static in vitro digestion models are useful because of their simplicity, reproducibility, and low cost. They consider the number of digestive tract regions, pHs, enzymes, temperatures, and residence times. We propose a static in vitro model simulating the digestive tract of black soldier fly larvae (BSFL). As the digestive tract of fly larvae presents similarities to that of humans, the starting point for the model was the INFOGEST international standard human digestion model. Based on the existing knowledge on BSFL biology, the most important organ for digestion is the midgut, composed of three regions varying in pHs, enzyme activities, and feed residence times: anterior, middle, and posterior midgut. Previous research has already identified pH values and the activity of the enzymes that should be used for the in vitro model. As a missing input parameter for the in vitro model, we determined the residence times for three artificial feeds varying in protein (7, 12 %DM) and carbohydrates (8, 47 %DM) in the three midgut regions (anterior: 15 min, middle: 45 min, posterior: 90 min). Then, we digested four artificial diets and five biowastes in the in vitro model and measured feed digestibilities (i.e. pellet DM following centrifugation) and nutrient parameters (e.g. amino acids, glucose) in the supernatant. To partially validate the simulated midgut digestion, the diets were ranked based on the supernatant nutrient results and compared to results of in vivo feeding experiments. This approach was broadly able to distinguish worst (cow manure) and best performing rearing substrates (vegetable canteen waste). But, for several diets, the performance estimated based on in vitro results did not match with the results in vivo. Future studies should try to establish a stronger correlation between in vitro and in vivo results by considering the nutrient requirements of fly larvae, hemicellulose digestion, and gut microbiota.