A comprehensive analysis of coffee silverskin bioconversion by Hermetia illucens larvae

Abstract Coffee silverskin, the outer layer of the green coffee bean, represents a major by-product of the coffee industry derived from the roasting process. In recent years the development of sustainable and circular strategies to manage and valorise organic wastes and by-products has become increasingly relevant and the potential of coffee silverskin in food industry, cosmetics, and bioconversion applications is gaining attention. In the present work we addressed the valorisation of coffee silverskin through insect bioconversion using the larvae of the black soldier fly Hermetia illucens, one of the most promising bioconversion agents among insects. These larvae grow on a huge variety of organic substrates due to their outstanding adaptability, that is conferred by the plasticity of the midgut physiology and associated microbiota. Our results demonstrate that black soldier fly larvae were able to grow and develop on coffee silverskin. The larvae reduced this by-product by 25% and a high protein insect biomass (i.e. 56 g per 100 g of dry matter) was obtained. Interestingly, 25% of the hemicellulose fraction of coffee silverskin was degraded by the larvae. In addition, the larval gut microbiota, which plays a key role in larvae digestion adaptability and bioconversion, was shaped by growing the larvae on coffee silverskin and bacterial taxa involved in complex polysaccharide degradation were selected. In conclusion, black soldier fly larvae may represent (1) a valuable tool for the development of new and sustainable strategies for coffee silverskin bioconversion and valorisation, and (2) an effective bioincubator for the selection and isolation of microbial strains with peculiar degrading capacity.