Microbiological safety throughout the production chain of the black soldier fly (Hermetia illucens)

The expanding global population and associated rising demand for animal proteins require additional production of sustainable protein sources. Mass production of insects as food and feed could be part of the solution, with the black soldier fly (Hermetia illucens, BSF) considered as most promising insect species for feed production. BSF larvae efficiently convert organic waste streams into their own high-value biomass, contributing to the circular bioeconomy the EU is striving for. However, despite the great potential, important knowledge gaps remain regarding the microbiological safety of reared BSF larvae. To fill these gaps, this PhD dissertation focused on different aspects throughout the entire production chain of BSF larvae; both microbiological aspects of the substrate, BSF rearing cycle and frass were studied.

First, the microbiological quality and safety of diverse organic waste streams were assessed through microbiological screening. Presence of (potential) food pathogens, such as Staphylococcus aureus, supported the need for microbiological analysis of waste streams before administration as substrate for rearing BSF larvae. Moreover, thermal pretreatments of supermarket food waste (SFW) without meat and fish, inoculated with Salmonella and S. aureus, showed that a heat treatment of 60 °C for 10 min was sufficient to ensure a microbiologically safe substrate for BSF larvae. Nevertheless, S. aureus did persist in SFW with meat and fish after the same heat treatment.

Next, fluctuations in the microbial counts and bacterial community composition underscored the dynamic nature of the microbiota across consecutive BSF life stages (larvae, prepupae, pupae and adults) and the associated substrate and frass throughout one rearing cycle. Notable microbial changes occurred during metamorphosis to the adult stage. Furthermore, a case study on the vertical transmission of E. coli across the different BSF life stages revealed that E. coli was detected in the frass, larvae and prepupae, but transmission to the adult stage did not occur.

Following the substrates and rearing cycle, the microbiological safety of BSF frass, a by-product of the rearing process, was assessed. Validation of a recently adopted EU regulation affirmed the alignment of a heat treatment of 70 °C for 60 min on BSF frass inoculated with Salmonella and Clostridium perfringens, with the imposed microbiological criteria for the use as plant fertiliser or soil amendment.

Finally, a case study in an industrial rearing facility examined the occurrence of the spore-forming foodborne pathogen C. perfringens during the rearing and processing of BSF larvae. Following the potential occurrence of C. perfringens in the production process of BSF larvae, a practical experimental design for challenge experiments was created based on preliminary experiments. This allows to investigate the dynamics of C. perfringens during BSF larvae rearing.

To conclude, this PhD dissertation demonstrates the ubiquity of microbiological risks during the rearing process of BSF larvae. When using organic waste streams as substrate for rearing BSF larvae and using BSF frass as plant fertiliser or soil amendment, microbiological screening is recommended to detect foodborne pathogens. Thermal pretreatment of both substrate and frass can be a viable solution to ensure microbiological safety, but potential presence of endospores remains a critical point of attention.