Impact of Heat Treatment on the Microbiological Quality of Frass Originating from Black Soldier Fly Larvae (Hermetia illucens)

SIMPLE SUMMARY: Industrially produced insects can convert organic waste streams into proteins and other valuable products. The black soldier fly (Hermetia illucens) belongs to the most promising insect species. The residue, called frass, which remains after the production of the larvae, shows great potential as a soil improver and plant fertilizer, but it also contains a large amount of microorganisms, potentially including pathogenic species for humans. To minimize safety risks upon consumption of crops fertilized by this frass, a treatment that reduces pathogenic organisms, such as heating, is required. In this study, the impact of a heat treatment of 70 °C for 60 min (as prescribed by legislation) on frass of black soldier fly larvae was evaluated. The treatment resulted in a small reduction of the total microbial counts, and bacterial endospores were not at all reduced. However, when the foodborne pathogens Salmonella and Clostridium perfringens were added to the frass, the heat treatment led to undetectable amounts for both pathogens, as well as for Enterobacteriaceae. Consequently, a heat treatment of 70 °C for 60 min is likely appropriate to meet the microbiological criteria for the application of insect frass as biofertilizer or soil improver. ABSTRACT: Since black soldier fly larvae (BSFL, Hermetia illucens) are being produced at substantial volumes, concomitantly large amounts of the resulting by-product, called frass, are generated. This frass can potentially be applied as valuable plant fertilizer or soil improver. Since frass carries high microbial counts, potentially including foodborne pathogens, safety problems for consumers should be prevented. A heat treatment of 70 °C for 60 min is proposed to reduce harmful organisms in insect frass, based on EU regulations ((EU) No. 2021/1925). This study evaluated for the first time the impact of the proposed heat treatment on BSFL frass. This was done by applying the treatment on uninoculated frass as well as on frass inoculated with Salmonella or Clostridium perfringens at 5.0 log cfu/g. The heat treatment resulted in a reduction (maximum one log-cycle) of total viable counts and did not noticeably reduce bacterial endospores. In contrast, Enterobacteriaceae counts were reduced to below the detection limit (10 cfu/g). Heat treatment of inoculated frass resulted in absence of Salmonella in 25 g of frass and reduction of vegetative C. perfringens to below the detection limit (1 cfu/g). The proposed heat treatment appears to be appropriate to meet the microbiological regulations for insect frass.