To treat or not to treat? Impact of hygienization on the microbiological safety of frass from black soldier fly larvae and yellow mealworm production aimed for fertilizer use.

Sustainable expansion of the insect farming industry requires valorization of the generated frass, a by-product with fertilizer and soil improver potential. Commercialization is currently restricted by Regulation (EU) No 2021/1925, considering insect frass as animal manure and imposing a pathogen-reducing heat treatment. This study investigated the microbiological safety of frass from industrial black soldier fly larvae (BSFL) and yellow mealworm (YM) production, untreated and hygienized by drying, pelleting or composting. Results showed that despite high microbial counts for both viable cells and aerobic endospores, Salmonella spp. were absent in untreated frass, Escherichia coli counts were low compared to other manure types, while the number of Enterococci was similar. The hygienisation procedures induced little change in the total number of viable cells and aerobic endospores and exerted more impact on temperature-sensitive Enterobacteriaceae compared to more tolerant Enterococcaceae. Drying, pelleting and composting were not equally suited for hygienization of frass from both insect species due to differences in initial microbial load and physical composition. Generally, it could be concluded that YM frass will meet the EU microbiological criteria more easily than BSFL frass and may even do so without hygienization. In case hygienization is necessary, reduction of the number of E. coli by the applied treatment will be the factor determining regulation compliance. Compared to Enterococcaceae, E. coli abundance in frass is lower and the species is more efficiently reduced. Oven drying is suited for both BSFL and YM frass but is expensive as it requires an external heat source. Pelleting seems applicable to YM frass, BSFL frass requires preliminary heating to reduce its moisture content. Composting of BSFL frass may produce a safe product if temperatures reached throughout the process are high enough. Composted YM frass can be considered microbiologically more unstable/unsafe than untreated YM frass, due to water addition at the process start. It may be concluded that flexibility in treatment procedures for insect frass, aimed for use as fertilizer, can be allowed. Microbiological safety is ensured after reference treatment (70 °C–60 min) but can also be achieved after milder processing or even without treatment, which would reduce costs and preserve frass quality.