Abstract
Insect farming generates substantial amounts of frass, a nutrient-rich by-product with valorization potential as fertilizer/soil improver. Marketing is restricted by EU regulations, imposing a reference heat treatment for 1 h at 70°C to reduce the presence of potential pathogens. This study evaluated the impact of heat treatments (50 → 80°C for 15 → 90 min) on microbiological quality and safety of black soldier fly larvae (BSFL) and yellow mealworm (YM) frass, as well as microbial dynamics during storage, before and after reference treatment. Fresh frass showed high microbial counts, but Salmonella was absent. Untreated BSFL frass did not meet the EU criteria to allow use as fertilizer, while some YM frass samples did. Reference heat-treated BSFL and YM frass complied with the EU criteria. Escherichia coli counts were below the 1000 cfu/g limit, and Salmonella (even when inoculated before treatment) was absent. Only in BSFL frass, Enterococcaceae counts were sufficiently reduced. Milder treatments (temperatures < 70°C and/or times < 1 h) of BSFL frass induced similar reductions of E. coli and (inoculated) Salmonella but Enterococcaceae counts remained above 1000 cfu/g. In some YM frass samples (inoculated) Salmonella was still detected after milder treatment. Refrigerated (4°C) or ambient temperature (26°C-28°C) storage up to 2 weeks of fresh and heat-treated frass did not increase bacterial counts. To ensure frass microbiological safety whilst preserving quality and reducing costs, tailored treatments seem appropriate. This may be no, milder, or more severe treatment, depending on the microbial load (counts and species type).