Effect of growth substrates and processing methods on the safety of black soldier fly larvae (Hermetia illuscens) and blue calliphora fly (Calliphora vomitoria) meal as livestock feed

Abstract

Safety concerns over potential spillage of microbial and chemical contaminants from organic wastes (OW) through insect meal into animal feeds and animal products could be a threat to animals and humans as primary and secondary consumers, respectively. This study evaluated the effect of type of OW substrates (bovine dung, pineapple residues, poultry manure, swine dung, brewer’s waste, chicken offal and chicken blood) on microbial and chemical contamination of black soldier fly (Hermetia illuscens) larvae and blue calliphora fly (Calliphora vomitoria). The effect of processing methods that included varying concentration of calcium hypochlorite, level of water washing and duration of steaming on microbial and chemical contamination of both H. illuscens larvae and the blue calliphora fly were also evaluated. Organochlorine pesticide residues (Lindane, DDE, Diedrin and Endosulfan sulphate) and Aflatoxins (B1, B2, G1, G2 and total aflatoxins) were within safe levels in both black soldier fly larvae (BSFL) and blue calliphora fly (BCF). Heavy metals including mercury and arsenic in both BSFL and BCF were below the 0.05 mg/kg limit of quantification. However, Lead and Cadmium in both BSFL and BCF varied (p<0.05) with the type of growth substrates used. Salmonella was neither detected in BSFL nor in BCF. While the type of growth substrates had no effect (P>0.05) on the levels of contamination (0 to 5.8 log cfu/g) of BSFL with Pseudomonas aeruginosa, no levels of contamination (< 1 log cfu/g) were detected in the BCF irrespective of the growth substrates type. The Staphylococcus aureus and Escherichia coli in BSFL varied (p<0.05) with type of growth substrates used. Enterobacteriaceae, E. coli and S. aureus in BCF were not affected (p>0.05) by type of growth substrate used. Total viable count, enterobacteriaceae and yeast and mould counts in BSFL meal decreased at a decreasing rate (P<0.05) with increasing calcium hypochlorite concentration. In the black soldier fly, the Total viable count, enterobacteriaceae and yeast and mould counts decreased at a decreasing rate (P<0.05) with increasing duration of steaming while in the blue calliphora fly only the enterobacteriaceae followed similar trends. The microbial contamination in both black soldier fly larvae and BCF decreased at decreasing rate (P<0.05) with the increasing level of water washing. However, combining water washing and steaming duration as a processing method was more effective for lowering the microbial contamination than calcium hypochlorite, water washing or steaming duration when applied singly. One level of water washing combined with 2.5 minutes of steaming was effective (<0.0001) in lowering microbial contamination in both black soldier fly larvae and blue calliphora fly meal to safe levels. In general, organic growth substrates can be used to produce safe insect feed with respect to heavy metal, organochlorine pesticide residues and aflatoxins. However, decontamination of the black soldier larvae and blue calliphora fly using a combination of water washing and steaming duration produced insect meal with microbiologically safe levels. Therefore, both substrate type and processing method influence the safety of insect and insect meal for animal feed.