Antibacterial properties of oil extracts of black soldier fly larvae reared on bread waste

Context Farming black soldier fly larvae (BSFL) has become an emerging agricultural sector for upcycling food waste into high-quality protein and oil biomass. Depending on the chemical composition of the food waste, the oil extracted from BSFL can reach high levels of lauric acid, a natural antimicrobial compound. Aims We aimed to evaluate whether feeding a bakery waste-based substrate can enhance the lauric acid synthesis of BSFL. In addition, we aimed to test the antibacterial activity of these BSFL oil extracts against E. coli in vitro. Methods In a 12-day-long feeding trial, we reared BSFL either on a ground corn-based control substrate (n = 6) or a substrate based on bread waste sourced from a local bakery in Hong Kong (n = 6). We studied the growth performance, crude fat and lauric acid content, and agar well-diffusion assay-based antibacterial activity of oil samples extracted from the larvae. Key results The BSFL reared on the bread waste had a higher average weight (P < 0.001) and average daily gain (P < 0.001), similar crude fat content (P = 0.17), and higher lauric acid content (P < 0.001) than did the control larvae. Both oil samples moderately inhibited the growth of E. coli in vitro; the inhibition zones were 1.92 ± 1.0 mm and 1.25 ± 0.75 mm (means ± s.d.) big in the bread waste-based and the control oil samples respectively (P = 0.28). Conclusions Our results indicated that locally sourced bread waste is a suitable substrate for farming BSFL, providing a sustainable alternative for waste management. Implications The inclusion of locally produced BSFL oil as a feed ingredient for the local livestock farming sector might be an avenue not only to achieve circular economy, but also to reduce antimicrobial drug use. However, the effects on animal health and productive performance warrant further in vivo experiments.