Towards more efficient biowaste utilization with black soldier fly larvae to produce more sustainable animal feed

Abstract

Today's food production has enormous negative impacts on the environment and does not provide nutritious food in sufficient quantities for all people. Protein, largely provided through animal source foods, is a key nutrient associated with these shortcomings. Environmental impacts from animal source foods stem among others from the use of unsustainable feed ingredients (e.g. soybean and fish meal). At the same time, poor people often do not have access to sufficient quantities of affordable quality feed. This affects their livestock production, which is crucial for income, livelihood, nutrition, food security and resilience. In light of these shortcomings and the predicted increase in global food demand, the production of livestock feeds and animal source foods must be dramatically improved. Larvae of the black soldier fly (BSFL), Hermetia illucens L. (Diptera: Stratiomyidae), promise benefits for the affordability, availability, quality, and sustainability of monogastric livestock feeds (i.e. poultry, fish, pigs, pets). BSFL are the natural food of some livestock and can replace and possibly outperform current feed ingredients. Producing BSFL that are more sustainable than current feed ingredients requires use of biowastes for growth. Thereby, BSFL rearing also has the potential to improve often inadequate waste management. However, unreliable and low rearing performance prohibits the use of many biowastes for efficient BSFL rearing. This thesis focuses on the development of solutions to improve BSFL rearing on biowastes by providing nutrients in ideal amounts and addition of microbial inoculants to the rearing substrate. Formulation of biowaste mixtures based on protein and non-fibre carbohydrates is a promising approach for more efficient and predictable BSFL rearing using a range of biowastes. In comparison to individual wastes, biowaste formulations resulted in a higher rearing performance and lower variability. Biowaste formulation is especially beneficial since it integrates wastes with an individually low performance (e.g animal manures or slaughterhouse waste) into efficient BSFL substrates. In comparison to previous trial-and-error testing of biowaste mixtures, the proposed approach is systematic and considers the existing and growing knowledge on BSFL digestion. The open-access web application ensures formulation of biowaste mixtures when reliable data on the composition of biowaste is not available. The rearing variability that remains despite formulation could be reduced by keeping other nutrients within narrower limits and by providing more accurate estimates of biowaste digestibility and larval nutrient and energy requirements. Development of the first in vitro simulation of BSFL digestion delivers such information. In addition, digestion of substrates in the in vitro model can indicate in vivo outcomes. Thereby in vitro simulation can be used for screening of different biowastes based on rearing performance. In order to improve the model accuracy, it is essential to promote an understanding of the complex digestion processes. This thesis showed that BSFL adjust the digestive residence time based on the substrate nutrient content, suggesting post-ingestive mechanisms and compensatory feeding by BSFL. The addition of microbial inoculants did not result in performance improvements. Inoculants were obtained from the residues of a previous rearing cycle, which were typically rich in bacteria considered beneficial for substrate decomposition (i.e. Providencia, Dysgonomonas, Morganella, and Proteus). The absence of these bacteria from two agri-food waste residues highlighted that the complex microbial processes involved in BSFL rearing are still poorly understood and further research is needed to reliably improve rearing performance. This is despite the growing knowledge for BSFL and five other fly species, reviewed in this thesis. This thesis sets another cornerstone for efficient BSFL rearing on biowaste that vary in nutrients and microbiota. Major global improvements in the sustainability of animal source foods through the use BSFL-based feeds require an increase in the supply of BSFL substrates through improvements in waste management. In parallel, it is essential to develop solutions managing food safety risks associated with microbial and chemical contaminants in biowaste.