Ammonia pretreatment of agri-food wastes to enhance black soldier fly larvae bioprocessing performance

Abstract

Black soldier fly larvae (BSFL) bioconversion is a promising bioprocessing technology. However, one major challenge in BSFL waste processing, is that agri-food wastes (e.g. animal manure, agricultural crops and residues) tend to be low in macronutrients and high in lignocellulosic fibre (e.g. cellulose, hemicellulose and lignin), resulting in longer larvae development times and lower larval mass compared to when reared on high-value substrates (e.g. food waste). This study aimed to increase BSFL process performance on four fibre-rich substrates by ammonia substrate pretreatment: brewers spent grain, grass clippings, oat drink-by-product and cow manure. We first conducted experiments to identify optimal aqueous ammonia dose (1 vs 5%, 25% conc.) and pretreatment time (3 vs 7 days) by comparing its effect on the lignocellulosic substrate composition (neutral, acid and lignin detergent fibre) compared to an untreated control. Once we determined the optimal pretreatment condition, we completed controlled feeding experiments with four replicates, using ammonia pretreated and untreated substrates in a climate chamber (9 days, 2.5 larvae/cm2, 28 °C and 44-70% relative humidity). Results demonstrated that for most substrates a dose of 5% achieved improved lignocellulosic degradation, decreasing neutral detergent fibre by 6 to 22%, when compared to the untreated control. A pretreatment time of 3 days performed best for all substrates, except grass clippings, which demonstrated better results at 7 days. Based on our results, aqueous ammonia can result in fibre degradation of BSFL substrates and influence larvae performance. Although not fully evident at this stage of the experiments, ammonia pretreatment of such fibrous wastes shows mixed results of larvae performance depending on the substrates in question. This research works to increase performance of abundant fibrous low-value waste streams for recycling within the food system.