Sugars influence substrate temperature and larval growth in black soldier fly bioconversion

Abstract

In black soldier fly larvae bioconversion, organic waste is upcycled into animal feed and fertilizer. Substrate temperature is a key factor for climate control and larval growth, and excessive heat can interrupt operation, yet its drivers are understudied. Digestible carbohydrates, a variable and abundant component of organic waste, could be a critical driver of temperature variation. Therefore, in this study, we investigated whether different digestible carbohydrates in organic waste affect temperature and larval growth. The okara-based substrate was supplemented with carbohydrates (20% based on dry matter). In a 7-day controlled feeding trial with industrial crates (60 cm × 40 cm crates, 8640 larvae each), we determined whether the sugar type (isomaltulose versus sucrose, i.e. high versus low digestion rate) and availability (day 0, day 3) affected larval dry mass, substrate temperature, metabolites, and pH as well as CO₂, and NH₃ emissions. We found that sugars strongly altered substrate temperature in a time-dependent manner. Sucrose addition on day 0 produced higher temperatures (mean increase: 2.1 °C, max.: 7.9 °C) and resulted in a lower final larval mass (∼ 20%) compared to sucrose addition on day 3. Isomaltulose addition resulted in a lower mean (0.4 °C versus sucrose) but highest peak (51.3 °C) temperature. pH and NH₃ concentration dropped after sucrose addition. Trends in metabolites suggested O₂-dependent metabolic shifts during early bioconversion. These results showed that the timing of sugar supply can shape thermal dynamics and larval yield, with implications for optimizing temperature control and efficiency in black soldier fly larvae bioconversion.