Production and carbon footprint of microbial oil from waste lemon peel extract: [version 2; peer review: 2 approved, 1 approved with reservations]

Background: The agricultural sector is one of the leading producers of agro-industrial solid organic waste. This waste, mainly disposed of by incineration or landfilled, could be used for the production of high-value chemicals. In this study, a fermentation process for the production of microbial oil from waste lemon extract (LE), an aqueous side-stream deriving from waste lemon peel and pulp processing, was developed and assessed for its impact. Microbial oil can have many diverse applications, from plasticizers in plastic and rubber compounds to moisturizers in cosmetic formulations. Methods and results: Characterization of LE revealed that its autoclaving process is effective for increasing the concentration of readily available glucose and fructose, reaching 28.77 ± 0.08 g L ^-1 and 25.68 ± 0.27 g L ^-1. Nitrogen content was measured too, revealing a C/N ratio of 85, optimal for triggering lipid accumulation in the selected microbial cell factory. Therefore, the oleaginous yeast Cutaneotrichosporon oleaginosum was cultivated in an unmodified LE-based medium in 2 L bioreactors, resulting in a lipid accumulation of 0.47 ± 0.08 g _oil g _CDW^-1. Finally, a new lipid extraction method using green solvents was developed, which allowed to extract and purify 11.79 g of oil, corresponding to 35% of the cell dry weight. The carbon footprint of this laboratory-scale production was estimated to be 71 – 434 kgCO ₂eq kg ^-1 of microbial oil, with electricity consumption of the fermentation step as the main factor. Sensitivity analysis suggests that the overall impact can be reduced with process optimization and scale-up. Conclusions: The proposed process is promising in terms of production and does not compete with edible resources and land use. However, the microbial oil yield and the downstream must be optimized to make the process sustainable.