Bioconversion of waste lipids into long-chain dicarboxylic acids : process insights and optimisation

A shift towards renewable materials and a circular economy is slowly becoming a necessity for the industry. Supply chains must reduce reliance on fossil resources while simultaneously revalorising waste streams. Within this context, long-chain dicarboxylic acids (LCDAs) are important components used in a wide variety of applications, yet their production still relies almost exclusively on non-renewable materials. To meet demand for these LCDAs in a sustainable way, alternative routes using renewable, non-food materials are essential. This thesis evaluates a biotechnological alternative, specifically the production of LCDAs from used cooking oil (UCO) and grease trap waste (GTW). Specifically, the conversion of these waste streams using Candida tropicalis ATCC 20962 and Starmerella bombicola Δpox1Δugta1Δfaa1, two yeasts that are naturally capable of producing these LCDAs, is investigated. Before the bioconversion could start, the waste streams were first subjected to a detailed compositional analysis to determine their lipid class distribution, fatty acid profile and level of non-lipid contaminants. Based on this characterisation, the effect of the GTW and its components on the growth of C. tropicalis was investigated, followed by the development of a bioconversion process for the production of LCDAs. This production process was subsequently optimised, primarily by adjusting feed rates. To further improve the process and mitigate the inhibition of the contaminants, a membrane ultrafiltration pretreatment of the GTW was then performed. Finally, S. bombicola was used for the bioconversion of both UCO and GTW, enabling an assessment of its suitability as an alternative production host. This yeast was chosen due to its higher lipase activity, which is required for the triglyceride-rich UCO. Using this yeast, the effect of oxygen availability on the conversion process, as well as the effect of lipids on oxygen transfer, was determined. Compositional analysis revealed that GTW mainly contained free fatty acids, whereas UCO was predominantly composed of triglycerides. In addition, the GTW contained high levels of contaminants such as surfactants and heavy metals. Evaluation of the effect of the GTW on C. tropicalis revealed that it considerably reduced cell growth. Subsequently, this reduction in growth was found to be mostly attributable to the anionic surfactants in the GTW. Despite the inhibition, a bioconversion process was developed, and the feeding rates of both GTW and glucose were optimised. This revealed that the optimal glucose feed rate was 0.38 g/(L.h), with higher feed rates favouring biomass with little LCDA formation, while lower feed rates limited cell viability and production. GTW feed rate optimisation revealed that high feed rates suppressed LCDA production, likely due to the contaminants in the GTW, limiting the feed rate to 0.5 g/(L.h). Under these conditions, a titre of 34.7 g/L of LCDA was achieved, with a 76% yield and a productivity of 0.42 g/(L.h). Pretreating the GTW using ultrafiltration with a C1-grafted 3 nm ZrO2 membrane effectively removed the surfactants and heavy metals. During bioconversion, this pretreated GTW resulted in a 25% increase in LCDA production, with a titre of 43.5 g/L and a productivity of 0.46 g/(L.h). This clearly showed the great potential of GTW as a feedstock for LCDA production. It was found that when using S. bombicola, oxygen mainly affected lipase activity and had only minor effects on conversion rates. However, production with S. bombicola remained limited compared to C. tropicalis, with titres being only 5.13 g/L when using GTW and 1.36 g/L for UCO. This difference is likely due to the need for lipase when using UCO. Overall, the work demonstrates that lipid wastes, especially GTW, could be economically viable feedstocks for LCDA production. A suitable bioconversion process was developed and optimised, reaching economically relevant production rates.