Sustainable Synthesis of Wax Esters from Plant Waste by Bacteria

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The aim of this project is to produce and extract wax esters from waste plant material, to replace petroleum based wax products and as sustainable alternatives to plastic in packaging applications. Waxes are esters of fatty acids and alcohols that can be derived from a variety of sources. In 2018 the global wax market was valued at over $10 billion,1 with growth driven by the desirable properties of waxes, including water and chemical resistance and non-toxicity. At this time, mineral (petroleum) waxes accounted for 75% of the market, with a further 20% from synthetic waxes (also fossil fuel derived) and around 5% naturally sourced from animals and plants.1 However, as society moves away from its dependence on fossil fuels, alternative carbon sources for chemical feedstocks must be considered. Current major uses of waxes are cosmetics, personal care products, lubricants and coatings, but increasing demand for alternatives to plastics has led to interest in wax for food packaging and storage. Plant sources offer an attractive alternative, as photosynthesis during plant growth takes in CO2 from the atmosphere, trapping atmospheric carbon in chemical feedstocks. However, the combination of large land requirements and high costs due to outdated extraction procedures and slow, difficult cultivations prohibit mass wax production directly from plants. Biocatalytic, microbial wax generation is a cheaper, more sustainable alternative, with using food and agricultural waste streams as feedstocks. The bacteria Acinetobacter baylyi is a known natural producer of wax esters, and hence has formed the starting point for previous investigations into microbe derived waxes. Synthetic biology also allows the engineering of bacteria to produce wax esters of varying saturation degree and length, demonstrating the potential for modifying biosynthetic pathways to ensure the controlled generation of the desired bioproduct. Previous work by this group has focussed on the generation of volatile fatty acids (VFAs) from the anaerobic digestion of waste plant material, a process which otherwise produces methane gas. These VFAs can be transformed into wax by Acinetobacter baylyi, a novel process not attempted previously. Physical factors of bacterial growth, including temperature, pH and concentration of substrate, will be varied and the properties of the waxes produced assessed. Process kinetics and time dependency of the wax yields will also be measured. Yields will be quantified, and the basic physical properties of the waxes, such as melting point and viscosity, analysed. Common chemical properties of waxes, such as their acid, iodine and saponification values, will be determined through known literature methods to assess the suitability of waxes generated for a variety of purposes. Novel extraction methods for the waxes produced will be explored to achieve maximum yields with minimal energy and solvent use. The effect of feedstock on wax properties and yield will also be investigated, using a range of different waste plant material. Appropriate substrates will be chosen based upon both their wax production capabilities and the economic sustainability of their source. This work is aligned with the EPSRC Synthetic Biology research area and falls within the Manufacturing the Future theme.