Rapeseed Meal Pretreatment for Improved Biopolymer Production

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Abstract

There are strong incentives for a transition of the current fossil fuel-based economy towards a bio-based one, stemming from the unsustainable nature of fossil fuels, climate change and difficult disposal of non-biodegradable products. As a result, there is increasing interest in waste valorisation, especially of agricultural waste, as this could provide a low-cost feedstock with potential for upgrading to valuable products, and/or contains high-value naturally occurring bioactive compounds. Waste valorisation could also address the problems of purposefully grown biomass feedstocks and the associated negative socio-economic impacts, such as high food prices and deforestation. Additionally, integration of multiple product valorisation processes via a biorefining scheme could lead to more viable economics from product diversification, and may also provide additional economic and environmental benefits via process integration and waste reduction. Rapeseed meal (RSM) is a potential candidate for valorisation due to its high content of potentially valuable phenolics, proteins and polysaccharides. This thesis details the progress made towards RSM valorisation within an integrated biorefinery context, with particular focus on the more sustainable aspects of the scheme, specifically green solvent extraction of RSM phenolics, and the production of biodegradable polymers from RSM polysaccharides. This thesis is presented in a journal format. The experimental phase of the PhD is roughly separated into four stages as follows: Stage I: preliminary work; Stage II: assessment of phenolic extraction and viability of integrated processing; Stage III: optimisation of protein extraction and acid hydrolysis, and in depth investigation into the use of RSM-derived acid hydrolysate for PHA synthesis; Stage IV: Optimisation of enzymatic hydrolysis of untreated and pre-treated RSM for use in PHA synthesis. The work conducted in Stages II, III and IV have been peer-reviewed and published in the Journal of Cleaner production (Wongsirichot et al., 2019), Waste Management (Wongsirichot et al., 2020b) and ACS Sustainable Chemistry and Engineering (Wongsirichot et al., 2020a), respectively. RSM phenolics are of interest due to their antioxidant activities, which could lead to high-value applications such as health supplements in the food industry, or as preservative in the chemical industry. Additionally, the antimicrobial activity of RSM phenolics means that their removal could lead to improved fermentation of other residual fractions. This PhD investigated the extraction of RSM phenolics using novel green deep eutectic solvents (DES). Thermodynamic simulations using the Conductor like Screening Model for Real Solvents (COSMO-RS) shown that DES extractions had greater hydrogen-bonding interaction between the sinapic acid and the DES compared to methanol which has been traditionally used in RSM phenolic extraction. This results in greater yields with DES compared to the methanol. Although this was subjected to the influence of mass transfer, leading to lower yields in the highly viscous glucose-based DES. The highest sinapic acid yield at 91.5 % was achieved using aqueous choline chloride: glycerol (1:1), which outperformed aqueous methanol (68 % yield) at the same conditions. RSM proteins are of interest to the food industry, due to their abundance, and the fact that RSM amino acids are complementary to the amino acid profiles recommended for human consumption. Beyond this, the results found in this thesis demonstrate that the extraction of RSM proteins was crucial for expanding the use of the residual RSM polysaccharides as a fermentation feedstock; as high residual proteins in the polysaccharide fraction hinder its use in nitrogen-limited applications, particularly the synthesis of polyhydroxyalkanoates (PHA) biopolymers. During the PhD, optimisation studies of protein extraction, acid hydrolysis and enzymatic hydrolysis were conducted to determine the most suitable condition for protein extraction and fermentation media synthesis. Maximum protein yield from alkali extraction was obtained (68.8 %) at 60 °C and 100 minutes. The use of consecutive extraction allowed for the creation of post-extraction RSM with high carbon and low nitrogen content. It was also demonstrated that without an integrated scheme to first extract the protein, neither acid or enzymatic hydrolysis of the RSM could produce media with a sufficiently high glucose and low nitrogen concentration for significant PHA accumulation. Both acid and enzymatic hydrolysis of the post-protein extraction RSM were assessed. Results demonstrated the difficulty in producing the high C:N ratio required for PHA synthesis with traditional diluted acid hydrolysis, especially after scale-up. An alternative scheme which uses oxygen-limitation lead to increased PHA accumulation compared to the fermentation with no nutrient limitation. However, the final PHA accumulation in RSM media could still be further improved. Finally, using a combination of post protein-extraction RSM and optimised enzymatic hydrolysis, suitable concentration of glucose and nitrogen in the media was produced and successful PHA accumulation achieved, at both shake flask and bioreactor scales with PHA accumulations of 9.34 and 7.11 % cell dried weight, respectively. During the PhD project, more traditionally studied aspects of RSM valorisation were greatly improved upon through optimisation and scale up of the protein extraction, as well as the application of green solvents and scale up of phenolic extraction. The less explored RSM polysaccharides were successfully upgraded via fermentation to produce sustainable biopolymers. Most importantly, the integrated biorefining scheme combining the valorisation of all three fractions is demonstrated to not only be viable but essential for expanding the possible scope of holistic RSM valorisation. This thesis not only provides a solid foundation for future development and implementation of RSM-based biorefining, but the insights gained during the PhD could also be applied to the valorisation of other wastes containing multiple valorisable fractions.

Keyword(s)

Biopolymer; Biorefining; Deep eutectic solvent; Fermentation; Lignocellulose hydrolysis; Phenolic Extraction; Polyhydroxyalkanoates; Protein Extraction; Rapeseed meal; Waste Valorisation

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