Natural Deep Eutectic Solvents as Promising Green Media for Cascaded Conversion of Beer Brewing Side Streams into Value-Added Products

Abstract

Supported by new legislature (EU SDGS 2030), considerable research efforts are currently focused on more efficient utilisation of biomass waste streams. This thesis presents a cascade valorisation approach for beer brewing biomass side-streams - namely Brewers’ Spent Grain (BSG) and Malt Dust (MD) - through a cascaded approach of extraction, pyrolysis, and activation, forming a semi-closed-loop biorefinery process. The experimental work comprises four main stages: selection and characterization of natural deep eutectic solvents (NADES), green solvent-based extraction of bioactive phenolic compounds, thermochemical valorisation of spent biomass via pyrolysis and CO2 activation, and the reuse of the resulting ACs for extract purification and separation from NADES. A range of NADES were assessed for use as extraction media, using environmentally benign and cost-effective components such as choline chloride, malic acid, and glycerol. Their performance as extraction solvents was evaluated through viscosity profiling. To obtain a deeper understanding of the function of NADES and their interaction with target solutes, solvent-solute interactions were elucidated via 1D and 2D NMR spectroscopy. It was found that the mechanism of solvation depends heavily on the type and chemical structure of NADES components: solute molecules containing sterically unhindered hydroxyl groups are able to engage more effectively in hydrogen bonding with the NADES, enhancing solvation and extraction efficiency. A critical insight from the study revealed a trade-off between viscosity reduction through water addition and the diminished solvent strength due to competitive H-bonding between water and NADES constituents. Secondly, a hydrothermal extraction method at 120°C was employed to isolate phenolic compounds from BSG and MD using two NADES formulations: GLY (choline chloride:glycerol, 1:2) and MA (choline chloride:malic acid, 1:1). These were benchmarked against conventional acetone-based maceration and microwave-assisted extraction. The NADES-based hydrothermal method demonstrated superior or comparable extraction yields, particularly for BSG, and provided a more sustainable and efficient route in line with green chemistry principles. BSG emerged as a more promising feedstock than MD, due to its higher lignin content, lower ash fraction, and better downstream suitability for pyrolysis and carbonization. To assess the feasibility of further valorisation of the “spent” biomass after extraction, pyrolysis followed by CO2-assisted physical activation was performed. Key parameters such as pyrolysis yield, specific surface area (as), pore size distribution (PSD) and inorganic content were quantified for both untreated and extracted biomass. Extracted biomass pre-treated with GLY retained high conversion efficiency and produced AC with comparable or superior surface area relative to untreated controls. In contrast, acidic solvents such as MA and MA:H2O caused a drastic decrease in solid residue, leading to lower carbon yields and suboptimal AC properties due to excessive mineral retention and mesoporosity. These observations suggest that NADES extraction impacts the thermal decomposition behaviour and structural integrity of biomass during pyrolysis, underscoring the importance of choosing the right solvent system not just for extraction, but for the entire cascade process. To complete the circular strategy, the potential of the produced ACs as biosorbents for extract purification was evaluated. Adsorption studies using model phenolic compounds in NADES matrices demonstrated that GLY-derived ACs had high affinity and favourable adsorption kinetics, often rivaling or surpassing commercial ACs. Importantly, desorption tests with ethanol and other mild solvents showed that these ACs could effectively release the adsorbed phenolics, indicating their suitability in semi-continuous extract purification processes such as solid-phase extraction (SPE). This "catch-and-release" behaviour holds promise for solvent regeneration strategies that limit solvent use and waste generation. This series of experimental investigations establishes a proof-of-concept for a cascaded biomass valorisation pathway using beer brewing waste. By integrating NADES-assisted extraction with thermochemical valorisation and extract purification using in-house pyrolyzed and activated carbons, the work advances the state-of-the-art in green biorefinery development. The findings also highlight the complexity and interdependence of process steps, and the need for tailored solvent and biomass selection to maximize performance across the cascade. Future directions should include life cycle and techno-economic assessments to assess the industrial viability of the proposed process.