Cuthill, Holly
(2020)
Emulsifying food microparticles prepared from cocoa by-product.
PhD thesis, University of Nottingham.
Abstract
This PhD research concerned the preparation of emulsifying food microparticles from a plant-based by-product of the food manufacturing industry, thereby increasing the efficient use of natural resources. In the early stages of this research, and at the time relevant to the product spectrum of the sponsoring industry partner, spent coffee grounds and cocoa shell were evaluated as feedstock. Coconut shell was considered for the higher natural lignin content. Cocoa shell was taken forward from results showing the largest droplet density on the surface of the biomass following hydrothermal pre treatment. The droplets were hypothesised to be lignin rich and therefore this densely covered surface was preferred prior to extraction. The process involved preparing a lignin-rich extract from the feedstock followed by anti solvent precipitation into spherical microparticles sized between 0.05 and 0.8 µm. As lignin is a largely hydrophobic polymer, these microparticles were hypothesised to stabilise emulsions through the Pickering mechanisms. It was further hypothesised that the conditions of the extraction protocol, and the microparticulation process would allow control over particle surface hydrophobicity, thus whether water-in-oil or oil-in-water emulsions would be preferentially stabilised, and particle size, impacting on emulsion droplet size. The rule of thumb is that Pickering stabilisation require particles an order of magnitude smaller in diameter than the desired emulsion droplet size spectrum.
Lignin-rich extracts were prepared by applying a hydrothermal pre treatment followed by ethanol extraction to the milled feedstock. Taking cocoa shell forward as the only feedstock, acid-assisted ethanol (AEE) or acid-assisted dioxane extraction (ADE) protocols were also utilised. Thereby, the energy intensive pre-treatment was removed. The hydrothermal ethanol extract (HEE), of the cocoa shell, contained around 50 % of lignin with the majority of the rest being lipid. A lipid removed HEE (LR HEE), containing 94 % lignin, was prepared through extracting the majority of the lipids with hexane for the purpose of evaluating the impact of the natural extracted lipids on the extract properties as well as the properties and performance of the precipitated microparticles. The chemical composition of the extracts was established using FT IR and NMR. The spectra revealed that the lipid removed from HEE included surface active phospholipids and ADE contained hemicellulose. AEE and HEE were compositionally similar and therefore only HEE was taken forward. Based on these results HEE, LR HEE and ADE were used to produce microparticles of varying composition to compare functionality. This work has shown that the method of lignin extraction, more specifically the solvent, impacts its functional properties with contact angle measurements revealing the acid-assisted dioxane extract (ADE) was slightly more hydrophobic than HEE, although both are considered intermediately hydrophobic. Microparticle production involved re dissolving HEE and LR HEE into ethanol and ADE into dioxane before solvent exchanging into water/oil to produce microparticles. As predicted, it was found through contact angle assessments that the surfaces of the extracts (HEE and ADE) were altered to either have increased or decreased wettability, following immersion in water or oil respectively, a phenomenon likely to occur in microparticle production.
Microparticles were successfully precipitated into water and were typically spherical and highly charged ( 40 mV) at neutral and high pH. Water based microparticles remained in the same size ranges (0.03 0.4 µm) over the 100 day storage period. However, larger microparticles were less stable. Larger microparticles were produced when the initial concentration of extract in solvent was highest or, to a greater extent, when the precipitation was carried out in an environment without agitation, which also resulted in irregular sized microparticles 0.1-0.8 µm. Microparticles were precipitated into sunflower oil and sedimentation was observed following 7 days of storage. When the microparticles were precipitated into a less polar solvent, dodecane, they sedimented rapidly. The differences were thought to be from the intermediately hydrophobic lignin rich microparticles preferring a more polar solvent, along with the higher viscosity environment of sunflower oil aiding microparticle suspension.
The aqueous microparticle suspensions were surface and interfacially active and utilising the microparticles as particulate emulsifiers was successful with limited coalescence observed over the 100 day storage period. The sunflower oil based microparticle suspensions were interfacially active and the produced w/o emulsions were stable for a limited time. The more hydrophobic surface of the ADE microparticles, revealed to produce somewhat more stable w/o emulsions compared to those from the HEE microparticles, as expected.
The major finding throughout the research was that lipid aided microparticle production in both water and oil continuous environments and helped to prevent microparticle aggregation. The LR HEE microparticle suspensions were less surface active than the HEE microparticle suspensions and this is thought to be from the removal of phospholipids and the produced emulsions were less stable to coalescence. Additionally, water and oil-soluble components taken from the extracts were found to be interfacially active, thought to be from free fatty acids, as the soluble components from LR HEE were typically less interfacially active.
This research has demonstrated the successful production of lignin rich microparticles and their ability to stabilise food emulsions along with the benefit of surface active lipid remaining in the system. Future research is required to understand the microparticle formation in the two processes applied including how the surface active material drives particle functionality.
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