Conventional and green extraction of polyphenols from brown seaweeds and their chemical characterisation

Abstract

Seaweeds have been reported as a rich source of nutrients and bioactive compounds including proteins, polyphenols, carbohydrates, lipids, alkaloids, and peptides. Amongst them, polyphenols have become materials of high interest from the past few years owing to their multifunctional potential as natural antioxidants, antimicrobials and other health benefits. However, the applications of seaweed polyphenols suffer due to some factors relating to their appropriate extraction and characterisation methods.

Therefore, for efficient extraction of polyphenols from seaweed, there is a great need to develop greener and sustainable methods. Additionally, the structural diversity of polyphenols makes it challenging to perform quantitative analysis thus making it difficult to quantify the effectiveness of extraction procedures. This thesis worked towards advancing the knowledge in greener extraction technologies using supercritical CO2 for brown seaweed polyphenols, improved quantitative 1H NMR spectroscopy and develop new qualitatively differential sensing assay methods used in their analysis. In this work, four brown seaweeds were studied- Ascophyllum nodosum, Fucus vesiculosus, Fucus serratus and Fucus spiralis. Initial work using conventional extraction methods (Soxhlet and maceration) were used as benchmarks for supercritical CO2. Conventional extracts were also used for method development of analytical methods, namely quantitative 1H NMR spectroscopy and qualitatively differential sensing techniques. Design of experiments (DOE) approach was taken to optimise the extraction parameters for supercritical CO2. The results for total phenolic content (TPC) values form supercritical CO2 were compared to the TPC from conventional maceration extracts to determine the efficiency of the supercritical extraction. The TPC content was characterised using 1H NMR spectroscopy. It was found that optimised supercritical extracts for A. nodosum and F. vesiculosus showed higher TPC values than conventional maceration extracts, thus ensuring better efficiency of the supercritical extracts. However, for F. serratus, the TPC was found lower in supercritical extracts than maceration extracts. Overall, it was concluded that each seaweed required different set of optimised parameters for supercritical CO2 extraction for better efficiency of the extraction. The crude extracts of all four seaweeds and extracts of A. nodosum seaweed in different solvent systems were also analysed qualitatively using differential sensing assay, which was the first time that this method was used for seaweeds. Previously, it was used for the characterisation of the tannins found in wine. Principal component analysis (PCA) and linear discriminant analysis (LDA) plots showed the clear distinction of phenolic profile for each seaweed extract. The results showed that this method was suitable for the identification of seaweed phenolics, thus offering a new qualitative approach in the field.

Thesis is embargoed until 31 July 2026.

Date of Award	Jul 2025
Original language	English
Awarding Institution	Queen's University Belfast H2020-Marie Skłodowska-Curie COFUND Programme
Supervisor	Pamela Walsh (Supervisor) & Gary Sheldrake (Supervisor)