Heffernan_2015_extraction.pdf (2.81 MB)
Extraction, characterization and seasonal variation of bioactive compounds (polyphenols, carotenoids and polysaccharides) from Irish origin macroalgae with potential for inclusion in functional food products.
thesisposted on 2022-09-19, 14:40 authored by Natalie Heffernan
Seaweeds contain polyphenols, carotenoids and polysaccharides with potential for use in functional foods. This study investigated these compounds using conventional and novel recovery techniques from seaweeds (Fucus serratus, Laminaria digitata, Gracilaria gracilis, Codium fragile, Fucus vesiculosus, Himanthalia elongata and Cystoseira nodicaulis) harvested from the Irish coast. The effect of seasonality on compound concentration, structure and bioactivity was assessed. Solid liquid extraction (SLE) and pressurised liquid extraction (PLE) were investigated for the optimal recovery of antioxidant compounds. Methods were evaluated based on extraction yield and antioxidant properties. SLE extracts possessed higher antioxidant activity compared to PLE extracts. Enriched fractions from SLE extracts were generated using membrane dialysis. An increase in activity compared to the crude extracts was achieved. Initial weak activity in the < 3.5 kDa fraction of F. serratus was further enhanced using reversed-phase flash chromatography separation. Quadrupole time-of-flight mass spectrometry (Q-ToF-MS) confirmed the presence of a high abundance of low molecular phlorotannins. Phlorotannins characterisation with ultra-performance liquid chromatography tandem mass spectrometry from brown macroalgae demonstrated isomeric complexity. The level of isomerisation differed substantially within the species studied. The seasonal variation on macro-composition of four seaweed over four seasons was investigated. Protein content was highest (28.70 %) in G. gracilis winter samples. Lipid content was low and ash content high in all species. Carbohydrate levels were highest in the brown seaweed (75-80 %). Insoluble dietary fibre ranged between 30-60% and was highest in spring. Soluble dietary fibre was highest in the spring/summer periods. Levels of phenolics in F. serratus were highest during the summer period and lowest in the autumn. This study demonstrated that seaweed composition is affected by seasonality and environmental factors. Brown seaweeds contain carotenoids (fucoxanthin and xanthophyll) known to possess antioxidant activity. The efficiency of three extraction techniques (conventional SLE, supercritical CO2 (SFE) extraction and supercritical CO2 with co-solvent) and seasonal variation was examined for these carotenoids. SLE gave a higher yield while SFE gave a higher purity of extract. The winter/spring period gave the highest purity of carotenoids from L. digitata, however, the summer was the peak period for F. serratus. The seasonal variation on structure and bioactivity of phlorotannins was also investigated in the brown algae F. serratus. Low molecular weight phlorotannins predominantly had a degree of polymerisation (DP) of phloroglucinol between 6-14 across all seasons/harvest sites. Winter samples exhibited the highest level of isomerisation (427 isomers) while summer samples had the lowest level of isomerisation (121 isomers). High levels of isomerisation were associated with higher bioactivity in the in-vitro tests. The seasonal variation laminarin from Laminaria digitata was investigated in relation to molecular weight, content and chain length. ESI-Q-TOF-MS analysis showed that summer was the optimum for harvest time for extraction of higher content of laminarin, no laminarin was detected in the winter. Variation in the molecular weight based in the number if glucose monomers was observed. This research highlighted the optimum extraction technologies for extraction of bioactive compounds (polyphenols, carotenoids and polysaccharides) from Irish macroalgae with potential for inclusion in functional foods. It has also demonstrated the seasonal variation of these compounds and its effect on structure and bioactivity.