Macroalgae represent a diverse and abundant resource, containing an array of unique chemicals with useful properties. These chemicals include: alginate, a long chain carbohydrate with gelling properties; laminarin, a carbohydrate consisting of glucose residues, which is readily fermented; mannitol a sugar alcohol that can be used as an artificial sweetener and fucoidan, a sulphated polysaccharide famed for its biomedical properties. Their current use in industry is minor, with the main focus being as a food source and for alginate extraction. However, there is great potential for this feedstock in chemical and fuel production, especially for biorefinery development, which makes use of the whole resource by providing multiple products from one feedstock. Brown macroalgae offer the most promising option in Northern Europe, being the largest and most fast growing of the seaweed species, as well as being plentiful around the coast of the UK with the potential for cultivation alongside harvesting from wild stock.
A potential barrier to the use of seaweed in industry is their seasonal variation in chemical content. In order to fully understand this, a study detailing the variations in carbohydrates, protein and ash, as well as a detailed study into the variation in composition and structure of fucoidan, identified as the most valuable of the potential extraction products due to its interest in the pharmaceuticals market, have been conducted. Three species of brown macroalgae, Fucus serratus (FS), Fucus vesiculosus (FV) and Ascophyllum nodosum (AN), have been analysed over a 12 month period. The results indicate that mannitol, laminarin and fucoidan are all highest at the end of the growing season in late summer and that ash, alginate and protein are highest during the winter months. The composition and structure of fucoidan is also seen to vary over the year, with FS having the highest sulphate content and results indicating a consistently more branched structure than was seen for FV and AN.
In order to make the best use of the macroalgal feedstock, a three step hydrothermal microwave assisted biorefinery is proposed, with utilisation of the waste as a feedstock for fuel production or as a fertiliser being considered. For this, a sample of FS, identified in the seasonal variation study to have the best potential for chemical extraction, has been used. A low temperature step at 50°C in water firstly removes mannitol and a portion of the salts, followed by processing at 120°C in water to extract fucoidan and alginate. Alginate is precipitated from the extract with calcium carbonate and fucoidan with ethanol. The final step is processed at 120°C with sodium chloride to extract the remaining alginate from the residue. A mass balance of the proposed biorefinery shows that 90% of mannitol, 79% of fucoidan and 79% of alginate have been extracted during processing. A study into the quality of the fucoidan extracted by microwave heating is comparable to that extracted from the raw biomass by conventional means. A comparison of microwave and conventional heating shows the benefits in using microwaves, with decreased extraction temperature and a full energy balance of the system significant energy reductions associated with microwave heating on a laboratory scale.
