Abstract
Each year, 95,000 tons (dry weight) of brown algae are harvested worldwide for alginate production. Alginates are highly valued in food, pharmaceutical, biomaterial and technical industries, due to their versatile properties such as their capacity to form thermostable hydrogels in the presence of divalent cations. Most alginates originate from brown algae (Phaeophyceae) where they serve as structure giving polysaccharides, and they consist of 1→4 linked β-D-mannuronic acid (M), and its C5-epimer α-L-guluronic acid (G). Several industrial applications rely on a high G-content in alginates, which confers good gelling properties. Because of its high natural G-content (FG = 0.6-0.75), the alginate from Laminaria hyperborea (LH) stipe has sustained a thriving 80-year-old industry in Norway. Alginates from other sources can be upgraded with mannuronate C-5 epimerases, that convert M to G, and this has been demonstrated in many studies. Nonetheless, these epimerases are not used in the seaweed alginate industry today. The present study demonstrates application of epimerases directly in the process of alginate extraction from cultivated Saccharina latissima (SL) and Alaria esculenta (AE), and the lamina of LH. Unlike conventional epimerisation, which involves extracting, precipitating, and purifying alginates before they are epimerized, this in-process protocol can decrease the time and costs necessary for alginate upgrading, making it more feasible in an industrial processing line. In-process epimerisation with AlgE1 enzyme enhanced G-content and hydrogel strength in all three examined species, with the greatest rise in SL (FG from 0.44 to 0.76, hydrogel Young's modulus from 22 to 34 kPa). As proof of concept, an upscaled in-process epimerisation of alginate from fresh harvested SL was successfully demonstrated, showing the potential of cultivated seaweed to serve as a source of high-G alginates.
