Seaweed resources – Characterisation, preservation and processing for sustainable utilisation

Abstract

What does our future seaweed nation look like? Norway is one of the largest harvesters of wild seaweed in the world, mainly for the production of alginate from Laminaria hyperborea and seaweed meal from Ascophyllum nodosum. However, 97 % of the global seaweed production comes from cultivation, and 97 % of all production takes place in Asia. Over the past decade, a new seaweed cultivation sector has emerged in Norway and Europe, primarily focusing on the two brown algal species Saccharina latissima and Alaria esculenta. We believe that the future seaweed production in Norway is based on both wild harvest and cultivation. However, there are several challenges that needs to be addressed before further growth in the seaweed sector is possible, such as developing new products, more effective and sustainable processing technologies, and more fundamental knowledge on cultivable seaweed species. This thesis can be seen as a contribution to a growing environmentally and economically sustainable seaweed industry in Norway, based on wild harvested L. hyperborea and cultivated S. latissima and A. esculenta. Cultivated S. latissima and A. esculenta are relatively new biomasses on the market, and despite an increasing number of studies, the knowledge of their chemical composition remains limited. Here, the seasonal variation of the seaweed composition and growth from April to June was investigated in both species, considering different harvesting and deployment dates. The latter distinguish this work from several earlier composition studies. It was shown that an earlier harvest of both species is beneficial when targeting end-products for food markets, with higher protein contents, including the highest content of the umami taste amino acid glutamate, and lower concentration of potentially harmful elements such as iodine, arsenic and cadmium. Interestingly, deployment date had little impact on the composition of A. esculenta through the harvesting season, whereas for S. latissima significant differences were observed between the two deployment groups (October and January), for several of the compounds. For instance, S. latissima harvested in June contained twice as much iodine when deployed in January compared to October. This indicates that a broader deployment window can be utilised for A. esculenta than for S. latissima with respect to biomass quality. A potential end-product from cultivated seaweed is alginate, a well-established ingredient already used in over 600 commercial products across markets such as pharmaceuticals, food, and textiles. However, where wild growing L. hyperborea has been utilised for alginate in Norway during decades, little is known about alginates from cultivated S. latissima and A. esculenta. Here, it has been explored how to extract alginates from S. latissima and A. esculenta, and it has been shown that with mild conditions (neutral pH and 20°C), yields between 20 and 25 % of the dry weight are obtained. Alginates from cultivated seaweed have a lower FG and shorter G-blocks, making them less valuable when compared with L. hyperborea stipe alginate. This challenge was met by implementing C-5 epimerases in the alginate extraction process to increase FG, a method termed “in-process epimerisation”. After in-process epimerisation with AlgE1 of S. latissima alginates, the FG was shown to increase from 0.44 to 0.76, giving the alginate similar chemical and physical properties as the high-value alginate from L. hyperborea stipe. In-process epimerisation of L. hyperborea lamina alginates have also been demonstrated, showing the potential for increased valorisation of already harvested biomass. Much focus has been put into preservation of the seaweed biomasses. Traditionally, formaldehyde has been used as a preservative in the alginate industry, since it acts like a biocide and prevents alginate discolouration by fixating polyphenols. However, due to the potential risk of health-related issues, stricter regulations concerning formaldehyde are expected and have already begun to be implemented in the EU. In this thesis, low molecular weight organic acids are explored as preservatives for L. hyperborea, S. latissima and A. esculenta. A high concentration of undissociated acid is crucial for the organic acids to act antimicrobial, and is a function of pH, acid concentration and pKa of the acid. Preservation at low pH will therefore be more efficient but will also lead to acid hydrolysis of the alginates. For L. hyperborea it was shown that acetic acid (0.1 M, pH 4.7) maintained a high alginate molecular weight (Mw > 400 kDa) for 8 weeks, when the biomass was stored at 13°C. Treatment with acetic acid also led to high alginate yields and alginate discolouration was avoided. For the cultivated species, formic acid (0.1 M, pH 3.2) was shown to maintain the molecular weight and crystallinity of cellulose, and maintain a high alginate molecular weight (Mw > 350 kDa) for 16 weeks when stored at 4°C. These results are promising for future biorefineries based on cultivated seaweed, since biomass can be available over a longer period than the short harvesting season.