Microwave-assisted depolymerization of carrageenans from Kappaphycus alvarezii and Eucheuma spinosum: Controlled and green production of oligosaccharides from the algae biomass

Algae biomass is nowadays considered as a unique reservoir of a huge variety of different molecules, such as lipids, carbohydrates, proteins and secondary metabolites, awaiting development in various fields. In particular, the valorization of the algae polysaccharide content is still in its infancy and intense research activity is starting to appear. Oligosaccharides with defined molecular weight and structures are extremely interesting for different applications such as plant biostimulation or biopesticide. Oligosaccharide are often prepared in medium to large scale upon chemical-based hydrolysis reactions from the extracted polysaccharides in harsh conditions (temperature and pH), leading to the loss of the fragile groups such as the sulfate functions in carrageenans. In the present work, microwave-based hydrolysis, as a green chemistry method, is optimized to produce bioactive oligosaccharides from carrageenans. Microwave hydrolysis process is first optimized on a commercially available carrageenan and further applied on carrageenans extracted from two algae, namely Kappaphycus alvarezii or Eucheuma spinosum that are produced in large quantities in the world and have a high content of carrageenans. We also develop a complete set of characterization methods, i.e. size exclusion chromatography, reducing sugar and residual sulfate determination and mass spectrometry, to perform the follow-up of the hydrolysis reaction. We here demonstrate that fast (5 min) and controlled hydrolysis of carrageenans can be achieved in pure water under microwave activation, while preserving the nature of the active sulfate groups (>95%).