Fermentation of cultivated brown algae Saccharina latissima and Alaria esculenta and effects on alginate quality

Abstract

Alginate, the main structural component of brown algae, is a family of polysaccharides containing $\beta$-d-mannuronate (M) and $\alpha$-l-guluronate (G) in varying ratios. Most alginate produced outside of Asia comes from wild harvested biomass, whereas cultivated seaweeds can provide a complementary supply to existing and new markets. Unfortunately, cultivated brown algae have a short harvest season, and cost-effective and sustainable preservation methods are required to ensure year-round access to high-quality biomass for alginate production. This study examines the effects of lactic acid fermentation for up to 95 days of cultivated Saccharina latissima (SL) and Alaria esculenta (AE) on alginate yield and quality in terms of purity, molecular weight (M_w) and monosaccharide composition. Alginate yield was relatively stable throughout storage time. Alginate lyase activity was observed in all the stored AE samples, while only in the SL alginate stored for 95 days. Both species showed a decrease in alginate M_w, presumably mainly due to acid hydrolysis during storage, but for AE, the decrease in M_w was also due to lyase activity. Enzymatic activity resulted in the AE alginate G fraction increasing from 53 % to 65 %, while the SL alginate G fraction remained constant during storage. The overall findings demonstrated that fermentation had a stabilizing effect on brown algae, maintaining a sufficiently high quality of the alginates for several applications. Rapid decrease in pH is crucial to prevent unwanted microbial growth, as well as lyase activity, but pH should not be too low, to avoid excessive depolymerization by acid hydrolysis.