Structure-Function Relationship of Rhamnan Sulfate Isolated from Commercially Cultured Edible Green Seaweed, Monostroma nitidum

The green seaweed, Monostroma nitidum, is widespread in Japan. In Okinawa Prefecture, production of seaweed is performed using culture-nets that are seeded artificially. The annual production of the algae in Okinawa was approximately 100t in 2016. Recently, because Monostroma nitidum is used in salads, soups and other items, its utilization in the food industry increased. The algae contain a soluble polysaccharide, rhamnan sulfate. To estimate the applicability of a rhamnan sulfate as a food additive or non-food additives, we investigated the rheological properties of the polymer that was isolated from commercially cultured Monostroma nitidum using a rheogoniometer. A soft gelation occurred at a concentration of 4.0%, and the elastic modulus stayed at a constant value after the temperature to 50°C, which was estimated to be a transition temperature, then decreased rapidly with further increase in temperature. Although a small decrease in elastic modulus was observed with the addition of urea (4.0 M), it remained constant with an increase in temperature up to 60°C, and then decreased. An increase in the elastic modulus was observed in a 0.05 M NaOH solution and soft gelation occurred. The elastic modulus remained large during the increase in temperature even at 90°C. A soft gelation also occurred when rhamnan sulfate was dissolved in a Tris buffer (pH 8.0) solution. The possible mode of intra- and intermolecular associations within and between rhamnan sulfate molecules were discussed.