Combined effects of seasonal variation and drying methods on the physicochemical properties and antioxidant activity of sugar kelp (Saccharina latissima)

Sugar kelp (Saccharina latissima), a marine macroalga, is a rich source of fibers, vitamins, minerals, and antioxidants. However, investigations on the effects of seasonal variation and the drying methods employed on the kelp composition and quality attributes are limited. In this study, the effects of seasonal variation and different drying methods (sun drying, freeze drying) and conditions (temperature, humidity) employed in a heat pump-based drying system on the physicochemical properties, phenolic activity, and antioxidant capacity of S. latissima grown in Maine, USA, were studied. Fresh sugar kelp samples were dried using sun drying, freeze drying, and heat pump-based drying systems at air temperatures of 30, 50, and 70 degrees C and relative air humidity levels of 25 and 50%. Seasonal variations in kelp harvest (i.e., early May and end of June) have significant effect (p<0.05) on the proximate composition. Significant effects of the drying methods (p<0.05) were also observed in the water and oil holding capacities (WHC and OHC), with the highest OHC and the lowest WHC exhibited in freeze dried sugar kelp. At lower drying humidity of 25%, the samples exhibited higher WHC and OHC as compared to higher humidity of 50%. Compared to the fresh kelp, irrespective of the drying method, the total phenolic content (TPC), antioxidant activity, and vitamin C content were observed to decrease by 5-10-fold. Overall, drying at lower temperature (<50 degrees C) and lower humidity was found to be suitable in terms of the processing cost, functional properties, and preservation of the bioactive compounds in dried sugar kelp.