Drying method influences the yield and activity of bioactive compounds extracted from the kelp species Macrocystis pyrifera and Undaria pinnatifida in New Zealand

Drying is a critical post-harvest processing step for macroalgal biomass, influencing product stability, processing efficiency, and the preservation of valuable bioactive compounds. This study evaluated how different drying methods influenced phytochemical composition and antioxidant activity of tissue from the native kelp Macrocystis pyrifera and invasive Undaria pinnatifida in Southern New Zealand. Blade and sporophyll tissue were exposed to seven drying protocols, freeze-drying, hot-air drying at 45, 60, and 80 °C, and microwave drying at 220, 440, and 660 W. Crude fucoidan yield, total phenolic content, and antioxidant capacity (using ABTS, DPPH, and FRAP assays) of the kelp extract was determined. Microwave drying resulted in the shortest drying times (0.07–0.35 h), whereas freeze-drying required substantially longer processing periods (21–33 h). Drying treatments influenced yield of target compound differently depending on the species, tissue type and target compound. The highest fucoidan yield was achieved in the sporophyll of Undaria (40.5 g F [100 g] −1 DS) using freeze-drying method, while the lowest yield was observed in all tissues of Macrocystis (9.9 – 17.4 g F [100 g] −1 DS) under freeze-drying. In addition, hot-air drying at 45 – 60 °C and controlled microwave drying, enhanced phenolic recovery in both tissues of Macrocystis and blade of Undari a compared to freeze-drying. Moreover, yield of antioxidant capacity differed between species and among tissue types (e.g., FRAP values were lower in Undaria sporophyll using microwave drying compared to freeze-drying, while the opposite trend was observed in Macrocystis blade). These findings demonstrate that drying conditions play a critical role in determining both processing method and retention efficiency of bioactive compounds for potential application in nutraceutical, pharmaceutical and healthy food industries.