Effects of drying processes on antioxidant properties and chemical constituents of four tropical macroalgae suitable as functional bioproducts

Seaweeds are a source of antioxidants and pigments, which can be used as functional ingredients for food, cosmetic, pharmaceutical, and industrial applications. As fresh seaweed has a large amount of water, it usually goes through a drying process in order to be commercialized, which facilitates transportation and storage and also increases shelf time. However, the drying procedures can change the chemical composition and antioxidant properties of seaweeds. Thus, the aim of this study was to evaluate the antioxidant properties and pigment and protein content of four Brazilian macroalgae (Gracilariopsis tenuifrons, Pterocladiella capillacea, Sargassum stenophyllum, and Ulva fasciata) suitable for use as functional bioproducts after processing with three different drying procedures (freeze-drying, oven-drying and silica-drying), using a fresh frozen treatment as control. Among the studied species, S. stenophyllum presented the highest antioxidant activity. For this species, freeze-drying was the procedure with the highest activity in the Folin-Ciocalteu and metal chelating assays, while for the ABTS, DPPH, and FRAP assays, no significant differences were observed between freeze-drying and silica-drying. Freeze-drying also showed low reduction of photosynthetic pigments in G. tenuifrons, P. capillacea, and U. fasciata and was the method that best-preserved protein content in the four species. In general, the antioxidant potential and pigment and protein content for the studied algae decreased in the following order: fresh frozen (control) > freeze-dried > silica-dried > oven-dried. Freeze-drying was the procedure that presented the lowest alteration in functional properties, as the low temperature prevents the degradation of heat-sensitive compounds. In addition, freeze-drying is a vacuum process and the absence of oxygen prevents oxidation reactions of functional ingredients.