Sequential chemical and enzymatic extraction of fucoidans from brown algae yields distinct fucoidan types

Fucoidans are sulfated fucose-rich polysaccharides produced by brown algae. They are traditionally extracted using neutral or acidic solutions at elevated temperatures. Recently, enzyme-assisted extraction (EAE) has been proposed as an alternative approach to obtain high yields while preserving fucoidans' native structures. In previous studies, EAE has replaced chemical extraction to yield a single fucoidan fraction. In this study, a new sequential approach was developed: fucoidans were first extracted under mild chemical conditions, yielding a fraction referred to as ‘easily accessible fucoidans’, followed by EAE to obtain a fraction defined as ‘interconnected fucoidans’. The process was applied to Alaria esculenta (AE) and Saccharina latissima (SL). Compositional and structural analyses by CHNS-analysis, HPAEC-PAD, SEC-MALS, and NMR revealed distinct differences between the two fucoidan fractions, including variations in monosaccharide composition, sulfation, and acetylation degrees. These differences, along with the varying extraction conditions required, suggest that the fractions represent distinct fucoidan types. We hypothesize that the chemically extracted fucoidans are not covalently bound to any structures and originate from the thallus surface or outer cell wall layers of the algae, while the EAE extracted fucoidans are more tightly bound (either covalently linked or entangled) to other cell wall components. Furthermore, a fraction of fucoidans from AE appeared to be covalently linked to polyphenols, as identified and characterized by NMR. To our knowledge, this is the first study describing potential covalent linkages between fucoidan and polyphenols in brown algae.