*3.2.1 Fucoidans*

Fucoidans are only found in brown algae, belong to the sulfated anion polysaccharide group. L-fucose is a basic unit in fucoidans. The primary linkage of (1 → 3)-α-L-fucopyranosyl and the linkage of alternating α(1 → 3) and α(1 → 4)-L-fucopyranosyls occur in fucoidans structure [21, 22]. Fucoidans are structurally diverse and different between various brown algae species, even in a season that leads to the difference in their content. Therefore, fucoidans exhibit bioactive diversity, for example, antioxidant [21, 23], antitumor, antibacterial, anticoagulant, anticancer [22], antivirus, immune activation, neuroprotective, and the protection of the stomach and liver [24]. Fucoidan contents in sterile tissue (dry matter) and reproductive tissue of kelp species ranged from 0.5–13% and 1.4–69%, respectively [25]. For brown algae grown in Vietnam, fucoidan content usually ranges from 0.8 to 3.5%, compared to dried algae. Fucoidans that extract from brown algae have a color range from light brown to dark brown, depend on their purification.
