A polysaccharide — a polymer of D-mannuronic acid — obtained from brown algae such as Macrocystis pyrifera and Laminaria, and the parent acid of the alginate family. It possesses strong hydrocolloidal properties, swelling in water to thicken, emulsify and stabilise, and is authorised for food use in various forms including the sodium, calcium and potassium alginates (E400–E404). The acid itself sees less direct food use than its salts, but every alginate gel, stabilised emulsion and restructured food traces back to this seaweed polymer extracted from kelp cell walls.