Crystalline Xylan: A Rising Star in Carbohydrate Materials

Carbohydrates are increasingly valued as renewable macromolecular resources, particularly because their semicrystalline architectures endow them with distinctive structural and functional properties. While hexose-based crystalline polysaccharides such as cellulose, chitin, and starch have been extensively studied, crystalline xylan, a pentose-based polysaccharide composed of xylose units, has only recently emerged as an underexplored member of this family. Debranching can reduce xylan substitution, enhance backbone regularity, and promote ordered crystalline domains, thereby distinguishing crystalline xylan from amorphous hemicelluloses. This review summarizes recent advances in crystalline xylan, covering its molecular and crystal structures, water-dependent unit-cell behavior, sources, and preparation strategies. Particular attention is given to its conversion into high-value xylan nanocrystals through top-down and bottom-up routes, with emphasis on the relationships among processing methods, nanoscale morphology, and colloidal stability. Finally, emerging applications in optical materials, dispersants, gels, plastics, and related functional systems are discussed, together with current challenges and future opportunities.