Curdlan is a linear, water-insoluble β-1,3-glucan exopolysaccharide produced primarily by the bacterium Alcaligenes faecalis (now classified within Agrobacterium spp.). It is distinguished by its unique thermo-irreversible gelling behavior and pronounced immunostimulatory properties. 

Molecular Structure

Curdlan consists of unbranched chains of β-(1→3)-linked D-glucose residues, with an average degree of polymerization of approximately 450 and a molecular weight near 74 kDa. In the solid state and in gels, the polymer adopts a rigid triple-helical conformation. Structural analyses using X-ray diffraction and transmission electron microscopy have revealed microfibrils composed of three intertwined glucan chains.

Under alkaline conditions or elevated temperatures, the ordered helical structure transitions into a random coil. Distinct crystalline forms have been described: Curdlan I (dry form) exhibits hexagonal packing, whereas Curdlan II (hydrated gel form) displays triclinic unit cells stabilized by extensive interchain hydrogen bonding.

Production and Physicochemical Properties

Industrial production of curdlan relies on aerobic fermentation using glucose or starch as carbon sources, typically conducted at 30 °C and pH 6–7. During fermentation, the insoluble polymer precipitates as granules, which are subsequently recovered, washed, and dried. Unlike some related β-glucans such as scleroglucan, curdlan does not dissolve in cold water but can swell in hot alkaline solutions.

Curdlan exhibits distinctive thermo-gelling behavior: heating above 55–80 °C induces formation of a resilient, elastic low-set gel, while heating above 80 °C in alkaline conditions yields a high-set, thermo-irreversible gel. Additional properties include high thermal and pH stability, low aqueous solubility, biodegradability by β-glucanases, and a granular morphology resembling starch doughnuts formed by wound microfibrils.

Biomedical Applications

Curdlan demonstrates notable biological activity through activation of macrophages via the Dectin-1 receptor, leading to immunomodulatory and antitumor effects, which are particularly enhanced when the polymer retains its helical conformation. These properties have generated interest in curdlan as an immune adjuvant and as a bioactive component in wound healing applications.

Chemical derivatives of curdlan have been developed to form nanoparticles and hydrogels suitable for drug delivery systems and tissue engineering scaffolds. Owing to its biocompatibility, structural stability, and capacity for functional modification, curdlan represents a versatile polysaccharide platform for advanced biomedical and pharmaceutical applications.