Gum karaya, also known as Sterculia gum, is a complex, partially acetylated polysaccharide exudate obtained from trees of the genus Sterculia (primarily Sterculia urens in India). It is valued for its exceptional swelling capacity and viscosity-enhancing properties.
Molecular Structure
Gum karaya consists of a branched galacturonorhamnan backbone featuring α-(1→4)-D-galacturonic acid and α-(1→2)-L-rhamnose residues in an approximate ratio of 43:15, with side chains composed of β-D-galactose, D-glucuronic acid, and acetyl groups (about 8–14%, molecular weight ≈ 9.5 million Da). The high uronic acid content (37–40%) and the presence of calcium and magnesium salts confer partial ionization. Acetyl esterification at the O-3 and O-2 positions limits hydrogen bonding, favoring rapid hydration rather than complete dissolution in aqueous systems.
Extraction and Properties
Gum karaya is harvested as tear-shaped exudates following bark incisions in S. urens. The raw material is cleaned, dried to a moisture content of 10–15%, and milled into powders of 40–200 mesh. In cold water (1–2%), it can swell up to 200 times its original volume, forming sticky, ropy mucilages. Partial hydrolysis of acetyl groups releases acetic acid, producing a characteristic vinegar-like odor. The polymer is resistant to organic solvents, exhibits non-Newtonian thixotropic behavior, remains stable over a pH range of 4–8, and tolerates temperatures up to 80 °C. Alkaline deacetylation yields more water-soluble derivatives.
Biomedical Applications
Owing to its strong water retention and mucoadhesive properties, gum karaya is widely used in ostomy adhesives, denture fixatives, and wound dressing formulations. Pharmaceutical-grade material is employed as a stabilizer in suspensions and emulsions and as a binder in tablet manufacturing. Additionally, its fermentability by gut microbiota supports prebiotic applications, although high daily intake (above approximately 20 g) may induce gastrointestinal discomfort such as bloating.
