Gellan gum is a microbial polysaccharide fermented by Sphingomonas elodea. Its molecular structure consists of linear tetrasaccharide repeating units of glucose, glucuronic acid and rhamnose at a molar ratio of 2:1:1. It is divided into two types according to acyl content. Native high-acyl gellan gum directly extracted from fermentation broth contains acetate and glycerate substituents, forming elastic and cohesive gels. Low-acyl gellan gum is obtained by deacetylation, producing gels with high strength and brittleness. The two types differ greatly in solubility and gelling behavior. High-acyl gellan gum swells in water to form a thick suspension and loses viscosity after heating hydration. Low-acyl gellan gum is partially soluble in cold water and needs heating above 70°C for full dissolution; it gels during cooling induced by ions, especially calcium ions. Gellan gum is highly sensitive to ionic environments. It is widely used in food industry such as bakery fruit fillings, confectionery, icings, dairy products, beverages and food coatings.