Gelation of gellan – A review

• Published in: Food hydrocolloids 2012-08, Vol.28 (2), p.373-411
• Main Authors: Morris, Edwin R., Nishinari, Katsuyoshi, Rinaudo, Marguerite
• Format: Article
• Language: English
• Subjects: Aggregation; Biological and medical sciences; Double helix; Food additives; Food industries; Fundamental and applied biological sciences. Psychology; Gelation; Gellan; General aspects; High acyl; Polyelectrolyte
• ISSN: 0268-005X; 1873-7137
• DOI: 10.1016/j.foodhyd.2012.01.004

Gellan is an anionic extracellular bacterial polysaccharide discovered in 1978. Acyl groups present in the native polymer are removed by alkaline hydrolysis in normal commercial production, giving the charged tetrasaccharide repeating sequence: → 3)-β-d-Glcp-(1 → 4)-β-d-GlcpA-(1 → 4)-β-d-Glcp-(1 → 4)-α-l-Rhap-(1 →. Deacylated gellan converts on cooling from disordered coils to 3-fold double helices. The coil–helix transition temperature (Tm) is raised by salt in the way expected from polyelectrolyte theory: equivalent molar concentrations of different monovalent cations (Group I and Me4N+) cause the same increase in Tm; there is also no selectivity between different divalent (Group II) cations, but divalent cations cause greater elevation of Tm than monovalent. Cations present as counterions to the charged groups of the polymer have the same effect as those introduced by addition of salt. Increasing polymer concentration raises Tm because of the consequent increase in concentration of the counterions, but the concentration of polymer chains themselves does not affect Tm. Gelation occurs by aggregation of double helices. Aggregation stabilises the helices to temperatures higher than those at which they form on cooling, giving thermal hysteresis between gelation and melting. Melting of aggregated and non-aggregated helices can be seen as separate thermal and rheological processes. Reduction in pH promotes aggregation and gelation by decreasing the negative charge on the polymer and thus decreasing electrostatic repulsion between the helices. Group I cations decrease repulsion by binding to the helices in specific coordination sites around the carboxylate groups of the polymer. Strength of binding increases with increasing ionic size (Li+