Sol-gel transition temperatures of high acyl gellan with monovalent and divalent cations from rheological measurements Emmanuel Flores-Huicochea a,1 , Adriana I. Rodríguez-Hernández b , Teodoro Espinosa-Solares c , Alberto Tecante a, * a Departamento de Alimentos y Biotecnología, Facultad de Química, Universidad Nacional Autónoma de México, Cd. Universitaria, D. F. 04510, Mexico b Centro de Investigaciones en Ciencia y Tecnología de los Alimentos, ICAP, Universidad Autónoma del Estado de Hidalgo, Av. Universidad km 1, Rancho Universitario, 43600 Tulancingo, Hidalgo, Mexico c Departamento de Ingeniería Agroindustrial, Universidad Autónoma Chapingo, P.O. Box 161, Chapingo 56230, Edo. De México, Mexico article info Article history: Received 25 February 2012 Accepted 9 November 2012 Keywords: Gellan Gels Polysaccharides Rheology Solegel transition abstract The solegel transition temperatures of 0.1e1.0% high acyl gellan (HAG) with 0e200 mM NaCl or KCl and 0e20 mM CaCl 2 or MgCl 2 were determined using rheological measurements. Transition temperatures for monovalent cations, Na þ and K þ , in the range of 50e80  C were not significantly different (p > 0.5). Absence of thermal hysteresis was the salient feature. However, thermal hysteresis (w4.4  C) was observed for 0.1% HAG without added salt, but disappeared on increasing HAG and counterion concentrations. Few concentrations of HAG and added monovalent and divalent cations showed thermal hysteresis not higher than 2.5  C. Transition temperatures for divalent cations were similar to those for monovalent cations although for considerably lower concentrations of Ca 2þ or Mg 2þ . Increasing concentrations of monovalent and divalent counterions give rise to higher transition temperatures but not to higher storage moduli. This was interpreted as a lack of cross-link formation in the three- dimensional network structure of the gels. A single solegel transition diagram for monovalent cations is proposed, in which different zones associated with the presence of ordered and disordered confor- mations serve to identify the conditions in which HAG can exist in aqueous media. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction Gellan is an anionic extracellular heteropolysaccharide produced by the bacterium Sphingomonas elodea, earlier classified as Pseu- domonas elodea (Jansson, Lindberg, & Sanford, 1983). The repeating unit is a tetrasaccharide formed by glucose, glucuronic acid, and L- rhamnose in a molar ratio of 2:1:1, respectively. The anionic char- acter is provided by the carboxylic group of glucuronic acid (Kang & Veeder, 1982; Kang, Veeder, Mirrasoul, Kaneko, & Cottrell,1982). In high acyl gellan (HAG), there is on average, one glyceryl substituent per repeating unit and one acetyl substituent per every two repeating units. The L-glyceryl and O-acetyl substituents are located at the C2 and C6 positions, respectively, on the O-3 glucose residue (Kuo, Mort, & Dell, 1986). Removal of these two acyl groups yields low acyl gellan (LAG) (Kang, Jolla, Veeder, & Colegrove,1983; Kang & Veeder, 1982). LAG forms brittle and non-elastic gels, whereas HAG produces elastic and soft gels (Sworn, 2000). The gelation mecha- nism of LAG is a two-step process in which random coils turn into double helices, which subsequently aggregate to generate a continuous three-dimensional network giving rise to a gel. The transition from random coils to double helices occurs when hot solutions are cooled, and the inverse transition occurs when gels are heated. The conformational change is therefore thermoreversible, and the temperatures of gelation (T g ) and melting (T m ) of the gel depend on the concentration of the polysaccharide and on the concentration and type of external counterions. Thermal hysteresis is absent if T g ¼ T m ; otherwise, T m is greater than T g . The presence of thermal hysteresis has been associated with aggregation of double helices (Milas & Rinaudo, 1996; Milas, Shi, & Rinaudo, 1990). The gelation mechanism of HAG is similar to that of LAG, but the pres- ence of acyl substituents introduces unique characteristics. Early studies examined the influence of acyl substituents on the rheological properties and some aspects of the solegel transition of HAG. The glyceryl groups are located inside the double helix, and the acetyl groups lie on its periphery. Additionally, the glyceryl * Corresponding author. Tel.: þ52 55 5622 5307; fax: þ52 55 5622 5309. E-mail address: tecante@unam.mx (A. Tecante). 1 Current address: Departamento de Desarrollo Tecnológico, Centro de Desarrollo de Productos Bióticos, Instituto Politécnico Nacional, Carr. Yautepec-Jojutla km 8.5, Col. San. Isidro, Yautepec, Morelos 6273, Mexico. Contents lists available at SciVerse ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd 0268-005X/$ e see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodhyd.2012.11.007 Food Hydrocolloids 31 (2013) 299e305