Rheology-structure properties of gellan systems: evidence of network formation at low gellan concentrations A.I. Rodrı ´guez-Herna ´ndez a , S. Durand b , C. Garnier b , A. Tecante a, * , J.L. Doublier b a Departamento de Alimentos y Biotecnologı ´a, Facultad de Quı ´mica “E”, UNAM, Cd. Universitaria, 04510 Me ´xico, DF, Mexico b Laboratoire de Physico-Chimie des Macromole ´cules, INRA, BP 71627, 44316 Nantes Cedex 3, France Received 12 June 2002; accepted 11 November 2002 Abstract The dynamic rheology and confocal laser scanning microscopy (CLSM) of gellan at low concentrations (0.005 – 0.05 wt%) in the presence of 10 mM CaCl 2 were investigated at 25 8C. DSC thermograms showed a conformational transition from coil to rigid helix at approximately 41 8C. Microscopy observations demonstrated that at these low gellan concentrations, the level of counter ions induced formation of a network, the compactness of which depended on gellan concentration. The reinforcement of the network structures by the increase of gellan concentration resulted in more elastic gels well evidenced through the rheology of the systems. q 2003 Elsevier Science Ltd. All rights reserved. Keywords: Gellan; Rheology; Confocal microscopy; Viscoelasticity; Gels 1. Introduction Gellan is an extracellular polysaccharide produced by aerobic fermentation of Sphyngomonas elodea (Sworn, Sanderson, & Gibson, 1995). The deacylated form is an anionic polysaccharide consisting of a linear tetrasaccharide repeating unit of [ ! 3)-b-D-Glcp-(1 ! 4)-b-D-GlcpA- (1 ! 4)-b-D-Glcp-(1 ! 4)-a-L-Rhap-(1 ! ](Jansson & Lindberg, 1983). It forms firm, hard, and brittle gels, with gelation being dependent on the type of cation, ionic strength, temperature, and polymer concentration (Sanderson, 1990). It has been reported that gellan is able to form gels at concentrations as low as 0.05 wt%, however, the concen- trations commonly used in food manufacture range from 0.2 to 0.4 wt% (Kelco International, 1991). Such low concen- trations and gum multifunctional properties allow to get a wide spectrum of textures and to use gellan in combination with other hydrocolloids to enhance stability, structure formation and flavor release in food systems. X-ray diffraction data conducted on Li þ -gellan indicated that the molecular structure is an extended, intertwined, threefold left-handed double helix where the polysaccharide is stabilized by interchain hydrogen bonds (Upstill, Atkins, & Attwool, 1986). X-ray examination of the K þ salt (Upstill et al., 1986) showed that one K þ ion is associated with the carboxylate group in every tetrasaccharide repeating unit surrounded by six ligands to induce the potassium coordination where both chains in the double helix participate. The ligands include two carboxylate oxygen atoms and a hydroxyl group in one chain, two hydroxyl groups in the other chain and a crystalline water molecule. All this structural organization is responsible for the high stability of the gellan double helix. The gellan sol–gel transition and solution properties have been studied by several authors (Crescenzi, Dentini, & Dea, 1987; Grasdalen & Smidsrod, 1987; Milas, Shi, & Rinaudo, 1990; Shi, 1990). These studies revealed the existence of a reversible transition between a random coil (disordered structure) and a helical arrangement (ordered structure) in solution. The sol – gel transition showed an ionic selectivity, corresponding to the order of ability of cations to promote gelation, similar to that proposed for gelation of k-carrageenan (Milas & Rinaudo, 1996). The gelation mechanism has been explained as a two-step process in which the first step is helix ordering followed by interaction between double helices (Chandrasekaran, Puigjaner, Joyce, & Arnott, 1988; Shi, 1990). The mechanical properties of gellan gels as a function of 0268-005X/03/$ - see front matter q 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0268-005X(02)00123-6 Food Hydrocolloids 17 (2003) 621–628 www.elsevier.com/locate/foodhyd * Corresponding author. Tel.: þ 52-55-56-22-53-07; fax: þ 52-55-56-22- 53-09. E-mail address: tecante@servidor.unam.mx (A. Tecante).