Carbohydrate Polymers 17 (1992) 121-126 Role of conformation and acetylation of xanthan on xanthan-guar interaction L. Lopes, C. T. Andrade Instituto de Macromol~culas, Universidade Federal do Rio de Janeiro, CP 68525, 20000 Rio de Janeiro. Brazil M. Milas & M. Rinaudo Centre de Recherches sur les Macromol~cules VOgbtales, CERMAV-CNRS, BP 53X, 38041 Grenoble, France (Received 24 July 1990; revised version received 18 January 1991; accepted 21 January 1991) The synergistic effect obtained by mixing xanthan and guar solutions were examined by low shear viscosity measurements in relation to the temperature. Native and deacetylated xanthan samples were used in mixtures in which the total polymer concentrations were 1 g/litre and 0.5 g/litre. Gelation was observed for temperatures lower than 15°C for the native xanthan-guar system (weight ratio 1/1) in 10-2 M NaC1 and at 22-24°C for the same system in water; in this last case, it is known that the xanthan is in the disordered conformation. For a mixture of deacetylated xanthan-guar, gelation was observed at a temperature below 26°C in water. The results confirm that there is a stronger interaction between deacetylated xanthan and guar than native xanthan and guar because of enhanced xanthan-guar gum backbone association in the former case. INTRODUCTION Xanthan gum is an anionic polysaccharide produced by the microorganism Xanthomonas campestris, whose primary chemical structure consists of (1 • 4)-/3-D- glucose repeating units, as the main chain, with a trisaccharide substituent on alternate glucose residues. Two mannose units and a glucuronic acid unit make up the side chain. The terminal fl-D-mannose unit of the side chain may have a pyruvic acid residue linked to its 4-- and 6-- positions. Also, the non-terminal D- mannose unit on the side chain may have an acetyl group at the 6-- position (Janson et al., 1975; Melton et al., 1976). The acetyl and the pyruvate contents vary due to the bacterial strain, culture and postfermentation conditions (Slonecker & Jeanes, 1962; Cadmus et al., 1976; Tait et al., 1986). Galactomannan polysaccharides occur as reserve materials in a wide range of legume seeds. Their structure consists of a (1 ~ 4)-fl-D-mannan backbone with single (1 • 6)-a-D-galactopyranosyl unit attached at the 0--6-- position of certain D-mannopyranosyl residues. Guar galactomannan has a high and irregular distribution of D-galactosyl residues (Mc Cleary et al., 1984). Both xanthan and guar gums are used as thickeners. Much effort has been made to elucidate the behaviour ofxanthan-galactomannan mixtures (Dea & Morrison 1975; Dea et al., 1977; Morris et al., 1977). It was shown that the observed synergistic interaction based on cooperative interaction depends on the mannose/ galactose (M/G) ratio and also on the fine structure of the galactomannan. Higher M/G ratios as well as unsubstituted D-mannose blocks seem to favour the interaction (Dea et al., 1972; Morris et al., 1977; McCleary, 1979; Dea et al., 1986). This is the case when carob gum is one of the components of the system. The mixture xanthan-carob gums also gelifies aqueous systems. These results have been explained by a model in which the helical xanthan interacts with the galactomannan backbone (Dea et al., 1977). Another model was suggested to explain the participation of both unsubstituted and D-galactosyl substituted blocks of the galactomannan in junction zones. In this model, the D-galactosyl residues would be located on the same side of the main chain, allowing interaction between the two backbones (McCleary, 1979). Dynamic viscoelasticity measurements have been Carbohydrate Polymers 0144-8617/91/$03.50 © 1991 Elsevier Science Publishers Ltd. 121