Rheological behaviour of tamarind seed gum in aqueous solutions K. Khounvilay 1 , W. Sittikijyothin * Department of Chemical Engineering, Faculty of Engineering, Burapha University, Chonburi 20131, Thailand article info Article history: Received 27 September 2010 Accepted 9 March 2011 Keywords: Tamarind seed gum Seed gum Rheological behaivour Intrinsic viscosity Critical concentration abstract Tamarind seed gum as seed polysaccharide from Tamarindus indica L. has been characterized for phys- icochemical and rheological properties in the present work. The structural analysis determined the presence of glucose:xylose:galactose in a molar ratio of 2.61:1.43:1.The Huggins and Kraemer plots obtained by capillary viscometry gave an intrinsic viscosity of 4.7 dl g 1 and the viscosity average molecular mass was calculated to be 9.18  10 5 g mol 1 using the MarkeHouwink relationship. The steady shear and dynamic viscoelasticity properties of tamarind seed gum in aqueous solutions at different concentrations were investigated at 20  C using a Haake Rheometer RS75. The tamarind seed gum solutions clearly exhibited shear-thinning flow behaviour at high shear rate and Newtonian region occurred at low shear rate range, however, at higher concentrations, pronounced shear thinning was observed. The value of zero shear viscosity (h 0 ) was estimated by fitting Cross and Carreau models. The specific viscosity at zero shear rate (h sp0 ) was plotted against the coil overlap parameter (C[h]) and the slopes of the lines in the dilute and semi-dilute regions were found to be w2.2 and 4.3, respectively. The value of the critical concentration (C * ) was about 4.23/[h]. While, the mechanical spectra in the linear viscoelastic region of tamarind seed gum solutions showed the typical shape for macromolecular solutions. Plots of h versus g and h * versus u were superimposable and hence obey the CoxeMerz rule. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Tamarind (Tamarindus indica L.), belongs to Leguminosae, is common tree of Southeast Asia and widely indigenous to India, Bangladesh, Myanmar, Sri Lanka, Malaysia, and Thailand (El-Siddig et al., 2006). Tamarind seed gum, a crude extract of tamarind seeds, is rich in polysaccharide (w65e72%) (Kumar & Bhattacharya, 2008), which contains glucose, xylose and galactose units, in a molecular ratio of w3:2:1 (Freitas et al., 2005; Patel et al., 2008; Wang, Ellis, Ross- Murphy, & Burchard, 1997)(Fig. 1). Its structure is based on a b(1 /4)-D-glucan backbone, substituted at position 6 of the glucopyranosyl units mainly by single a-D-xylopyranosyl residues as well as by disaccharide side chains composed of b-D-gal- actopyranosyl-(1 / 2)-a-D-xylopyranosyl residues (Patel et al., 2008). In addition, tamarind seed gum is a high molecular weight polysaccharide (720e880 kDa) (Freitas et al., 2005; Sims et al., 1998; Wang et al., 1997), which forms viscous solutions when dissolved in water as many polysaccharide gums extracted from plant materials. Presently, it has potential for commercial applica- tions for example in the pharmaceutical industry for controlling drug release (Sumathi & Alok, 2002) and in the textile printing as a thickener (Abo-Shosha, Ibrahim, Allam, & El-Zairy, 2008). Particularly in the food industry in Japan, refined tamarind seed gum as permitted food additive has been used for modifying texture as a thickening, stabilizing and gelling agent (Nishinari, Yamatoya, & Shirakawa, 2000). Since the information of the rheological properties of poly- saccharide gum in aqueous solutions is useful and plays an important role in developing structureefunction relationships for the systems of polysaccharide solutions, the concentration regimes of several polysaccharide gums in aqueous solutions have been observed (see, for instance, Kapoor et al., 1998; Sittikijyothin, Torres, & Gonçalves, 2005 for galactomannans; Xu, Liu, & Zhang, 2006 for Aeromonas gum; Wang et al., 1997 for gum from Deta- rium senegalense Gmelin). In the polysaccharide gum solution, the onset of coil overlap and entanglement depends both on the number of coils present (proportional to concentration) and on the volume that each occupies (proportionally to intrinsic viscosity, [h]) and can there- fore be characterized by the (dimensionless) “coil overlap param- eter”, C[h]. For most random-coil polysaccharides, Morris, Cutler, Ross-Murphy, Rees, and Price (1981) showed that plots of viscosity against C[h] were closely superimposable, irrespective of * Corresponding author. Tel.: þ66 38 102222x3352; fax: þ66 38 745900x3350. E-mail address: wancheng@buu.ac.th (W. Sittikijyothin). 1 Present address: Faculty of Engineering, National University of Laos, Vientiane, Lao Democratic People’s Republic. Contents lists available at ScienceDirect Food Hydrocolloids journal homepage: www.elsevier.com/locate/foodhyd 0268-005X/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2011.03.019 Food Hydrocolloids 26 (2012) 334e338