Phase behaviour of oat b-glucan/sodium caseinate mixtures varying in molecular weight Jacob K. Agbenorhevi a , Vassilis Kontogiorgos a,⇑ , Stefan Kasapis b a Department of Chemical and Biological Sciences, University of Huddersfield, Queensgate, Huddersfield HD1 3DH, UK b School of Applied Sciences, RMIT University, City Campus, Melbourne, Vic 3001, Australia article info Article history: Received 30 May 2012 Received in revised form 2 September 2012 Accepted 2 October 2012 Available online 12 November 2012 Keywords: b-Glucan Phase separation Mixed systems Sodium caseinate Rheology abstract The isothermal phase behaviour at 5 °C of mixtures of sodium caseinate and oat b-glucan isolates varying in molecular weight (MW) was investigated by means of phase diagram construction, rheometry, fluores- cence microscopy and electrophoresis. Phase diagrams indicated that the compatibility of the b-glucan/ sodium caseinate system increases as b-glucan MW decreases. Images of mixtures taken at various bio- polymer concentrations revealed phase separated domains. Results also revealed that at the state of ther- modynamic equilibrium, lower MW samples yielded considerable viscosity in the mixture. At equivalent hydrodynamic volume of b-glucan in the mixtures, samples varying in molecular weight exhibited sim- ilar flow behaviour. A deviation dependent on the protein concentration was observed for the high MW sample in the concentrated regime due to the size of b-glucan aggregates formed. Results demonstrate that by controlling the structural features of b-glucan in mixtures with sodium caseinate, informed manipulation of rheological properties in these systems can be achieved. Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The phase behaviour of protein–polysaccharide mixtures con- tributes significantly to the stability, structural, rheological and textural characteristics of food products. It is well established in the literature that phase separation depends on the molecular characteristics of biopolymers. The interplay of parameters such as molecular weight, chain conformation and charge density as well as the mixing conditions (biopolymer concentration and mix- ing ratio, temperature, cooling rate) and solvent pH and ionic strength will determine the thermodynamics of phase separation (de Kruif & Tuinier, 2001; Doublier, Garnier, Renard, & Sanchez, 2000; Perez, Carrara, Sanchez, Rodríguez Patino, & Santiago, 2009; Schmitt, Sanchez, Desobry-Banon, & Hardy, 1998; Syrbe, Bauer, & Klostermeyer, 1998; Tolstoguzov, 2003; Turgeon, Beau- lieu, Schmitt, & Sanchez, 2003). Mixed linkage (1?3)(1?4)-b-D-glucans found in cereals (oats, barley, rye and wheat) have received significant consumer and re- search attention because of their health benefits, including lower- ing cholesterol levels and glycaemic index response. b-Glucans are linear cell wall homopolysaccharides of consecutively linked (1?4)-b-D-glucosyl units that are separated by single (1?3) bonds. The structure, molecular weight and concentration of b-glu- cans are known to influence their physical and functional proper- ties in solution or when used as ingredients in various formulated food products. These parameters as well as their phys- ical properties, applications and physiological effects have been discussed previously in detail (Brennan & Cleary, 2005; Lazaridou & Biliaderis, 2007; Lazaridou, Biliaderis, & Izydorczyk, 2003; Wood, 2007). Approval of oat b-glucans as functional bioactive ingredients has stimulated new product development activity over the years (Brennan & Cleary, 2005). Incorporation of b-glucans into milk or dairy products presents a potential application of this polysaccha- ride as a delivery method so as to provide the associated health benefits to consumers. However, the required amount (P0.75 g per serving) for health claims, makes it difficult in the development of food formulations as b-glucans exhibit thermodynamic incom- patibility when mixed with milk proteins that results in phase sep- aration (Kontogiorgos, Tosh, & Wood, 2009a, 2009b; Lazaridou & Biliaderis, 2009). It is important, therefore, to understand the phase behaviour of b-glucans with food proteins before we attempt to make products with desired rheological properties. Determination of phase diagrams is often employed to describe the thermodynamic compatibility of mixed biopolymer systems. In our previous study, the phase behaviour of mixtures of b-glucan with whey proteins under different solvent conditions was investi- gated and it was shown that a decrease of pH from 7.0 to 3.0 and sucrose addition enhanced miscibility (Kontogiorgos et al., 2009b). The present work, builds on our previous investigation assess- ing the effect of b-glucan molecular weight in mixtures with casein 0308-8146/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.foodchem.2012.10.073 ⇑ Corresponding author. Tel.: +44 1484 472488; fax: +44 1484 472182. E-mail address: v.kontogiorgos@hud.ac.uk (V. Kontogiorgos). Food Chemistry 138 (2013) 630–637 Contents lists available at SciVerse ScienceDirect Food Chemistry journal homepage: www.elsevier.com/locate/foodchem