Characterisation of gum tragacanth (Astragalus gossypinus)/sodium caseinate complex coacervation as a function of pH in an aqueous medium Sara Ghorbani Gorji a , Elham Ghorbani Gorji b , Mohammad Amin Mohammadifar a, * a Department of Food Science and Technology, National Nutrition and Food Technology Research Institute, Faculty of Nutrition Sciences and Food Technology, Shahid Beheshti University of Medical Sciences,193954741 Tehran, Iran b Department of Food Science and Technology, Faculty of Agriculture, Islamic Azad University, Science and Research Branch, Tehran, Iran article info Article history: Received 17 May 2012 Accepted 23 October 2012 Keywords: Gum tragacanth Sodium caseinate Complex coacervation Protein/polysaccharide interaction Particle size measurements abstract The formation of electrostatic complexes between sodium caseinate and gum tragacanth (Astragalus gossypinus, A.g) as a function of pH (7.00e2.50), the biopolymer mixing ratio and the total biopolymer concentration was studied by spectrophotometric and light scattering measurements. This multi-methodological approach allowed us to demonstrate the critical structure-forming pHs associated with the formation of soluble and insoluble complexes for the sodium caseinate/A.g mixture. The phase transition of sodium caseinate/A.g complexed/coacervated system was explained by moni- toring the absorbance profiles as a function of time. Particle size measurements revealed a progressive decrease in the complex/aggregate size while lowering the pH until reaching a minimum (pH 4) at which nanoparticles (70 nm) formed. As the mixing ratio of protein to polysaccharide increased from 1:1 to 3:1, the critical pHs shifted towards higher pH values. Conversely, for a constant mixing ratio, higher levels of total concentration resulted in an increase in the maximum turbidity but had no meaningful effect on pH C (w5.89) and pH F1 (w4.00). Ó 2012 Elsevier Ltd. All rights reserved. 1. Introduction The food industry has recently demonstrated great interest in finding novel materials and techniques to develop value-added structures that can be employed as functional food ingredients for the adjustment of the texture, mouthfeel and optical properties of foods or as encapsulation devices for bioactive components. In this respect, biopolymer particles formed from proteins and poly- saccharides can meet various industrial needs to enhance the stability and viability of the active food ingredients. However, the various interactions between these two biopolymers play a key role in food systems because they are responsible for the structure, stability, and rheological characteristics of various processed foods (Kilara, 2006; Samant, Singhal, Kulkarni, & Rege, 1993; Sanchez, Schmitt, Babak, & Hardy, 1997; Tolstoguzov, 1997). Thus, the control or manipulation of these biomacromolecular interactions and a deeper understanding of the mechanisms of their interac- tions are crucial factors in the development of novel food pro- cessing methods and food products. The interactions between proteins and polysaccharides can be either repulsive or attractive. Complex coacervation is the asso- ciative phase separation in a solution of oppositely charged mac- roions, such as polyelectrolytes (Spruijt, Sprakel, Stuart, & van der Gucht, 2010). Generally, complex coacervation involves two primary pH-induced structure-forming events associated with the formation of soluble and insoluble complexes. The former event (denoted as pH C ) occurs at a pH corresponding to the first experi- mentally detectable increase in turbidity during a pH titration. This is followed by a second structure-forming event at pH F1 that is associated with the formation of insoluble complexes. The insol- uble complexes concentrate in liquid coacervate drops, leading to a phase separation of the mixture into two liquid layers (Weinbreck, De Vries, Schrooyen, & De Kruif, 2003). Optimal complex formation is considered to occur at a pH where both bio- polymers reach their electrical equivalence, denoted as pH opt , which is followed by the dissolution of the complexes at a lower pH (pH F2 ) due to the protonation of the reactive groups on the * Corresponding author. Tel.: þ98 2122548120; fax: þ98 2122357487. E-mail addresses: sarah.ghorbani@gmail.com (S.G. Gorji), elham.ghorbani@ gmail.com (E.G. Gorji), mohamdif@ut.ac.ir (M.A. Mohammadifar). 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.2012.10.019 Food Hydrocolloids 34 (2014) 161e168