Gum Arabic-Chitosan Complex Coacervation Hugo Espinosa-Andrews, Juan G. Ba ´ ez-Gonza ´ lez, Francisco Cruz-Sosa, and E. Jaime Vernon-Carter* DIPH and DBT Universidad Auto ´ noma Metropolitana-Iztapalapa, San Rafael Atlixco # 186, Mexico City 09340, Mexico Received December 7, 2006; Revised Manuscript Received February 14, 2007 The formation of electrostatic complexes of gum Arabic (GA) with chitosan (Ch), two oppositely charged polysaccharides, as a function of the biopolymers ratio (R GA/Ch ), total biopolymers concentration (TB conc ), pH, and ionic strength, was investigated. The conditions under which inter-biopolymer complexes form were determined by using turbidimetric and electrophoretic mobility measurements in the equilibrium phase and by quantifying mass in the precipitated phase. Results indicated that optimum coacervate yield was achieved at R GA/Ch ) 5, independently of TB conc at the resulting pH of solutions under mixing conditions. High coacervate yields occurred in a pH range from 3.5 to 5.0 for R GA/Ch ) 5. Coacervate yield was drastically diminished at pH values below 3.5 due to a low degree of ionization of GA molecules, and at pH values above 5 due to a low solubility of chitosan. Increasing ionic strength decreased coacervate yield due to shielding of ionized groups. Introduction Macromolecules are the main components of formulated food products, and the control of structural properties of proteins and polysaccharides is a wide topic of investigation. 1,2 Interactions between food macromolecules can be either repulsive or attractive, underlining two opposite phenomena: biopolymer incompatibility and complex formation. 1-4 The complexes formation can be either soluble or insoluble. 5 The insoluble complexes concentrate in liquid coacervate drops, leading to a phase separation of the mixture into two liquid layers. 5 The word “coacervate” is derived from the Latin “co” (together) and “acerv” (a heap) to signify the preceding union of the colloidal particles. 6,7 IUPAC defines coacervation as the separation into two liquid phases in colloidal systems (the phase more concentrated in colloid component is the coacervate, and the other phase is the equilibrium solution). 8 The phenomenon can be divided into “simple” and “complex” coacervation. Briefly, simple coacer- vation usually deals with systems containing only one colloidal solute, while complex coacervation usually deals with systems containing more than one colloid. Simple coacervation is a process involving the addition of a strongly hydrophilic substance to a solution of a colloid, which causes two phases to be formed: one phase rich in colloidal droplets and the other poor in such droplets. This process is dependent primarily on the degree of hydration produced, a variable difficult to control. On the other hand, complex coacervation has been found to be primarily dependent on pH and has been reported to occur in systems containing two dispersed colloids of opposite electrical charge. The optimum conditions for complex coacervation are achieved when pH is adjusted to a point where equivalents of oppositely charged molecules of the two colloids are present, because the greatest number of salt bonds form at this point. 9,10 These complexes have many applications, including carbon- less copy paper, 11 fat substitution, 12 protein separation, 13 microencapsulation, 14-17 cosmetics, 18,19 food, 20,21 and enzyme immobilization. 22 A number of studies have shown that complex coacervation could be obtained in protein-polysaccharide mixtures, provided that external parameters triggering electrostatic interactions (i.e., pH, ionic strength, biopolymers ratio, total biopolymers con- centration, temperature, charge density, and polyelectrolyte stiffness) are accurately controlled. 6,23-29 Complex formation is driven by the increase of entropy due to expulsion of small ions from the double layers around the individual polyelectrolyte chains, while in the case of weak polyelectrolytes, the polyelectrolyte is able to increase the charge of the polyelectrolyte groups, which implies a further decrease of the free energy. 30 The nature of protein-polysaccharide complexes also is influenced by entropic factors, such as flexibility and/or transitions between globular and extended conformations, and by enthalpy contributions, which in turn are regulated by the protein-polysaccharide ratio, the nature, and density of charges on the biopolymers. There is scarce informa- tion about factors influencing ionic polysaccharide-polysac- charide interactions. Recently, it was reported that mesquite gum, a polysaccharide very similar in chemical composition to gum Arabic, formed soluble complexes with chitosan at mineral oil-water flat interfaces 31 and around mineral oil-in-water emulsions. 32 In both cases, the systems exhibited viscoelastic properties that were highly dependent on the mesquite gum- chitosan ratio. Chitosan (Ch) is the second polysaccharide most abundant in the world and is obtained by alkaline N-deacetylation of chitin. The use of chitosan in the food industry is particularly promising because of its biocompatibility and nontoxicity. 33 Chitosan is a heterogeneous binary polysaccharide that consists primarily of 2-acetamido-2-deoxy--D-glucopyranose and 2-amino-2-deoxy--D-glucopyranose residues, the latter residue being responsible for its cationic charge at acidic pH values. The properties of chitosan in solution depend on molecular weight, the deacetylation degree, pH, and ionic strength. 34,35 The pK a value of the glucosamine segments is 6.3-7. 35 At low pH and low ionic strength, the intrinsic viscosity of chitosan increases rapidly, due to strong electrostatic segment-segment repulsion, adopts an extended conformation, and the rotational * To whom correspondence should be addressed. Phone: (+52)- 5558044648. Fax: (+52)5558044900. E-mail: jvc@xanum.uam.mx. 1313 Biomacromolecules 2007, 8, 1313-1318 10.1021/bm0611634 CCC: $37.00 © 2007 American Chemical Society Published on Web 03/22/2007