Rheological Behavior of WPI Dispersion as a Function of pH and Protein Concentration LAURENT BAZINET,* ,† MAHER TRIGUI, ‡ AND DENIS IPPERSIEL ‡ Institute of Nutraceuticals and Functional Foods (INAF) and Centre de Recherche en Science et Technologies du Lait (STELA), Universite ´ Laval, Pavillon Paul Comtois, Department of Food Sciences and Nutrition, Sainte-Foy, Quebec, Canada G1K 7P4, and Agriculture et Agro-Alimentaire Canada, Centre de Recherche et de De ´veloppement sur les Aliments, 3600 Boulevard Casavant Ouest, St. Hyacinthe, Que ´bec, Canada J2S 8E3 Physical and flow properties of proteins can provide information necessary for the optimal design of unit processes and quality control of the manufacturing process and final products. Therefore, the purpose of this investigation was to characterize the rheological behavior of a whey protein isolate (WPI) (BiPRO) dispersion as a function of pH and protein concentration. A rotational viscometer was used to determine the apparent viscosity, shear rate, and shear stress of WPI dispersions. Both the consistency index (k) and the flow behavior index (n) were sensitive to changes in pH and protein concentration. Mathematical relations obtained from experimental values of k and n allowed the determination of a model for apparent viscosity (η) of WPI dispersions as a function of pH and protein concentration. At 5 and 10% BiPRO, whatever the pH, the rheological behavior appeared to be a newtonian fluid, while at 20% BiPRO, the rheological behavior appeared to be a nonnewtonian pseudoplastic fluid. Furthermore, at 20% Bipro, the apparent viscosity presented an increase in viscosity from 5.6 to 5.4, followed by a decrease from pH 5.4 to 5.0 at all shear rates. The highest viscosity was obtained at 20% pH 5.4, with an approximate value of 0.25 Pa.s, 10 times higher than the one obtained at 5 and 10% BiPRO. KEYWORDS: Whey protein isolate; apparent viscosity; model; pH; protein concentration; rheological behavior INTRODUCTION Some proteins are known to form large aggregates, sometimes up to several microns in diameter, without phase separation at relatively high concentrations (1). Low concentrations, usually below 1%, are used in scientific works to avoid interference from aggregation. However, proteins are often used or treated in food industry at concentrations favoring aggregation (2). The rheological properties of protein solutions are governed by composition, molecular mass, size, shape, flexibility, degree of hydration, and intermolecular interactions (3). Most of these factors are in turn influenced by concentration, temperature, pH, ionic strength, and previous processing treatments (4, 5). Intermolecular interactions between protein molecules may be especially important with respect to rheological properties: Proteins are charged particles, and it was shown that the presence of charges on particles increases the viscosity of dispersions (6). Rheological properties of whey protein concentrate (WPC) solutions have been investigated by many workers (7-11), but a very small number of studies appears on whey protein isolate (WPI) dispersion. Furthermore, recently, Bazinet et al (12) demonstrated the feasability of bipolar membrane electroacidi- fication (BMEA) for whey protein separation from a WPI solution and the influence of the initial protein concentration on the purity and yield of the separated fraction. At 5% WPI initial concentration, this technology allows the separation of 98% pure -lactoglobulin (-lg) fraction with a 44.0% recovery yield. However, with a 20% WPI solution, it was possible to reach pH 4.65 with conductivity control at 350 µS/cm, but protein precipitation was still low in comparison with 5% WPI (13). The changes in viscosity as pH decreases observed at 20% WPI would decrease the final precipitation rate of -lg, since the viscosity of the 20% WPI dispersion was very different. Consequently, the design of the spacers actually used in BMEA for protein precipitation was not adapted. In fact, the change in protein conformation and aggregation of these proteins may lead to a fouling of the spacers, as already observed by Bazinet et al. (14). In this context, flow properties of proteins can provide information necessary for the optimal design of BMEA unit process. Therefore, the purpose of this investigation was to characterize the rheological behavior of a WPI dispersion as a * To whom correspondence should be addressed. Tel.: 418 656-2131, ext. 7445. Fax: 418 656-3353. E-mail: Laurent.Bazinet@aln.ulaval.ca. † Universite ´ Laval. ‡ Centre de Recherche et de De ´veloppement sur les Aliments. 5366 J. Agric. Food Chem. 2004, 52, 5366-5371 10.1021/jf049893v CCC: $27.50 © 2004 American Chemical Society Published on Web 07/23/2004