#### Abstract

We have determined the density, rheological behavior and surface tension of whey protein concentrate (WPC) solutions. Densities (r) were measured at concentrations of 0.05–0.40 w/w at temperatures of 20–35C. The results were expressed as a function of temperature and mass fraction (w). This function fit the data with deviations of less than 0.4%. Apparent viscosities (ha) for WPC solutions with mass fractions w 0.20 at temperatures of 10–40C and high shear rates, 50–1,200/s, were found to be independent of shear rates, implying that the rheological behavior of WPC solutions is Newtonian. Dynamic viscosity (h) data were fitted to an empirical function of the WPC mass fraction and temperature with a mean deviation of 4.7%. Surface tensions (s) were determined for mass fractions between 0.01 and 0.30 at 25C. At this temperature and w = 0.05, there was a critical surface tension, sc = 42.5 mN/m. When w 0.10, the arithmetic mean of s at 25C was 46.3 mN/m. The surface tension values were similar to those published for skimmed milk at 25C. In addition, for w = 0.05 and w = 0.20, we found that at temperatures between 20 and 40C, the surface tension decreased linearly with temperature. These linear equations fit our experimental data with an average deviation lower than 0.4%. 3 Corresponding author. TEL: +34-958-243310; FAX: +34-958-248992; EMAIL: german@ugr.es Journal of Food Process Engineering 32 (2009) 235–247. All Rights Reserved. © Copyright the Authors Journal Compilation © 2008 Wiley Periodicals, Inc. DOI: 10.1111/j.1745-4530.2007.00213.x 235 PRACTICAL APPLICATIONS Density, rheological behavior and surface tension are required to design and control processes with momentum, heat and mass transfer. The process of producing protein concentrates from milk or whey by ultrafiltration uses spiral-wound membranes. The cross-flow pressure drop and permeate mass flow are a function of fluid density and viscosity, which in turn depend on concentration and temperature. The ultrafiltration process used to concentrate solutions with mass fractions of about 0.10–0.20 w/w must then be treated to avoid physicochemical or microbiological alterations. Spray drying is usually used as the preservation technique. In the spray-drying design, these physical properties are necessary to calculate the mean droplet diameter and droplet size distribution.