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The Nonequilibrium Phase and Glass Transition Behavior of ß-Lactoglobulin

Roger Parker ^*, Timothy R. Noel ^*, Geoffrey J. Brownsey ^*, Katrin Laos  and Stephen G. Ring ^*

^* Institute of Food Research, Norwich Research Park, Norwich NR4 7UA, United Kingdom; and Department of Food Processing, Tallinn Technical University, Tallinn 19086, Estonia

Correspondence: Address reprint requests to Dr. Steve Ring, Institute of Food Research, Norwich Research Park, Colney Lane, Norwich NR4 7UA, UK. Tel.: 44-0-1603-255031; Fax: 44-0-1603-507723; E-mail: steve.ring{at}bbsrc.ac.uk.

Concentrated solutions of bovine ß-lactoglobulin were studied using osmotic stress and rheological techniques. At pH 6.0 and 8.0, the osmotic pressure was largely independent of NaCl concentration and could be described by a hard sphere equation of state. At pH 5.1, close to the isoelectric point, the osmotic pressure was lower at the lower NaCl concentrations (0 mM, 100 mM) and was fitted by an adhesive hard sphere model. Liquid-liquid phase separation was observed at pH 5.1 at ionic strengths of 13 mM and below. Comparison of the liquid-liquid and literature solid-liquid coexistence curves showed these solutions to be supersaturated and the phase separation to be nonequilibrium in nature. In steady shear, the zero shear viscosity of concentrated solutions at pH 5.1 was observed at shear rates above 50 s^–1. With increasing concentration, the solution viscosity showed a progressive increase, a behavior interpreted as the approach to a colloidlike glass transition at 60% w/w. In oscillatory shear experiments, the storage modulus crossed the loss modulus at concentrations of 54% w/w, an indication of the approaching glass transition. Comparison of the viscous behavior with predictions from the Krieger-Dougherty equation indicates the hydrodynamic size of the protein decreases with increasing concentration, resulting in a slower approach to the glass transition than a hard sphere system.