FOOD HYDROCOLLOIDS Food Hydrocolloids 22 (2008) 752–762 Effects of dextran sulfate, NaCl, and initial protein concentration on thermal stability of b-lactoglobulin and a-lactalbumin at neutral pH B. Vardhanabhuti à , E. Allen Foegeding Department of Food Science, Southeast Dairy Foods Research Center, North Carolina State University, Box 7624, 238 Schaub Hall, Raleigh, NC 27695-7624, USA Received 21 September 2006; accepted 8 March 2007 Abstract The effects of NaCl, dextran sulfate (DS), and initial protein concentration on thermal stability of b-lactoglobulin (b-LG) and a- lactalbumin (a-LA) at pH 6.8 were investigated. NaCl was the biggest factor in accelerating protein aggregation as shown by an increase in turbidity, molecular size, and a decrease in protein solubility. DS at low concentration showed a protective effect against aggregation. Evidence suggested some degree of interaction between b-LG and DS. At higher concentration, phase separation between biopolymers favored aggregation, which resulted in an increase in turbidity and a reduction in protein solubility. r 2007 Elsevier Ltd. All rights reserved. Keywords: b-Lactoglobulin; Dextran sulfate; Thermal stability; NaCl; SEC–MALLS; Aggregation 1. Introduction The annual manufacture of whey in the US from 1999 to 2003 has averaged more than 1000 million pounds per year (USDA, 2005). Whey proteins are among the major ingredients in sports drinks, which have shown a strong growth along with other functional beverages. Stability of whey protein during thermal processing and extended storage are essential to product quality and shelf life. Thermal stability of proteins is a general term that describes the ability of a protein solution to survive a thermal process without detrimental changes. Understand- ing the factors regulating the thermal stability of whey proteins will aid processors in developing whey protein- based drinks. b-lactoglobulin (b-LG) and a-lactalbumin (a-LA) are the major proteins in whey protein concentrates and isolates, so they contribute greatly to the thermal behavior of these ingredients (Cayot & Lorient, 1997; Swaisgood, 1982). Primary structure of b-LG comprises 162 amino acid residues (18,300 M w ) with two disulfide bonds and one sulfhydryl group. There has been extensive research on heat denaturation and aggregation of b-LG. The denatura- tion mechanism clearly depends on pH (Hoffman & van Mil, 1999), ionic strength and the nature of ions (McPhail & Holt, 1999), concentration and purity of the protein (Qi, Brownlow, Holt, & Sellers, 1995), dielectric constant (Narizhneva & Uversky, 1998), temperature (Roefs & de Kruif, 1994) and genetic variants (Manderson, Hardman, & Creamer, 1998; Sawyer, 2003). Under ambient condi- tions, b-LG mainly exists as a non-covalently linked dimer at neutral pH, and by increasing the temperature the equilibrium is shifted toward the monomeric form (Georges, Guinand, & Tonnelat, 1962). Upon further heating (450 1C), the protein undergoes a conformational change, with increased exposure of previously buried hydrophobic groups and the thiol group. However, the native-like secondary structure is retained and this situa- tion can be pictured as a ‘‘molten-globule state’’ (Ptitsyn, 1995). Roefs and de Kruif (1994) proposed a kinetic aggregation model based on thiol/disulfide exchange reactions leading to the formation of polydisperse, disulfide-linked aggregates. Apart from the importance of thiol/disulfide interchange, it is also evident that non- covalent interactions play an important role in aggregation ARTICLE IN PRESS www.elsevier.com/locate/foodhyd 0268-005X/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.foodhyd.2007.03.003 à Corresponding author. Tel.: +1 919 513 2247; fax: +1 919 515 7124. E-mail address: bvardha@ncsu.edu (B. Vardhanabhuti).