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M. K. MCGUFFEY and E. A. Foegeding. Dept. of Food Science, North Carolina State Univ., 238 Schaub Hall, Box 7624, Raleigh, NC 27695-7624 The utilization of whey protein mixtures in neutral pH nutritional beverages is currently limited by their high thermal reactivity and rapid insolubilization. However, industrially-isolated a-lactalbumin (a-La) is the least reactive whey protein, has reduced allergenicity and is the main component in human milk and, therefore, has tremendous potential in these applications. The objective of this study is to quantitatively describe the aggregation of an industrial a-La preparation of a defined protein composition in a simulated, nutritional beverage ionic environment. The contributions of disulfide bond formation, net charge and hydrophobicity to the loss of monomer, size distribution, solubility and turbidity development throughout the aggregation process will be described. All experiments were performed with 50 g/L total protein at 95°C. Solubility was determined by heating for 1 hr., centrifuging then quantitation by spectrophotometry. Size exclusion chromatography combined with multiangle laser light scattering was used to determine molecular weight (Mw) and radius of gyration. Reactive thiol groups were determined with Ellman's reagent. At pH ³ 6.85, the solubility of a-La was greater than 90%, but drops dramatically to 35% at pH 6.80, indicating that a specific ionizable group(s) may regulate solubility. a-La demonstrated first order monomer loss kinetics. The aggregate average log Mw grows linearly with time from 600 kDa (t=1.9 min.) to 20000 kDa (t=15 min.). The shape of these distributions indicates that two growth processes occur throughout the heating process. The solution turbidity when held at 95°C fully develops within 35 min. When cooled, the turbidity is remarkably reversible depending on the net quantity of reactive thiol groups evolved. With sufficient net negative charge, a-La heated to 95°C forms soluble aggregates through two growth processes. Initially, small aggregates are formed through disulfide bond formation. Then, larger thermo-reversible aggregates are formed through hydrophobic-mediated association.
Session 64, Dairy Foods: Milk proteins
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