Properties and Molecular Interactions of Whey Protein Concentrate upon Storage

Accelerated storage tests of whey protein concentrate were carried out to characterize changes of the proteins that may affect its nutritional quality or functional properites. After 42 days at 37"C and 75% relative humidity, pH 4.6 soluble protein decreased only slightly (14% loss), and there were no significant changes of sulfhydryl or disulfide content. Hydroxymethylfurfural increased dramatically (17 to 192 /~moles/100 g whey protein concentrate). "Free" lactose, and total, dinitrobenzenesulfonate-available and pepsin-pancreatin-digestible iysine contents also decreased (17, 34, 57, and 72% losses). Samples were darker after storage. Diffuse reflectance spectra showed increased absorbance at 420 as well as at 280 nm. Gel filtration high performance liquid chromatography indicated formation of high molecular weight (> 200,000) aggregates that could be detected at 225 and 420 nm in the absence or presence of mercaptoethanol. Polyacrylamide gel electrophoresis also suggested formation of large aggregates unable to penetrate the gel. Protein-carbohydrate products were suggested in a negatively charged fraction with high electrophoretic mobility, which was stained with the glycoprotein stain, periodic acid fuchsin sulfite.