Use of multi-angle laser light scattering and size-exclusion chromatography to characterize the molecular weight and types of aggregates present in commercial whey protein products.

In this study, a range of commercial whey protein products were characterized by the use of size-exclusion chromatography coupled with a multi-angle laser light scattering (MALLS) detector. The MALLS system detected some very large-sized material that eluted close to void volume in all samples; this material was hardly detected by concentration detector. It was demonstrated by chitosan treatment that this peak was very small lipid globules or phospholipids, which gave the residual "cloudy" appearance in upper layers after ultracentrifugation of whey products. Composition, molecular weight, and the photo diode array (PDA) spectrum (200 to 400 nm) of the major protein peaks, including: beta-lactoglobulin (BLG), alpha-lactalbumin (ALA), bovine serum albumin (BSA), immunoglobulin G (IgG) and some minor components were analyzed. The molecular weight of BLG, ALA, BSA, and IgG peaks in whey protein isolates (WPI) were 2.3 to 3.7 x 10(4), 1.4 to 1.6 x 10(4), 4.8 to 6.7 x 10(4), and 1.2 to 2.5 x 10(5) Da, respectively. Compared with WPI, WPC has similar major proteins, but more large-sized residual lipid material and different minor constituents such as lactose and nonprotein nitrogen, depending on various commercial samples and protein content. An improved TCA-precipitation method was applied to quantify glycomacropeptide (GMP) in whey proteins, which demonstrated that there was a very low concentration of GMP in WPI manufactured using an ion-exchange process. The molecular weight of GMP was found to be approximately 8600 Da. Size-exclusion chromatography MALLS was demonstrated to be a powerful technique for detailed analysis of the molecular weight of various proteins, aggregates and minor components, such as GMP, in whey protein products.