Characterization of commercial and experimental sodium caseinates by multiangle laser light scattering and size-exclusion chromatography

A range of sodium caseinate samples were characterized by a multiangle lase r light scattering (MALLS) system or by the use of MALLS as an on-line dete ctor with size-exclusion chromatography (SEC). Sodium caseinate solutions, analyzed using a MALLS system alone, gave weight-average molar mass (M-w) v alues in the range 1200-4700 kDa and z-average root-mean-square radius (R-g ) values ranged from similar to 50 to 120 nm. When these solutions were ult racentrifuged at 90000g for 1 h, a cloudy top layer was formed; the subnata nt was carefully removed and analyzed by SEC-MALLS. The M-w values were fou nd to be in the range similar to 30-575 kDa, and R-g values ranged from sim ilar to 22 to 49 nm. During SEC, the MALLS system detected some very large- sized material that eluted close to the void volume; this material was hard ly detected by the concentration detectors, i.e., ultra-violet (UV) and dif ferential refractive index (DRI). The intensity of the light scattering (LS ) signal from this very large sized material was greatly reduced in the sub natant. SEC of sodium caseinate samples revealed two main peaks with M-w of similar to 420-750 kDa and 39-69 kDa, respectively. The R-g values were ve ry large for a protein molecule, and initial calculations suggested that th e shape of caseinate molecules was likely to be highly elongated.