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 laser light scattering (MALLS) system or by the use of MALLS as an on-line detector with size-exclusion chromatography (SEC). Sodium caseinate solutions, analyzed using a MALLS system alone, gave weight-average molar mass (M-w) values 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 ultracentrifuged at 90000g for 1 h, a cloudy top layer was formed; the subnatant was carefully removed and analyzed by SEC-MALLS. The M-w values were found to be in the range similar to 30-575 kDa, and R-g values ranged from similar 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 hardly detected by the concentration detectors, i.e., ultra-violet (UV) and differential refractive index (DRI). The intensity of the light scattering (LS) signal from this very large sized material was greatly reduced in the subnatant. 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 very large for a protein molecule, and initial calculations suggested that the shape of caseinate molecules was likely to be highly elongated.