Determination of the size distribution of bovine casein micelles using photon correlation spectroscopy

Abstract

Photon correlation spectroscopy may be used to determine particle diffusion coefficients. By calculations involving two parameter Schulz-Zimm size distributions contaminated with trace amounts of larger particles, it has been demonstrated that the effects of the large particle component on the measured diffusion coefficient are observed first at small angles and that diffusion coefficients at wider angles depart from their zero contaminant values only at much higher levels of contamination. Provided the particles described by the Schulz-Zimm distribution are large enough to exhibit intramolecular interference, it has been confirmed that size distribution parameters can be extracted from the slope of a linear fit to this wide-angle data and its intercept at zero-angle. The casein micelle system from skim-milk contaminated by much larger fat globules exemplifies the systems described in these model calculations. Diffusion coefficients have been measured as a function of the scattering angle for casein micelles subjected successively to a series of fat-removal techniques. Integrated scattering measurements in the same angular range confirmed the loss of fat and also showed micellar scattering unaffected by the procedures. Though still causing the diffusion coefficients measured at angles less than 70° to increase, the final stages of fat removal had no effect on the values obtained at angles ⩾80°. These wide-angle data were then used to compute Schulz-Zimm distribution parameters to describe the casein micelle size distribution.