Optical parameters of milk fat globules for laser light scattering measurements

Abstract

This study presents milk fat globule refractive index (ni) and absorption coefficient (ka) values that can be used reliably for particle size distribution measurements by Laser Light Scattering at two different wavelengths. Aka ≈10-5 was obtained for liquid milk fat. The ni of milk fat globules at 20 °C was measured at 589 nm and was calculated to be 1.470 at 466 nm and 1.460 at 633 nm. Because the software of the LLS equipment does not allow for differences in ni,water at the two different wavelengths, ni,fat was corrected to reflect the difference between the refractive indices of the fat globule and its surrounding medium at each wavelength. For example, if ni,water is taken to be 1.33, then for milk fat globules the corrected value is 1.458 at 633 nm and 1.460 at 466 nm. These optical parameters led to reliable milk fat globule size distributions. In any case, ka must never be overestimated, since this leads to artefactual peaks.