Optical parameters of milk fat globules for laser light scattering measurements

This study presents milk fat globule refractive index (n(i)) and absorption coefficient (k(a)) values that can be used reliably for particle size dist ribution measurements by Laser Light Scattering at two different wavelength s. A k(a) approximate to 10(-5) was obtained for liquid milk fat. The n(i) of milk fat globules at 20 degreesC was measured at 589 nm and was calculat ed to be 1.470 at 466 nm and 1.460 at 633 nm. Because the software of the L LS equipment does not allow for differences in n(i,water) at the two differ ent wavelengths, n(i,fat) was corrected to reflect the difference between t he refractive indices of the fat globule and its surrounding medium at each wavelength. For example, if n(i,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. Th ese optical parameters led to reliable milk fat globule size distributions. In any case, k(a) must never be overestimated, since this leads to artefac tual peaks.