Broadband Optical Properties of Milk

Abstract

Dairy products play an important role in our daily nutrition. As a turbid scattering medium with different kinds of particles and droplets, each alteration of these components changes the scattering properties of milk. The goal of this work is the determination of the amount of main scattering components, the fat droplets and the casein micelles, by understanding the light propagation in homogenized milk and in raw milk. To provide the absolute impact of these milk components, the geometrical and optical properties such as the size distribution and the refractive index (RI) of the components have to be examined. We determined the reduced scattering coefficient [Formula: see text] and the absorption coefficient [Formula: see text] from integrating sphere measurements. By use of a collimated transmission setup, the scattering coefficient [Formula: see text] was measured. Size measurements were performed to validate the influence of the fat droplet size on the results of the scattering properties; also, the RI of both components was determined by the said coefficients. These results were used to determine the absolute impact of the milk components on the scattering behavior. By fitting Mie theory calculations on scattering spectra [Formula: see text] and [Formula: see text] from different raw milk samples, it was possible to get reliable values for the concentrations of fat and casein and for the size of the fat droplets. By destroying the casein micelles, it was possible to separate the influence of the different scattering components on scattering behavior.

Dependency of the optical scattering properties of human milk on casein content and common sample preparation methods

“…5). Also the Mie calculated values for g show a maximum around the fat globule size distribution of the samples that underwent 1 min of sonication, which explains the experimental results for g. Our experimental results for sonicated samples agree well with the study on bovine milk by Stocker et al, 15 who measured the same trends for μ s and μ 0 s with sonication time. Compared to gently inverting, we observed a 20% reduction in μ 0 s for the vortexed samples, whereas μ s only showed a decrease of 5%.…”

“…The fifth milk sample was used to investigate the contribution of casein micelles to the scattering properties of milk. Hereto, we adapted the method from Stocker et al 15 to denature the casein micelles by adding an excess of 75 μl of 0.5-M ethylenediaminetetraacetic acid (EDTA) to the 5-ml milk sample. This denatures the casein micelles by destroying the calcium phosphate nanoclusters inside the casein micelles.…”

“…18 After denaturation, casein micelle scattering becomes negligible. 15 The samples containing EDTA were homogenized by gently inverting the sample by hand. Following all sample preparation methods described above, we prepared 5 samples per donor, yielding a total of 25 samples.…”

“…Sonication is a crucial preparation step for NIR and MIR human milk analyzers, as it reduces the average size of the milk fat globules, and thereby reduces the cross talk from optical scattering to the measured absorption. 11,15,16 Human milk sonication methods described in the literature are highly variable in terms of sonication power and duration. 11,15,16 Next to fat globules, casein micelles (with radii around 100 nm) are the second most dominant scattering particles in human milk.…”

“…11,15,16 Human milk sonication methods described in the literature are highly variable in terms of sonication power and duration. 11,15,16 Next to fat globules, casein micelles (with radii around 100 nm) are the second most dominant scattering particles in human milk. Therefore, the second aim of this study is to also quantify their contribution to the scattering properties of human milk.…”