Sensitive real-time measurement of the refractive index and attenuation coefficient of milk and milk-cream mixtures

Abstract

We demonstrate a first simultaneous measurement of both the refractive index and the attenuation coefficient (defined as the sum of the scattering and absorption coefficients) of highly turbid milk and milk-cream mixtures. We achieve this by observing the real-time reflectance profile of a divergent laser beam made incident on the surface of the milk sample. The experiments were carried out on commercial milk samples with fat volume concentrations of 0.5 or less, 1.6, and 3.3%, and on milk-cream mixtures with fat volume concentrations of 10 and 33.3%, without any dilutions of these samples. We find that the reflectance data are well described, for the first time without any empirical fit-parameters, by Fresnel theory that correctly includes the effect of angle-dependent penetration into the turbid medium on the total internally reflected signal. Therefore, our method provides the most accurate determination to date of the refractive index and attenuation coefficient of milk and milk-cream mixtures. Our sensor is compact, portable, and inexpensive.

Polymorphism, microstructure and rheology of butter. Effects of cream heat treatment

“…For determination of zeta potential the refractive index (RI) of the cream must be known. For all samples it was assumed that the RI was 1.39 (Calhoun, Maeta, Roy, Bali, & Bali, 2010). The zeta potential generated from the samples is a combination of the signal from the fat globules as serum phase in which they are dispersed.…”

Accuratein situmeasurement of complex refractive index and particle size in intralipid emulsions

“…We initially tested our theoretical model for TIR in a highly turbid medium [i.e., Eq. (2)] in milk-cream mixtures, 16,17,18 and now have performed a more rigorous test in intralipid emulsions. However, both milk and intralipid are polydisperse media for which no reference data exists on various optical parameters, including the complex refractive index and particle size.…”

“…It is noteworthy that, despite all the attention, a precise in situ determination of several important optical properties of intralipid emulsions, such as the particle size, refractive index, and attenuation coefficient (which is commonly expressed as the imaginary part of a complex refractive index [13][14][15][16][17][18][19][20][21] ), has continued to elude researchers. 28 The chief difficulty arises from the fact that intralipid emulsions, being highly turbid, have a large attenuation coefficient.…”

“…29 Other attempts to extract the complex refractive index by modeling the reflectance data either introduce extraneous fitting parameters (beyond the two parameters of interest, namely, the real and imaginary parts of the refractive index) resulting in overfitting of the data 19,20 or focus on only the critical angle region, which is just a small subset of the reflectance data. [14][15][16][17][18] . Both approaches have built-in arbitrariness causing wide variation in extracted values.…”

“…Our method employs a real-time sensitive measurement of total internal reflection (TIR) of a divergent laser beam from the sample surface, over the full range of incident angles, i.e., for reflectivity values going from unity in the TIR regime to nearly zero in the non-TIR regime where almost all the light is transmitted. We employ a new empirical model we introduced earlier 16,17,18 that incorporates angle-dependent penetration of the incident light into the medium during TIR. However, in contrast to Refs.…”