Using the USAXS technique to reveal the fat globule and casein micelle structures of bovine dairy products

Cows' milk is a commodity used worldwide to make many dairy products. We have used the ultra small angle X-ray scattering (USAXS) technique to reveal the fat globule and casein micelle structures of some dairy products. USAXS covers the q-range 5 × 10^-4 Å^-1 < q < 10^-1 Å^-1, thereby allowing the study of micron-scale structures present in those dairy products. We measured the USAXS intensity, Iq, as a function of the scattering vector magnitude, q, for samples of skim milk, non-homogenized whole milk, homogenized whole milk, half and half and heavy cream, at two temperatures, 7 °C and 45 °C. The data collected from the scattering experiments were fitted using the Unified fit model run under the IRENA software from the Advanced Photon Source, Argonne National Laboratory (Illinois, USA). The fittings were carried out when the data were plotted as log[I(q)] vs log[q]. We observed a combination of linear regions (LRs) and knees. Two parameters of interest were obtained from the fittings, a radius of gyration, R_g, and a Porod exponent, P. Unified fit allowed us to fit up to four structural levels. One of the knees was centered at q ≈ 8 × 10^-3 Å^-1 for all samples measured at 7 °C, but vanished at 45 °C. Two LRs were identified as being either due to casein micelles (CMs) or to fat globules (FGs). The porod exponent obtained from these LRs allowed us to describe the surface morphology of CMs and FGs. Two of the R_g values gave a rough estimate of the FGs and CMs sizes. FGs were identified for samples of homogenized whole milk, half and half and heavy cream in the q-region 2 × 10^-4 < q < 8 × 10^-4 Å^-1. We found that, in the absence of chymosin, or changes in pH, CaCl₂ concentration or temperature changes, skim milk and non-homogenized whole milk displayed a Porod exponent that indicated a behavior characteristic of aggregation. Using computer simulations, we found that, seemingly, bovine CMs spontaneously formed approximately 1-dimensional aggregates possibly analogous to swollen randomly branched polymers.