Multiscale structure analysis reveals changes in the structure of casein micelles treated with direct-steam-infusion UHT

Direct-steam-infusion ultra-high temperature (dUHT) technology has been paid much attention to in recent years because of its short heating time, which helps retain more nutritional ingredients and extend shelf life of dairy products. However, protein instability is an urgent problem for dUHT milk to solve. As the main component of protein, it is unclear whether and what structural changes casein micelles (CMs) undergo after dUHT treatment. In this study, CM dispersions were treated with dUHT technology (140 °C/3 s, 145 °C/3 s, 150 °C/3 s, 155 °C/3 s, and 157 °C/0.116 s), and unheated sample was taken as the control. As the temperature increased, hydrodynamic diameter of the CMs reduced from 190 nm to 156 nm, zeta potential did not change significantly, and turbidity decreased significantly. After dUHT, contents of casein, calcium, and phosphorus in the serum phase increased. Furthermore, dUHT treatment would lead to disruption of intermolecular hydrogen bonds, rearrangement of secondary structure, and unfolding of CM. The 31P NMR spectra demonstrated that colloidal calcium phosphate solubilized and electrostatic repulsion within micelles increased with dUHT treatments. Further analysis of molecular structure of CMs using small-angle X-ray scattering and a cryo-transmission electron microscope found that after dUHT treatment, CMs dissociated in the form of casein clusters (20–40 nm) and became loose and porous in structure. In conclusion, dUHT treatment destroys the calcium bridges and increases electrostatic repulsion inside micelles, leading to dissociation of casein clusters and a smaller, looser, and more porous structure of the CMs.