Rehydration characteristics of milk protein concentrate powders monitored by electrical resistance tomography

Electrical resistance tomography (ERT) is a robust and low-cost method offering real-time visualization of processes. In this work, we developed an ERT-based method to characterize the rehydration behavior of milk protein concentrate (MPC) powders. Circular-type and linear configurations were used to achieve high resolution in the radial and axial directions, respectively. To evaluate the rehydration profile, MPC powders were reconstituted to 2.5% (wt/wt) total solids at room temperature, and the rehydration behavior of the MPC powders [MPC with 85% protein (MPC85) and milk protein isolate with 90% protein (MPI90)] was monitored for a dissolution time of 30 min using the ERT system. The MPC powders were characterized in terms of overall mean conductivity, area under the mean conductivity curve, slope at a dissolution time of 3 min, and the relative dissolution index. Additionally, the focus beam reflectance measurement (FBRM) was used as a reference method to follow rehydration characteristics. Particle count changes from the FBRM measurements showed that MPI90 had higher larger particle counts and more resistance to dispersing in water. As the dissolution time proceeded, mineral ions and proteins were released and consequently increased the overall conductivity, confirming the transfer of water into MPC particles. At lower protein contents, the particle dispersion rate was higher and an increase in overall mean conductivity was observed, indicating better powder dissolution. Both configurations were able to effectively monitor differences in the dissolution behavior of MPC powders. In the ERT circular configuration, MPC85 and MPI90 showed maximum conductivity of 0.201 ± 0.006 and 0.162 ± 0.001 mS/cm, respectively. In the linear probe configuration, MPC85 and MPI90 showed maximum conductivity of 0.161 ± 0.001 and 0.136 ± 0.001 mS/cm, respectively, suggesting increasingly inhibited water transfer as the protein content of the powder increased. In this study, we demonstrated the capability of ERT using the circular and linear probe configurations to offer, in addition to qualitative tomographic images, reliable quantitative data by which to characterize the dissolution behavior of high-protein dairy powders.