Magnetic resonance imaging (MRI) to quantify the swelling and drying of whey protein hydrogels

Magnetic resonance imaging (MRI) is a very powerful technique increasingly used in food engineering, yet examples where the local water content is quantified are scarce. Homogeneous whey protein hydrogels were utilized as a model system, far simpler than most foods. The normalized proton density intensity of hydrogels was predicted using experimental correlations of the spin-lattice and spin-spin relaxation times, T₁ and T₂ respectively. Using a typical echo time T_E of 20 ms, the intensity is maximum at a volumetric swelling ratio Q ∼15, and MRI is suitable to study the drying of hydrogels at lower Q values. Swelling can be studied by adjusting T_E to target the Q range of interest, e.g. ∼200 ms. Whereas reasonable agreement is found between predicted normalized intensities and from drying and swelling experiments, local quantification of Q in unknown conditions will suffer from noise and relatively poor repeatability. Deviations from predictions are observed in the swelling at high NaCl concentrations, and at high alkaline pH, that need to be studied further. Opaque particulate hydrogels can be studied equally well compared to stranded transparent gel, a clear advantage against optical techniques.