Low-Field NMR Relaxometry for Elucidating the Effect of Temperature on the Rehydration Kinetics and pH and Ionic...

Rehydration of texturized vegetable proteins (TVPs) is decisive for their functional properties, but little is known in relation to how rehydration conditions impact their ability to absorb and retain water. The present study aimed to investigate how rehydration dynamics and intrinsic water mobility of texturized soy protein are affected by temperature, pH, and ionic strength under dynamic or equilibrium conditions. Low-field NMR T₂ relaxation measurements were conducted dynamically on soy TVP during rehydration under different combinations of pH (5.4, 6.2, or 7.0), ionic strength (0.29, 0.46, or 0.71 M), and temperature (15, 25, or 35°C). Rehydration could be tracked from a decrease in the T₂ relaxation time constant, reflecting a reduced mobility of water protons when water molecules were immobilized in intrinsic structures. Kinetic modeling revealed a significant effect of temperature on the rate of rehydration, whereas pH and ionic strength did not affect the rate of rehydration. To evaluate equilibrium conditions, 24 h soaking experiments were conducted, which revealed significant effects of pH on the rehydration capacity and NMR T₂ relaxation. Increasing pH increased the rehydration capacity and T₂ relaxation time constants, indicating that water compartmentalization in TVPs can be modulated through pH effects on electrostatic repulsion. In conclusion, NMR relaxometry enables kinetic studies of soy TVP rehydration, and this study demonstrates that rehydration kinetics and equilibrium rehydration capacity can be decoupled. Temperature primarily governed the rate of water uptake, whereas pH influenced final rehydration capacity, while ionic strength in the range applied here did not affect rehydration capacity.