The Effect of Structure and Imbibition Mode on the Rehydration Kinetics of Freeze-dried Carrots

Water ingress is studied in freeze-dried winter carrot pieces, prepared with different thermal pre-treatments before freeze-drying, as a model system for rehydration of dried vegetables in the food industry. Various drying protocols for carrot pieces result in different microstructures and its rehydration properties. Water ingress at room temperature is measured in real-time by in situ 3D magnetic resonance imaging (MRI) and NMR relaxometry. Two modes of imbibition were studied: water ingress into dried carrot pieces by full submersion and floating at the air-water interface. Differences in the time scale and degree of water ingress are observed by MRI, where floating samples show faster and more extensive rehydration. These alterations were not observed in NMR relaxometry at the same field strength. The contrasting observations are explained by susceptibility effects that strongly affect the MRI results, even at low field strength, due to the presence of air-water boundaries in samples rehydrated by full submersion. Clear effects on rehydration kinetics of non-blanched carrot tissue are observed, dependent on pre-treatment induced changes in the meso/microstructure. For blanched samples rehydration kinetics is strongly increased, which is explained by an extra, more dominant effect that enhances water ingress. Moreover, NMR relaxometry shows a slow decreases of T₂ of the water surrounding the carrot pieces, due to leaching of sugars and other solutes. Since no effect on the leaching kinetics is observed for different pre-treatments and the process is much slower than rehydration, it is concluded that this leaching is a diffusion driven process following rehydration.