Rehydration kinetics of freeze-dried carrots

• Rehydration kinetics of blanched carrot is fast because sugar mobilization results in less swelling.
• Rehydration kinetics of non-blanched carrot is determined by the interplay between capillary suction and swelling.
• Imbibition mode does not affect water uptake but has effect on air entrapment.
• Rapid carrot rehydration is followed by slow, diffusion-driven leaching of solutes.

Rehydration kinetics by two modes of imbibition is studied in pieces of freeze-dried winter carrot, after different thermal pre-treatments. Water ingress at room temperature is measured in real time by in situ MRI and NMR relaxometry. Blanched samples rehydrate substantially faster compared to non-blanched samples, independent of their porous microstructure. It is proposed that for non-blanched tissues immobilized sugars result in nearly complete swelling of the solid matrix, hindering the ingress of water through the porous network. Non-blanched carrot pieces frozen at − 28 °C rehydrate faster compared to those frozen at − 150 °C, due to blocking of smaller pores by swelling. In blanched tissues the mobilization of sugars results in a more homogeneous sugar distribution, leading to less swelling of the solid matrix and allowing fast ingress of water via capillary suction.Industrial relevanceThe dried fruits and vegetables that are currently available on the market are a poor compromise between convenience (rehydration kinetics) and sensorial quality. This is a major bottleneck for consumers to “Make the Healthy Choice the Easy Choice” and this also negatively impacts market growth. Currently, rational optimization of drying processes is impeded by lack of insight which structural features determine rehydration kinetics (convenience) and texture (sensorial quality) upon rehydration. We therefore started a program to quantitatively assess and model microstructural features and rehydration behavior of freeze-dried carrots as a model system.