The impact of freeze-drying on microstructure and rehydration properties of carrot

The impact of freeze-drying, blanching and freezing rate pre-treatments on the microstructure and on the rehydration properties of winter carrots were studied by μCT, SEM, MRI and NMR techniques. The freezing rate determines the size of ice crystals being formed that leave pores upon drying. Their average size (determined by μCT) can be predicted in a quantitative manner by considering dendritic growth and freezing rates. Blanching as a pre-treatment, however, did not affect pore size distribution induced by freeze-drying. Upon rehydration of the freeze-dried carrots, PFG NMR and MRI show that cellular compartments were not restored and instead a porous network with permeable barriers is formed. Blanching pre-treatment introduced a less connected and more anisotropic porous network if followed by fast freezing, indicating that more of the native cell wall morphology is preserved.

► Fast freezing is beneficial to structure preservation. ► Slow freezing allows ice crystals to grow outside cells. ► Fast freezing forces ice crystals to grow inside cells with little cell separation. ► Ice crystals in carrots have a dendritic growth.