Effect of air-dehydration pretreatment before freezing on the electrical impedance characteristics and texture of carrots

The impedance characteristics, rehydration ratio and texture of dried-rehydrated carrot and rehydrated carrot after dehydrofreezing-thawing were evaluated to investigate the effect of air-drying on the dehydrofreezing of carrots and to determine how cell membrane damage affects the texture of sample tissues. Air-drying of carrot samples was applied at several temperatures to remove various amounts of water as a pretreatment to freezing. Electrical impedance analysis using an equivalent circuit model for biological tissues indicated there was cell membrane damage in the rehydrated samples after drying at the temperature of 80 °C. In addition, the sample dried at the higher temperature had a low rehydration ratio and the texture of the sample was deteriorated. On the other hand, no cell membrane damage was observed in the sample dried at a lower temperature of 40 °C and the texture was the same as that of the fresh sample. However, the texture, especially the elastic properties, of the dehydrofrozen-thawed samples was significantly deteriorated. Also, the electrical impedance analysis revealed that the cell membrane of the dehydrofrozen-thawed samples dried under any condition were significantly damaged. The results suggested that freezing damage of cell membrane associated with the loss of cell turgor is one of the main factors that affect the texture of fruits and vegetables. The samples dried to a moisture content of 4.0 and 2.0 (dry basis, g-water/g-dry) showed a partial improvement in texture compared with the fresh frozen sample, and this is thought to be due to a limitation of ice crystal development by dehydration prior to freezing.