Rehydration characteristics of compressed freeze-dried carrot tissue

Abstract

The effects of treatments such as pressure, dwell time, storage, moisture addition, temperature of rehydration, cook time, piece size, chemical additives, and irradiation on rehydration rate of freeze-dried carrot slices and bars were studied. Rehydration was measured by means of rehydration ratio. This value was calculated by dividing the freeze-dried sample weight into the final weight of the rehydrated product. Increases in pressure and dwell time during compression of bars decreased rehydration ratios. Bars packed under 25 in. vacuum and stored at temperatures below 33°F exhibited maximum rehydration ratios. Maximum rehydration rates for freeze-dried compressed carrot bars were obtained from carrot slices pre-cooked for 8 minutes and freeze-dried to 10% plasticizing moisture. The addition of NaCl was shown to be an effective agent to increase rehydration rates. A 24 hr soak in a 0.1 and 0.2 m solutions of NaCl prior to freeze-drying gave maximum rehydration over other salt concentrations and chemical additives. Histological examination of freeze-dried carrot tissue with the scanning electron microscope illucidated rehydration mechanisms. Observation of NaCl crystals depositions and bar interface microstructures related to rehydration characteristics.