Transport Properties Of Cellular Food Materials Undergoing Freeze-Drying

Abstract

Both sliced and mashed apples have been freeze..Jried at various surface temperatures under the usual pressure range of commercial opera/ions. The surface of sliced samples could no/ be maintained at above IOtt: in order /0 prevent the frozen layer from melting. while tha.t of mashed samples was allowed to heat up to 70"C. Thermal conductivities and permeabilities were determined by applying the uniformly-retreating-ice front model to the dried layer of the samples undergoing freeze-drying. The effects of freezing rate on transport properties were found to be critical for the mashed samples. The results Indicated that the drying rate of sliced samples was limited by the transfer rate of water vapor flOWing through the dried layer. A cellular structural model was proposed for predicting the permeability of the dried layer. based on the resistance of a cell membrane to the molecular transfer of water vapor.