1st Nordic Drying Conference - NDC'01, Trondheim, Norway, June 27-29, 2001

OPTIMUM HEATING TEMPERATURE OF CELLULAR FOOD MATERIALS UNDERGOING FREEZE-DRYING Tetsuya ARAKI and Yasuyuki SAGARA The University of Tokyo, Dept. of Global Agricultural Sciences, Yayoi 1-1-1, Bunkyo-ku, Tokyo, 113-8657, Japan, E-mail: aa97153@mail.ecc.u-tokyo.ac.jp or asagara@mail.ecc.u-tokyo.ac.jp Keywords: freeze-drying, structural model, apple, permeability, process optimization ABSTRACT Both sliced and mashed apples have been freeze-dried at various surface temperatures under the usual pressure range of commercial operations. The surface of sliced samples could not be maintained at above 10°C in order to prevent the frozen layer from melting, while that 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 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. The model provides an optimum heating program for the surface temperature of a cellular food material whose drying rate is limited by mass transfer rate. Optimum heating temperature was determined for the surface of sliced samples by applying its actual transport properties to the model, and was found to be 8.5°C under this experimental pressure range.

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