Advances in Transport Phenomena during Freeze-Drying of Food Materials : Fundamentals and Applications

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Measurement method and values of thermal conductivity and permeability of water vapor were presented for the dried layer of food materials undergoing freeze-drying. Some structural models were developed for predicting the permeability of water vapor flowing through the dried layer. In a cellular food model, the resistance of a cell membrane to the molecular transfer of water vapor was determined from both value of permeability and microscopic observation of average diameter of cells. The model was considered to play an important role in predicting optimum heating program for the surface temperature of materials. A micro-slicer image processing system (MSIPS) has been developed for measuring the three-dimensional (3-D) structure and distribution of ice crystals formed in frozen food materials. The system has functions to reconstruct the 3-D image based on the image data of exposed cross sections obtained by multi-slicing of a frozen sample with the minimum thickness of 1 μm and to display the internal structure as well as an arbitrary cross section of the sample choosing observation angles. The effects of freezing conditions on the morphology and distribution of ice crystals were demonstrated quantitatively from the observations of raw beef stained by fluorescent indicator of muscle fibers. The scale-up procedures were described for optimizing the design and operation of industrial freeze-dryer to produce dried-egg soups.
2001-08-01