Abstract
Abstract An account is given of some of the more purely physical aspects of the process of drying sensitive materials in vacuo at a low temperature. A relation is established on theoretical and experimental grounds, between the mean air pressure in the desiccator, the rate of drying, and the difference between the vapour pressures at the evaporator and condenser. Considerations arising from the shape and size of the desiccator, evaporator and condenser ate discussed. The process whereby sensitive materials are dried in the frozen state in a moderately high vacuum is now well established and has been used on a commercial scale for the preparation of dried blood plasma and penicillin.1,2,3,4 In view of its high cost it has not been applied to foodstuffs to any considerable extent, although the results are in most cases superior to those of any other method of drying, and rival those of quick‐freezing.5,6,7 However, good results have been obtained with foodstuffs at higher temperatures and pressures than those usually considered necessary; this paper describes some experiments and observations on the process under such conditions which are independent of the material being dried; a second paper will describe the results obtained with green peas. The process, as it will be considered here, consists essentially in the sublimation of the ice contained in the frozen material (the “evaporator”) and its condensation as a layer of ice upon a cold surface (the “condenser”); it takes place in a vessel known as the “desiccator,”, the pressure in which is wry considerably reduced. The rate of drying is controlled almost entirely by the rate at which the evaporator is heated, since apart from losses due to radiation and conduction, which arc very small until the concluding stage is reached, all the heat applied must be used as latent heat of sublimation. In order that the final product shall be of satisfactory quality, an upper limit must be fixed to the temperature of the material at each stage of the process. Since the principal factor influencing the temperature of the evaporator is the vapour pressure around it, one requirement of a satisfactory plant is that the desired rate of drying may take place with a sufficiently low vapour pressure at the evaporator.
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