Freeze-drying of food

Fig. 2.52. Schematic comparison of two commonly used tunnel freeze drying systems for the freeze drying of food and luxury food.

Di Matteo, P, Donsi, G. and Ferrari, G., The role of heat and mass transfer phenomena in atmospheric freeze-drying of foods in a fluidised bed, /. Food. Eng., 59 (2003) 267-275. 

Sublimation plays a major role in the freeze-drying of foods. See also Freeze-Drying. 

G. W. Oetjen, W. Ehlers, U. Hackenberg, J. Moll, and K. Neumann, Temperature measurements and control of freeze-drying processes, in Freeze-Drying of Foods, Natl. Acad, of Sciences, Natl. Res. Council, ed., Washington, D.C., 1962, pp. 25-42. 

C. J. King, Freeze-Drying of Foods. CRC Press, Cleveland, 1971. 

Freeze drying Freeze drying of foods takes place in a freeze dryer at vacuum levels of 0.4 to 1.3 mbar absolute, corresponding to sublimation temperatures from -30 to -17°C depending on the product requirements. The main components of the freeze dryer are... 

Suppose we move back to solid at point (355 torr and - 10°C). If we now hold the temperature at — 10°C and reduce the pressure, we move vertically down along EJ. At a pressure of 2.1 torr we reach the suhlimation curve, at which point the solid passes directly into the gas phase (sublimes) until all the ice has sublimed. An important application of this phenomenon is in the freeze-drying of foods. In this process a water-containing food is cooled below the freezing point of water to form ice, which is then removed as a vapor by decreasing the pressure. 

Toumy, J. M. Freeze Drying of Foods for the Armed Forces, Technical Report 70-43-FL, U.S. Army Natick Laboratory ... 

Sublimation is the transfer of a substance from the solid to the gaseous state without formation of an intermediate liquid phase, usually at a relatively high vacuum. Major applications have been in the removal of a volatile component from an essentially nonvolatile one separation of sulfur from impurities, purification of benzoic acid, and freeze drying of foods, for example. The reverse process, desublimation (16), is also practiced, for example in the recovery of phthalic anhydride from reactor effluent. The most common application of sublimation in everyday life is the use of dry ice as a refrigerant for storing ice cream, vegetables and other perishables. The sublimed gas, unlike water, does not puddle and spoil the frozen materials. 

Economical microwave equipment suitable for the requirements of industrial freeze drying of foods and pharmaceuticals on a large continuous scale is not yet available. 

By far the largest number of the industrial freeze dryers in operation is of the vacuum batch type with freeze drying of the product in trays. There are two main types, depending on the type of condenser used. In the first type, the condenser plates are alongside the tray-heater assembly and in the same chamber in the second type the condenser is in a separate chamber joined to the first by a wide, in general, butterfly valve. This latter type of plant is always used in pharmaceutical industries, but it can also be used for the freeze drying of foods. Because of the wide variety and complexity of the... 

K.E. Yassin and H. Gilbert, Atmospheric freeze-drying of foods in a fluidized bed of finely divided absorbent. In Proceedings of the Sixth International Congress on Food Science Technology, No. I, 1983, pp. 208-209. 

Fisher, R. R. Freeze Drying of Foods. National Academy of Sciences, National Research Council, Washington D. C. 1962. 

Diffusion of gases in small pores occurs often in heterogeneous catalysis where gases diffuse through very small pores to react on the surface of the catalyst. In freeze drying of foods such as turkey meat, gaseous HjO diffuses through very fine pores of the porous structure. 

Sublimation can be used to purify solids that readily vaporize. Figure 11.3 shows a simple way to purify iodine by sublimation. Impure iodine is heated in a beaker so that it vaporizes, leaving nonvolatile impurities behind. The vapor crystallizes on the bottom surface of a dish containing ice that rests on top of the beaker. Freeze-drying of foods is a commercial application of sublimation. Brewed coffee, for example, is frozen and placed in a vacuum to remove water vapor. The ice continues to sublime until it is all gone, leaving freeze-dried coffee. Most freeze-dried foods are easily reconstituted by adding water. 

Kan, B., 1962. Methods of determining freezedrying process end-points, in Freeze-drying of foods (ed. F. R. Fisher). National Academy of Sciences - National Research Council, Washington, D. C., USA, pp. 163-177. 

Neumann, K. H., 1968. Determining temperature of ice, in Freeze-drying of Foods and Biologicals (ed. R. Noyes). Noyes Development Corp., Park Ridge, USA,... 

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