Defrosting of Internal Vapor Condenser during Final Drying and Its Application to Food Production Freeze Dryers

Abstract

A vapor condenser located internally in the drying chamber without shut-off valve having many advantages, it is believed that the condenser must be regenerated while the dryer is not in service.

It is, however, interesting to note that ice temperature for defrosting must be 0°c and its equilibrium vapor pressure corresponds to the relative humidity of 6.5 to 2.5% which is at the final drying temperature of foods, 45°c to 65°c, and is sufficiently low in humidity as compared with the water activity of 0.2 suitable for storage of dried foods. If, therefore, ice build-up on the condenser is melt off by use of low temperatured (near 0^°c) vacuum steam, without affecting dried product with spray or mist incidental to defrosting, the condenser will be completely regenerated in the final drying stage. Based on this concept, we have developed a new defrosting system for the internal condenser.

In the final drying stage, small amount of defrost water is poured in the bottom of the drying chamber which is steam jacketed, and is heated to generate vacuum steam which defrosts the condenser coils set on both sides of the shelf stack. The moisture content of the dried food taken out from the chamber shows very satisfactory value and has no difference from that of dried products processed with the dryers with external condenser.

Food production freeze dryers of batch type with this new defrosting system are already in operation in large numbers.