214 EARL W. FLOSDORF 



water vapor and by rapid application of heat to the frozen product, 

 rather than keeping it in an icebox, the process could be made more 

 practical, with resulting improvement in products. Even while 

 being heated, products may be maintained at any desired range of 

 temperatures such as —10° or around — 40°C. simply by regulating 

 the degree of vacuum and rate of evaporation. 



There are two stages in drying by sublimation. In the first, ice 

 is evaporated from a frozen mass. In the second, moisture is re- 

 moved from the final dry solid to lower the residual content to a 

 minimal level. During the first stage, depending on the particular 

 product, some 98 to 99% of all water is removed. In the second, 

 the residual moisture content is reduced to 0.5% of the final product 

 or less; this represents final removal of 99.95% of the original con- 

 tent of water (assuming 10% solids originally). In the first stage, 

 temperatures are well below 0°C. Actual temperature varies with 

 the prodi^ct, as will be discussed later. Upon passing from the first 

 stage to the second the temperature gradually rises and finally 

 reaches that of the room or higher, depending upon whatever final 

 ambient temperature is used. 



Basically, during the first stage of drying a maximal rate of evapo- 

 ration for a given temperature of the frozen product is desirable. To 

 achieve this under some circumstances it may be necessary to heat the 

 frozen product but without causing it to soften or melt. At the same 

 time a maximal rate of flow away from the evaporating surface must 

 be estabhshed. To accomplish this rapid flow adequate passageways 

 must be provided for vapor, and this must then be condensed or 

 evacuated efficiently. 



C. POTENTIAL APPLICATIONS 



1. Quick- Freezing Materials for Storage 



Frequently in carrying out an investigation with an unknown bio- 

 logical system, it has not been established whether there are labile 

 components present or not. Manipulations may extend over a mat- 

 ter of days or even months. By utilizing quick-freezing and storage 

 at low temperature at all times when not actually working with 

 preparations, loss of possible labile components is avoided. This 

 type of investigation was encountered in studies with the antigenic 

 structure of Hemophilus pertussis (3). Eventually it was found that 



