VII. QUICK-FREEZING AND FREEZING-DRYING 223 



undergoing desiooation. It is pressure of water that is important 

 and this is a question of the vapor pressure of the ice at the tempera- 

 ture of the product being dried. Air must be removed to a pressure 

 of 100 to 200 ju of mercury and in efficient commercial installations to 

 50 fx. Penicillin is usually dried at about — 30°C., where the pressure 

 is 300 iJL. For blood plasma, —25° or 0.5 mm. of mercury pressure is 

 suitable and serum can be dried at — 10° or 2 mm. These conditions 

 must be determined experimentally for each material. A McLeod 

 gage can be used for measuring total pressure including water vapor, 

 but it must be suitably trapped (3). Time required for desiccation 

 depends upon the quantit}^ in the container and depth of the frozen 

 layer. Plasma in 3C0 ml. quantities requires twenty to forty hours. 

 Cultures in the small quantities usually dried (0.1 to 0.5 ml.) require 

 only five or six hours, but usual practice is to allow overnight drying 

 for convenience. 



Viruses are in general more difficult to dry than other substances 

 and a lower temperature must be maintained during dehydration 

 than is frequently necessary with many other products, preferably 

 well below — 20°C. (8). Some bacterial and virus cultures undergo 

 as much as 95% loss in percentage viability, whereas in certain other 

 cultures as much as 90% survival is obtainable. The menstruum is 

 important as well as the rate of drying in obtaining maximum sur- 

 vival. Delimiting of optimal conditions for the many organisms to 

 which freeze-drying is applied has been carried out for only a rela- 

 tively few. In many cases, as for the maintenance of a library of 

 stock cultures without the necessity for continual subculturing or 

 animal passage, extended partial loss of viability is not of serious 

 consequence. Particularly in the case of live vaccines the percentage 

 of survival is very important and it is here that most work has been 

 centered (2). 



Histological and Cytological Preparations. In the application 

 of the freezing-drying method to animal and plant cytology (18,45), 

 tissue is frozen at a very low temperature, such as in liquid air, which 

 results in killing the tissue. Then it is dried at a temperature suf- 

 ficiently low to avoid appreciable diffusion or displacement of cell 

 constituents. 



Hoerr (45) and other workers (l) have attempted to improve the 

 quality of tissue fixation b}^ resorting to more rapid freezing of the 

 tissue by means of pentane chilled to — 131°C., or isopentane chilled 

 to —195°. Dehydration was carried out quite slowly with the prod- 



