222 EARL W. FLOSDORF 



paring fibrinogen and prothrombin, there may be inactivation of the 

 product as a result of loss of carbon dioxide, thus producing alkalinity. 

 This inactivation may be prevented, however, by buffering the orig- 

 inal solution quite strongly or by acidulating just before drying. 

 Lyophilic colloids are well preserved, but lyophobic colloids lose their 

 original properties more or less in proportion to the extent of their 

 lyophobic characteristics. 



The complementary activity of guinea pig serum is one of the bet- 

 ter examples of a product with a limited life even though its duration 

 is extended manyfold as a result of freeze-drying. Refrigeration tests 

 carried out as long as five years from the date of preparation show 

 little or no loss, but at room temperature the potency begins to fall 

 off before this time. A similar situation exists with viruses. The 

 final residual content of moisture in the product is of utmost impor- 

 tance in many cases in determining the life of the preparation. In the 

 case of products in the intermediate class, which have a limited degree 

 of preservation, the residual moisture is usually more critical than 

 otherwise. In all cases it is well to try to obtain the final content of 

 moisture in the range of 0.5 to 1% for maximal life. 



Concentrated globulin preparations dissolve very slowly in re- 

 constitution. Instead of being a matter of minutes, the time may be 

 extended to one-half hour or more. In this case, however, there is no 

 loss in biological activity after the product has become dissolved com- 

 pletely. 



With regard to the percentage of viable cells remaining in cul- 

 tures of bacteria after desiccation there is wide variation, depending 

 upon the species and also the particular strain ; this frequently may 

 be as little as 5%, but, since the surviving cells keep well, there is 

 adequate survival for the purpose of maintenance of stock cultures. 



With viruses, there is usually less activity after processing, but, 

 as with bacteria, the final desiccated product is well stabiUzed and no 

 further loss occurs during storage (7). Wooley (16) has reported 

 that lymphocytic choriomeningitis and St. Louis encephalitis after 

 freeze-drying had been preserved for 378 and 833 days, respectively, 

 tests for longer periods not having been made. 



2. Optimum Dehydrating Conditions 



Cultures and Miscellaneous Materials. The actual degree of 

 vacuum required is a function of the temperature of the material 



