4 H. O. HALVORSON 



cells to repair this damage before the leaching and other sub- 

 sequent undesirable changes become lethal. The manufacturers 

 of bakers' yeast have been able to dry yeasts and retain a high 

 degree of viability by carefully controlling the strain, growth 

 conditions, method of desiccation, and method of rehydration. 

 The problem is complex, and it is likely that optimum conditions 

 will vary with each variety of cell. 



In bacterial cells, there seems to be some correlation between 

 the size of the cells and the ease with which they can be preserved 

 by desiccation. In general, the smaller cells can be preserved in 

 this manner more successfully than the larger ones. It is easier, 

 therefore, to keep cocci alive in a dry state than yeasts, and the 

 vegetative cells of the Gram positive spore-forming bacteria are 

 more easily damaged by drying than the smaller Gram positive 

 lactic acid bacteria^. 



With most dried cells there is a slow 'die off' during storage. 

 Here again, protective additives can have a marked effect on the 

 stability of the cells. It is felt by many that the dam.age during 

 storage may be due in part at least to oxidation and, therefore, 

 that cells will be more stable if stored in an inert gas than in air 

 or oxygen. This has been found to be true particularly with the 

 viruses. Here, storage in the presence of air is quite damaging^. 

 With these, to attain long time livability, it is necessary to dry 

 and store the cells in an inert atmosphere at all times. Exposure 

 to air even for a short period may be quite injurious, because, if 

 the air comes in contact with the cells, enough oxygen is 

 absorbed to produce products that are lethal. This lethal effect 

 may be due to the formation of peroxides or hydroperoxides, 

 which are known to be very toxic. 



Cells of protozoa, fungi, and bacteria that are properly dried 

 can be kept viable for long periods of time. By this, we do not 

 mean that all the cells in the population remain alive, but 

 enough do, to allow one to recover an active culture when the 

 material is rehydrated. This, of course, can happen even though 

 only a very small fraction of the initial population remains 

 viable. In our own laboratories, we have recovered active 



