WATER AS A PHYSICAL FACTOR 267 



butter is to be prevented by salt, it is unnecessary to give any attention 

 to the fat; the bacteria live only in the water and not in the fat globules. 

 In adding 3 per cent of salt to a butter with 15 per cent of moisture, a 

 brine of 3 parts of salt in 15 parts of water is produced; in other words, 

 a 20 per cent brine, because salt does not dissolve in the fat. Similar 

 considerations will come up in the preservation of fruit, vegetables, meat, 

 milk, and other food substances by drying or condensation. 



DESICCATION. Microorganisms do not die immediately after the 

 removal of the water, and they do not die all at once after a given time. 

 Death through drying is a slow and regular process. Paul and his 

 associates found that the number of bacteria dying in the unit of 

 time is, under constant conditions, proportional to the number sur- 

 viving. If we had 1,000,000 cells per gram in the beginning, and the 

 death rate were 90 per cent per day, there would be, at the end of each 

 day, 10 per cent of the original number surviving. This would give the 

 following numbers for one week: 



Beginning 1,000,000 cells per gram. 



After i day 100,000 cells per gram. 



After 2 days 10,000 cells per gram. 



After 3 days 1,000 cells per gram. 



After 4 days 100 cells per gram. 



After 5 days 10 cells per gram. 



After 6 days i cell per gram. 



After 7 days o.i cell per gram. 



This table shows graphically the mode of death of dried bacteria. The 

 number of cells approaches zero without ever (at least theoretically) 

 reaching it. From one cell per gram after six days we do not come to 

 o on the seventh, but to one cell in 10 g. and on the eighth day one 

 cell in 100 g. The total number dying in the first day is much larger 

 than that dying on the sixth day, but the rate is constant, 90 per cent 

 of the number surviving. This regularity has been found with bacteria 

 dying from various causes, and it is commonly compared with the 

 simplest chemical processes, the monomolecular reactions. 



Paul and his associates found further, that the death through drying 

 is caused by an oxidation process; in pure oxygen bacteria died much 

 faster. The poisonous effect of oxygen upon moist bacteria has already 

 been pointed out on page 228. 



Most resistant to drying are the spores of bacteria; mold spores, 



