MOISTURE. 151 



volved. Butter is an excellent medium for many bacteria, yeasts, and 

 molds, though it contains only 12 to 15 per cent of moisture. If butter fat 

 were soluble in water, the concentration of 85 parts of solids in 15 parts of 

 liquid would certainly prevent any growth whatever, but fat is insoluble, 

 and the fat particles do not interfere at all with the growth of microorgan- 

 isms in the droplets of buttermilk distributed all through the butter. 

 The concentration in these small droplets is the deciding factor. If the 

 growth of microorganisms in butter is to be prevented by salt, it is un- 

 necessary 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 preserva- 

 tion of fruit, vegetables, meat, milk, and other food substances by drying 

 or condensation. 



DESICCATION. Microorganisms do not die immediately if water is not 

 present. The majority of bacteria will remain alive for a few days if 

 dried on glass or filter-paper. Some will live for a month or more, while a 

 few, like the B. carotamm, die within twenty-four hours after drying. 

 Spores of bacteria are extremely resistant to desiccation. Certain spores, 

 like those of Bad. anthracis and B. mesentericus, have been kept alive for 

 many years. The mycelium of molds is ordinarily killed if dried com- 

 pletely, while the spores survive. Yeasts show a varying resistance. 



The resistance of microorganisms is influenced greatly by the medium 

 on which they are placed for drying. Hansen found that yeast cells 

 dried on cotton were still alive after two to three years, while if dried on 

 platinum wire some died in five days and others lived as long as 100 days. 

 Compressed beer yeast mixed and dried with powdered charcoal kept as 

 long as ten years; Pseudomonas radicicola dried on a cover-glass or filter- 

 paper died within twenty-four hours; on seeds, this same organism was 

 still alive after fourteen days and in the dried nodules of legumes a few 

 cells were able to reproduce after more than two years. Soil containing 

 an average number of 17,000,000 bacteria per g. was dried for two years; 

 the total number of organisms averaged then 3,250,000; 20 per cent of the 

 bacteria, therefore, could resist desiccation. Dried cultures of microorgan- 

 isms are commonly sold for several purposes, as dairy-starters and the so- 

 called "magic yeast" used for bread-making. Such cultures are dried on 

 milk, sugar, starch, flour or similar porous and absorbing material. Starters 



