WATER AS A PHYSICAL FACTOR 265 



spores in salt solutions. Molds will sometimes show a good growth in 

 concentrated sugar solutions but fail to produce spores. Bact. anthracis 

 loses its virulence in sea water. Often, the form of microorganisms is 

 affected by concentrated solutions. Some bacteria grow more spherical, 

 others become elongated or distorted. The deforming influence is not 

 due to the osmotic pressure only, but depends mainly upon the chemical 

 character of the salt; magnesium salts especially have a tendency to 

 produce such involution forms. 



Salt and Sugar Solutions. Most experiments on the influence of 

 concentrated solutions have been carried on with sodium chloride, be- 

 cause of its wide application in the preservation of foods. Most micro- 

 organisms, especially the rod-shaped bacteria, are suppressed by a salt 

 concentration of 8 to 10 per cent. At 15 per cent only few cocci develop 

 slowly, while some species of Torula grow without a very noticeable re- 

 tardation. Above 20 per cent the Torula are practically the only 

 organisms which can develop. They are, therefore, found in all food 

 products which are preserved by salt, as salted pork, beef, fish, butter, 

 and pickles, often in nearly a pure culture. It seems that they are 

 easily overpowered by other organisms in the absence of salt, but in 

 salted food, this competition is eliminated. 



The selective influerfce of salt is used in some fermented products to 

 prevent undesirable fermentations. This is true in sauerkraut and 

 brine pickles, where the desirable bacteria can grow in the presence 

 of salt while the undesirable ones are kept away. Possibly the salting 

 of butter has the same effects. 



Another compound of great practical importance is cane sugar,' 

 which is the standard preservative for fruits and condensed milk. Its 

 action has been studied mainly upon molds. Theoretically, dextrose 

 should be expected to have twice as strong a preserving action as saccha- 

 rose because it has only half the molecular weight and consequently 

 produces twice as strong an osmotic pressure in the same percentage of 

 concentration. Its preserving effect is indeed a little higher than 

 that of saccharose, but the proportion is not nearly 1:2. The common 

 molds are extremely resistant to strong sugar solutions, about 60 to 70 

 per cent of cane sugar seems to be the limit of growth for Pemcillium 

 and Aspergillus species. Yeasts can also grow and ferment in very con- 

 centrated solutions while bacteria in general do not tolerate solutions 

 higher than 15 to 40 per cent, though many exceptions are known. 



