MOISTURE. 149 



The effect of salts upon microorganisms is therefore not due to the os- 

 motic pressure only; the chemical constitution of the salts also plays an 

 important role. 



The different functions of life are influenced in different degrees by 

 concentrated solutions. Some bacteria will multiply but not form spores 

 in salt solutions. Molds will sometimes show a good growth in concen- 

 trated sugar solutions but fail to produce spores unless the medium is 

 diluted. 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 8-10 per cent. At 15 per cent only few cocci develop 

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

 tardation. Above 20 per cent the torulae 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 this has a selective action, 

 preventing nearly all other organisms but the torulae. 



The selective influence 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. But though its preserving action is a little higher than 

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



