228 Agricultural Bacteriology. 



Sweetened condensed milk is an illustration of this 

 method of preservation. Part of the water of the milk 

 is evaporated by heating in a vacuum, thereby increasing 

 the density of the milk. The addition of sugar is then 

 made which still further increases the density to a point 

 where bacterial growth is impossible. Such milk may 

 contain living organisms. Dilution of the milk with 

 several volumes of sterile water reduces the concentra- 

 tion to a point where bacterial growth becomes possible 

 and the milk then undergoes fermentation. 



Fruits are protected from decomposition by placing 

 them in concentrated sugar solutions in the making of 

 preserves. In the preparation, the fruit is cooked so 

 slightly in the sugar that it is not rendered sterile, but 

 the bacteria that are not destroyed ar^ unable to grow in 

 the concentrated medium. Marmalades, jams, and jel- 

 lies owe their keeping qualities to the same factors. 



Preservative action of acids. The bacteria are un- 

 able, as a rule, to grow in a strongly acid medium. This 

 fact is made use of in the preservation of many foods and 

 even fodders. One of the most common uses of acids in 

 the preservation of foods is in the preparation of pickles 

 with the use of vinegar, which is itself a product of bac- 

 terial action. The acid reaction of the vinegar is so 

 strong that the ordinary bacteria can not grow in it, thus 

 if certain vegetables, as cucumbers, are placed in the 

 vinegar they will be protected against the attacks of bac- 

 teria. Some of the molds find favorable conditions for 

 growth in vinegar, and may destroy so much of the acid 

 of the vinegar as to allow the bacteria present to grow 

 with the result that the pickles are spoiled. Frequently 

 spices are added to the vinegar to be used for pickles, 

 since they improve the flavor. They also have a preset 



