172 AGRICULTURAL BACTERIOLOGY 



the different groups of microorganisms. As a group, the 

 bacteria are the least resistant to the action of solutions of 

 high osmotic pressure; the molds are the most resistant. 

 This differentiation in the ability of organisms to grow in 

 concentrated solutions is of great importance in food preser- 

 vation. Concentration alone can not usually be relied 

 upon to protect food materials, but must be supplemented 

 by some other agent or process. Frequently heat is ap- 

 plied in concentrating the material, as in preparing syrups 

 from the sap of the cane, the beet, or the maple; or heat 

 is applied in the preparation of the food after the concen- 

 tration has been raised by the addition of sugar or salt. 

 The yeasts and molds that are most likely to grow in such 

 materials are destroyed by the heat; and, unless recontam- 

 ination occurs, the food should keep. The exclusion of air, 

 thus preventing mold growth, is another supplementary 

 factor. 



Syrups owe their keeping qualities to their concentration. 

 If these sugary liquids are insufficiently concentrated, they 

 undergo most commonly an acid fermentation, caused by 

 bacteria. Condensed milk is prepared by the concentration 

 of fresh milk and by the addition of cane sugar. Jellies 

 and jams are protected from bacteria by their concentra- 

 tion. The heating they receive in preparation frees them 

 from yeasts, and mold growth is prevented by covering the 

 surface with a layer of paraffin. The addition of salt 

 to meats and fish, coupled with partial drying, is a com- 

 mon practice. The same materials may also be preserved 

 by placing them in a saturated solution of salt. 



Preservatives. Many chemical substances exert an in- 

 jurious effect on microorganisms, inhibiting their growth 

 when present in such minute quantities that their effect 

 can not be ascribed to physical action. They act chemi- 



