178 AGRICULTURAL BACTERIOLOGY 



milk the development of the acid-forming bacteria is 

 stopped at these temperatures; consequently the milk may 

 not undergo the customary souring change, but other 

 bacterial types that act on the casein can grow slowly at 

 these low temperatures, rendering the milk unfit for use 

 within a few weeks. It is believed that many of the cases 

 of poisoning due to ice cream have been caused as a result 

 of the storage of cream, a practice that has now been largely 

 abandoned. 



Even in the case of foods that are not contaminated, 

 by microorganisms, changes may go on that limit the time 

 that the food can be held in storage. These changes are 

 due to the enzymes that are normally present in many 

 uncooked foods, and are usually termed autolytic changes. 

 The ripening of meats is an example. In the case of eggs 

 the white becomes less viscous, and loses to some extent its 

 beating properties. Water also passes from the white into 

 the yolk; the membrane surrounding the yolk weakens, so 

 that when the egg is broken, it is difficult to avoid mixing 

 the yolk with the white. 



Preservation of eggs. In preserving eggs it is necessary 

 to prevent the invasion by bacteria and to limit the loss of 

 water from the eggs. Eggs are practically sterile when 

 laid, but, owing to the porous nature of the shell, bacteria 

 are able to penetrate it readily if the surface of the egg is 

 moist. These organisms penetrate the shell in the same 

 manner as they do the wall of a porcelain filter, i. e., by 

 growing through it. If the eggs are left in a dirty nest, 

 or placed in a damp cellar, the adhering moisture may be 

 sufficient to enable the bacteria on the shell to multiply and 

 so penetrate the shell. In cold-storage rooms it is essen- 

 tial that the temperature be kept constant, so that moisture 

 will not condense on the surface of the eggs. 



For home use, eggs may be preserved by placing them 



