Bacteria, Helpful and Otherwise 445 



the ingredients have been mixed to form a solid sticky mass, 

 called dough, the yeast is added, and the material set aside in a 

 warm place to rise. The yeast plant grows rapidly, giving off 

 carbon dioxide. This gas fills the dough full of small bubbles, 

 causing it to swell. The dough is then baked and becomes bread. 

 If baked without yeast, bread is very hard and compact. 



Bacteria and Food. Bacteria of decay render invaluable 

 service to mankind by the decomposition of useless organic mate- 

 rial so that it may become available to plants for the production of 

 new food. They are, however, a constant and common source 

 of annoyance, as they quickly render food, especially protein 

 food, unfit for eating, if they have access to it. Nearly all kinds 

 of food, especially milk and meat, spoil in a short time if not 

 protected from their activity. When they attack protein food, 

 it often happens that poisonous substances, known as ptomaines, 

 are formed, which cause serious illness to those who eat the food. 

 To-day all classes of people are interested in protecting food 

 from the decaying action of bacteria. There are several methods 

 of preventing their attacks. 



Preservation of Food. Preservation of food is closely related 

 to the study of bacteria. Largely on this account the study of 

 bacteriology is considered an essential part of the preparation of 

 all who expect to deal with the problems of domestic science. 



Food may be kept from spoiling for a long time by any one 

 of five methods : canning, drying, smoking, pickling, or cold stor- 

 age. In all these methods the important thing is to prevent the 

 activity of bacteria, as by doing this the chief cause of the decom- 

 position or spoiling of food is stopped. It is well known that a 

 high degree of heat will kill bacteria. It is also a well-established 

 fact that bacteria require a considerable degree of moisture to 

 insure their reproduction, growth and development, and it has 

 been learned from experiments that they will not develop at all 

 in foods that are fairly dry. The preservation of food, then, 

 depends either on the killing of bacteria or on making their en- 

 vironment such that they will cease to act. 



Canning. In ordinary canning of meat or fruit three steps 

 are necessary. First, the substance, after being washed, is 



