44 



HOUSEHOLD BACTERIOLOGY. 



The Gas 

 Produced 



Compressed 

 Yeast 



The gas will be formed, replacing the water in the 

 vial. If a burning match is held in the mouth of the 

 tube as it is removed from the' water, the flame will 

 be extinguished. This indicates that the vial con- 

 tains the gas carbon dioxide ; or a teaspoonful of 

 clear lime water may be poured into the vial and 

 shaken about in it. The carbon dioxide present will 

 turn the lime water milky from the insoluble car- 

 bonate of lime (calcium) formed. 



Compressed yeast is sim- 

 ply a mass of yeast plants 

 mixed with some form of 

 starch and pressed into 

 cakes. A two-cent cake is 

 said to contain over half a 

 billion yeast plants. 



As these cakes are made 

 .or a special purpose they 

 should contain only one species of yeast. They do, 

 however, contain bacteria and if the dough is allowed 

 to rise too long or at too high a temperature they 

 grow and produce an acid which makes the bread 

 sour; so that sour bread results from the growth of 

 bacteria and not from the yeast. When pure yeast is 

 used and all conditions of cleanliness are carefully 

 looked after no sour bread results. A dusty kitchen 

 or unclean utensils may increase the danger from bac- 

 terial growth. If the bread be made with milk, this 

 should be scalded to kill the bacteria always present. 

 As we have seen, milk is rich' in dust-plants, especially 



FIG. 25. A YEAST CELL CON- 

 TAINING FOUR SPORES. 



