RELATION OF BREAD RAISING TO TEMPERATURE 89 



the dough are produced by mechanically mixing air with 

 the dough. The result is a bread that is light enough 

 but lacks the peculiar flavor present in ordinary raised 

 bread. In another type of bread {quick biscuits) chemical 

 means are relied upon to produce the gas. A small quan- 

 tity of cream of tartar a-nd saleratus is mixed with the 

 dough. These two materials act upon each other chem- 

 ically and give rise to a quantity of carbon dioxide gas, 

 which appears very quickly, and rapidly fills the dough 

 with bubbles of gas. The dough, when subsequently 

 baked, is light, but has a flavor quite different from that 

 which would be produced in the same dough if it were 

 raised by the action of yeast. No other method of pro- 

 ducing lightness in the dough gives quite so good flavors 

 as can be obtained by the use of yeast, and none is thought 

 to make bread quite so easy of digestion. 



Relation to Temperature 



The growth of yeast, and hence the raising of bread, is 

 very closely dependent upon temperature. Yeasts grow 

 readily in warm temperatures, less readily in low tempera- 

 tures, and not at all if the temperature is in the vicinity 

 of freezing. Common yeast grows best if kept between 

 75" and 90" F. At higher temperatures the yeast does 

 not produce such good results, since certain other injurious 

 microorganisms (bacteria) are then likely to grow. If 

 the dough is kept at a temperature above 90°, there is 

 almost sure to be trouble from the growth of undesired 

 organisms which give rise to unpleasant flavors. Bread 

 made from such dough is very apt to be sour. The 



