ANIMAL LIFE AND ANIMAL PRODUCTS. FOODS 431 



neutralize the gastric juice (acid) of the stomach and so to inter- 

 fere with digestion. 



To obviate these difficulties sour milk (containing lactic acid) 

 is sometimes used in making the dough. Occasionally vinegar 

 (p. 420) is added. Most frequently a baking powder, containing 

 an acid substance along with the soda, is employed. The acid 

 substances contained in baking powders are alum (p. 469), acid- 

 phosphate of calcium or sodium, or potassium-hydrogen tartrate. 

 The last, known commonly as cream of tartar HKC 4 H 4 O6, acts as 

 follows : 



HKC4H 4 6 + NaHC0 3 -> NaKC 4 H 4 O 6 + H 2 CO 3 -> H 2 + C0 2 . 



The sodium-potassium tartrate which remains is better known 

 under the name of Rochelle Salt. 



It is important that the soda and the acid substance should be 

 used in the correct proportions, which can be calculated from the 

 equation. In commercial baking powders a little corn-flour is 

 added, to keep the particles of the other compounds apart and 

 prevent that gradual interaction which otherwise would be bound 

 to occur. The acid substance should also be somewhat insoluble, 

 so that, even when wet, it will not act upon the soda until ample 

 time has been allowed for complete mixing with the dough. 



Bakers' Bread. The " raising" of bakers' bread is effected by 

 adding yeast. The batch is " set " in a warm place for some hours 

 to permit the yeast to propagate and to act upon the sugar in the 

 flour. In this action, as we have seen (p. 417), carbon dioxide and 

 alcohol are produced. A little sugar, molasses, or malt extract 

 is added to the dough, to afford a larger supply of the sugar re- 

 quired for the production of the carbon dioxide. 



The whites of eggs " raise " cake, without the presence of any 

 soda, because of the expansion under heat of the bubbles of air 

 entangled with the albumen when the eggs are " whipped." 



