5i6 THE POPULAR SCIENCE MONTHLY. 



making is easily understood. To the water with which the flour is to 

 be moistened some yeast is added, and the yeast-cells, which are very 

 much smaller than the grains of flour, are diffused throughout the 

 water. The flour is moistened with this liquid, which only demands 

 a temperature of about 70° Fahr. to act with considerable energy on 

 every granule of flour that it touches. Instead, then, of the passive, 

 lumpy, tenacious dough produced by moistening the flour with mere 

 water, a lively " sponge," as the baker calls it, is produced, which 

 " rises " or grows in bulk by the evolution and interposition of millions 

 of invisibly small bubbles of gas. This sponge is mixed with more 

 flour and water, and kneaded and kneaded again to effect a complete 

 and equal diffusion of the gas-bubbles, and finally the porous mass of 

 dough is placed in an oven previously raised to a temperature of about 

 450°. 



The baker's old-fashioned method of testing the temperatui*e of his 

 oven is instructive. He throws flour on the floor. If it blackens with- 

 out taking fire, the heat is considered sufficient. It might be supposed 

 that this is too high a temperature, as the object is to cook the flour, 

 not to burn it. But we must remember that the flour which has been 

 prepared for baking is mixed with water, and the evai^oration of this 

 water will materially lower the temperature of the dough itself. Be- 

 sides this, we must bear in mind that another object is to be attained. 

 A hard shell or crust has to be formed, which will so incase and sup- 

 port the lump of dough as to prevent it from subsiding when the fur- 

 ther evolution of carbonic-acid gas shall cease, which will be the case 

 some time before the cooking of the mass is completed. It will hap- 

 pen when the temperature reaches the point at which the yeast-cells 

 can no longer germinate, which temperature is considerably below the 

 boiling-point of water. 



In spite of this high outside temperature, that of the inner part of 

 the loaf is kept down a little above 212° by the evaporation of the 

 water contained in the bread ; the escape of this vapor and the expan- 

 sion of the carbonic-acid bubbles by heat increasing the porosity of 

 the loaf. 



The outside being heated considei*ably above the temperature of 

 the inner part, this variation produces the differences between the 

 crust and the crumb. The action of the high temperature in directly 

 converting some of the starch into dextrin will be understood from 

 what I have already stated, and also the partial conversion of this dex- 

 trin into caramel, which was described in Nos. 13 and 14 of this series. 

 Thus we have in the crust an excess of dextrin as compared with the 

 crumb, and the addition of a variable quantity of caramel. In lightly 

 baked bread, with a crust of uniform pale-yellowish color, the con- 

 version of the dextrin into caramel has barely commenced, and the 

 gummy character of the dextrin coating is well displayed. Some 

 such bread, especially the long staves of life common in France, ap- 



