THE GRANULE OF STARCH. 689 



put into a trough with about half its weight of water, and sufficient 

 salt and yeast or leaven, then thoroughly mixed up into what is known 

 as the " sponge." (Here we may remark that the best flour takes up 

 the largest quantity of water ; and a rough test of the quality of two 

 samples of flour may be made by comparing the quantity of water re- 

 quired to obtain a dough of similar consistency.) After the sponge is 

 made, it is left for about five hours in a warm place to ferment, after 

 which it is kneaded with the rest of the flour, and again left to rest 

 some time. The dough is then weighed into lumps, which are put in 

 tins, and set aside till they have risen to twice their previous bulk. It 

 is to the yeast or leaven that the raising of bread is due, and the action 

 is identical with that of the fermentation of beer. The flour contains 

 a small amount of a nitrogenous substance which changes a portion of 

 the starch into sugar ; the yeast then attacks the sugar, splitting it 

 into alcohol and carbonic-acid gas, the little bubbles of which try to 

 escape from the mass of the dough, but get entangled by the gluten 

 and gum which the flour contains ; and thus every part of the bread 

 becomes penetrated with little cavities. Eventually the fermentation 

 would cease, and the bubbles of gas would find their way to the out- 

 side, thus leaving the dough much less light and spongy than we wish 

 it to be ; but the baker guards against this by putting it at the proper 

 time into a hot oven, the heat of which at first increases the fermenta- 

 tion. In a few minutes, however, the temperature becomes sufficiently 

 high to kill all the yeast-germs ; the fermentation is thereby stopped ; 

 and, by continued heating, the starch-granules are burst and the mass 

 is fixed in the porous form it has then attained. A little of the alco- 

 hol is retained in the bread ; but practically almost the whole of it 

 in London amounting to some three hundred thousand gallons per 

 annum is driven off by the heat. During the baking, the starch of 

 the outer portions of the bread has been browned by the heat, and 

 thereby changed into a sugar known as dextrine or British gum ; and 

 perhaps this fact accounts for the fondness of some children and even 

 grown-up people for crusts. 



Of late years a system for making what is called aerated bread has 

 proved very successful, and is free from the slightest objection. The 

 dough is made by mixing the flour with water saturated with carbonic- 

 acid gas, which on heating is expelled from the water, and thus dis- 

 tends the dough, producing a light, spongy bread, with no loss of 

 starch or sugar, and without any injurious or objectionable ingredient 

 having been introduced. 



Having dealt with the baking of the bread, let us now briefly con- 

 sider its further progress in being adapted for the wants of the body. 

 As soon as a piece of bread is put into the mouth, an abundant flow of 

 saliva takes place ; and in fact it needs no actual tasting to induce this 

 flow, for even the sight or smell of anything nice is quite sufficient to 

 " make the mouth water," as we express it. The saliva is poured into 

 vol. xxin. 44 



