PRACTICAL PAPERS —BREAD MAKING. 
823 
ing the process of digestion, starch is changed into sugar by 
the action of the gastric juices, and any substance that will 
prevent the conversion of starch into other compounds before 
it is taken into the stomach, will prevent it afterwards, and so 
interfere with ready digestion. Besides this action upon the 
bread itself, alum has a direct injurious action upon the mucus 
membrane of the alimentary canal. Being actively astringent 
in its nature, it not only interferes with the process of absorp¬ 
tion, but has a tendency by long use to produce chronic con¬ 
stipation. Alum is also objectionable where bread is sold by 
weight, as in Europe, as it causes the retention of a larger 
amount of water within the loaf, and so increases the weight 
of bread from a given quantity of flour. This substance may 
be detected in bread by moistening a portion of the interior of 
the loaf with an alcoholic solution of logwood, made by digest¬ 
ing for twenty-four hours, 120 grains of chip logwood in eight 
ounces of alcohol. This solution gives to bread free from 
alum a straw yellow color, but a dark red, when alum is pres¬ 
ent. Sulphate of copper, on account of its very poisonous na¬ 
ture, should never be used, even in the smallest proportions. 
When the loaf is subjected to the heat of the oven, fermen¬ 
tation is stopped, although the loaf continues to raise by the 
expansion of the gas contained within its pores, and at the 
same time the dough diminishes in weight from loss of water, 
which in bread from twelve to twenty-four hours alter baking 
constitutes about two-fifths of the weight of the loaf. 
The chemical change produced by the process of baking is 
confined to the outer portions of the loaf. When starch is 
subjected for some time to a temperature of 300 deg., it is 
changed into dextrin, a substance having the same composition 
but different physical and chemical properties, being soluble in 
water, and of light brown color. The crust of the loaf is sim¬ 
ply that portion of the dough in which this change has taken 
place, and if of a dark brown color, has been heated to incipi¬ 
ent decomposition. Toast owes its wholesome properties and 
sweetish, agreeable taste, to the formation of dextrin, by ex- 
