174 THE CHEMISTRY OF SOME FOODS. [CH. VII. 



possible that it contains something essential to growth 

 and general nourishment. It is not quite so digestible as 

 white flour. The bran in it stimulates the intestine and 

 so acts as a mild laxative. 



209. Mix some wheat flour with a little water to form a stiff 

 dough. Allow this to stand for a short while, preferably at 37 C. 



Wrap a piece, the size of a chestnut, in muslin, and knead it 

 for a few minutes in a basin of water ; pour the suspension into a 

 beaker, and note the white deposit of starch grains that settles down 

 on standing. Examine this microscopically, noting that the grains 

 differ considerably from those of potato-starch in being smaller, 

 circular, and with a central hilum. Make a drawing of the grains. 

 Boil a little with water, cool, and add a drop of iodine. The deep 

 blue starch reaction is obtained. 



210. Knead the dough thoroughly under the tap until no more 

 starch comes through the muslin. A yellowish, sticky mass, known 

 as gluten, is left behind. Test portions of this by the usual protein 

 colour reactions : they are all obtained, gluten being a protein. 



F. Bread. 



The dough formed by adding water to flour is imper- 

 vious to the digestive juices. Before it can be used it has 

 to be aerated and the gluten rendered porous. 



A pure culture of yeast is mixed with warm water, 

 flour and salt. The dough thus formed is thoroughly 

 kneaded, and the mass kept warm for some hours. During 

 this time the yeast cells multiply and convert some of the 

 starch into glucose and this into alcohol and CO 2 . Also 

 the ferment of the flour called diastase becomes active and 

 converts some of the starch into glucose. More flour is 

 added and the process allowed to proceed for some hours 

 longer. The gas formed causes the mass to rise. The 

 dough is weighed out into loaves, which after being allowed 

 to rise once more for a certain time are heated to about 

 232 C. for an hour and a half. The heat kills the yeast, 

 expands the gas bubbles, and causes the outer part of the 



