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PKACTICAL PHYSIOLOGY 



ferment for another hour, when it is weighed out into loaves, which are then 

 placed in pans and heated to about 232 C. in an oven for an hour and a half. 

 The heat kills the yeast, but at the same time causes the enclosed bubbles of gas 

 to expand, so that the dough becomes filled with little cavities. The heat also 

 causes the outer part of the dough to become hardened by coagulating the 

 protein, and at the same time it converts the starch into dextrin and soluble 

 starch, and so forms the crust. The crust is glazed because of the dextrin, and 

 it is coloured and its taste different from the rest of the bread, because of the 

 caramel produced by the action of the heat on the sugar which is developed. 



EXPERIMENT IV. Shake up some bread thoroughly with cold 

 water and filter off the extract. Test the residue for starch by adding 

 iodine, and for protein by the colour tests. Test the filtrate for sugar 

 by Trommer's test. All the reactions are positive. If a similar 

 extract be made of the crust it will be found to give a purplish colour 

 with iodine, due to the soluble starch and dextrin which it contains. 



The colour tests for protein and starch can also be well demonstrated 

 by pouring the necessary reagents upon different areas of a piece of 

 bread. Do this, taking care to use a minimum amount of the reagents. 

 To bring out the red colour of Millon's test, it is necessary to toast the 

 bread slightly over the Bun sen flame. 



PERCENTAGE AVERAGE COMPOSITION OF SOME OF THE MORE 

 IMPORTANT FOOD-STUFFS. 



(Adapted from various sources.) 



