214 PEACTICAL PHYSIOLOGY 



hydrolyses some of the starch of the flour into sugar. The yeast cells 

 then act on this to produce alcohol and carbonic acid gas, so that a 

 fermenting mass is obtained. More flour is now added to this, and the 

 process allowed to proceed five or six hours longer, until the developed 

 gas causes the top to burst, after which the remainder of the flour is 

 added. The mass is now called dough. It is thoroughly mixed by 

 'machinery, and allowed to 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 proteid, 

 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 II. Shake up some bread with water and filter off the 

 extract. Test the residue for starch by adding iodine, and for proteid 

 by the xanthoproteic, or Millon's 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. 



CHAPTER XL 

 DIGESTION. FERMENTS. 



THE organic food-stuffs proteids, fats, and carbohydrates exist mainly 

 as very large molecules which are incapable of diffusing through animal 

 membranes. Before the food can be absorbed by the gastro-intestinal 

 mucosa it is necessary that these large molecules be resolved into- 

 smaller ones. Digestion is the process by which this resolution is 

 effected in the animal body, and although absorption is not a mere 

 physical process of osmosis it can nevertheless only proceed efficiently 

 with simple molecules. 



The agencies which act on the food-stuffs during digestion, and which 

 produce this resolution, are called ferments. 



These ferments are of two kinds organised and unorganised. An 



