FREE INSTITUTE OF SCIENCE 



91 



THE CHEMISTRY OF BREAD-MAKING 



Among the nitrogenous constituents of value as yeast food is asparagin, 

 a moderately widespread plant principle, found in largest amount in asparagus, 

 whence it gets its name. This principle exists in small amount in potato but 

 not in the cereals, which may account for the empiric use of potato water in 

 preference to other Hquids in making yeast extemporaneously, as is some- 

 times done in the home. 



It was at one time beheved that yeast and other organized ferments, as 

 they were called, acted in a manner essentially different from the enzyms or 

 unorganized ferments. This view is now known to be erroneous, and it has 

 been proved that yeast acts through the agency of enzyms secreted during its 

 life and growth. 



Yeast breathes or takes in oxygen and exhales or gives off carbon dioxid 

 when surrounded by a favorable medium for its activity. The soluble carbo- 

 hydrates constitute the food element needed in largest amount for the exercise 

 of its functions. If the sugar is present as a disaccharid, such as cane-sugar, 

 known as an indirectly fermentable sugar, it is necessary first for one of the 

 yeast enzyms (the one named invertase) to transform the disaccharid into the 

 directly fermentable monosaccharid, which it does by the following reaction. 

 This reaction, as will be seen, is one of hydrolysis, in which the enzym plays 

 no part that can be expressed : 



C12H22O11 + H2O — > CeHiaOe + CeHioOe 



Cane-sugar Water Dextrose Levulose 

 (sucrose) 



The dextrose and levulose of the foregoing reaction are isomeric sugars dif- 

 fering in their optical activity, and the mixture is known as invert sugar. 



Having a monosaccharid at its disposal, and assuming that it has been pro- 

 vided with proper nourishment, the yeast acts upon the sugar and produces 

 alcohol and carbon dioxid, according to the following reaction: 



The carbon dioxid produces the leavening effect; the alcohol is mostly 

 driven off by the baking, so that in freshly baked bread the proportion is 

 rarely above 0.25 per cent., and this rapidly diminishes to zero after the loaf 

 is cut. As may be readily appreciated, yeast works more actively at some 

 temperatures than at others. Too great activity is not desirable, for abnormal 



