58 



CARBOHYDRATES 



amylase 



(CeHioOs)^. +^H20 *■ -2Ci2H2iOii 



Starch , dextrins, Maltose 



or glycogen 



This conversion, however, is generally not complete because the amylases 

 can attack only a-l,4-linkages between glucose units. Thus amylose is 

 completely hydrolyzed into maltose, but the breakdown of amylopectin 

 stops when a-l,6-linkages (branch points) are reached. This discon- 

 tinuance results in the formation of limit dextrins which consist mostly 

 of glucose tri- and tetrasaccharides, each containing at least one a-1,6- 

 linkage. The amount of limit dextrins formed is usually about 10 to 20 

 per cent of the amount of starch hydrolyzed. 



The amylase enzymes have received various common names according 

 to the place where they are found. Thus the amylase of saliva is called 

 ptyalin; that in pancreatic juice, amylopsin; and the very active amylases 

 present in sprouting cereal grains and other plant sources are frequently 

 named diastase. Takadiastase is another amylase preparation, obtained 

 from a mold fungus, Aspergillus oryzae, w^hich has long been used in the 

 Orient for making certain fermented foods. However, in all of these 

 variously named preparations there are only two basically different types 

 of amylases, which are designated merely as alpha and beta amylase, 

 respectively. The manner in which each of these attacks starch is ex- 

 plained in detail in Chap. 10. 



The removal of starch from textiles and starched goods by means of 

 amylase is an application of enzyme action to an industrial problem. 

 The fabric is not attacked as when alkali or acid is used, and hence the 

 enzyme is to be preferred to other means of starch removal. Many textile 

 mills now make use of such commercial amylase preparations. 



Dextrins 



As already noted, dextrins are intermediate products in the hydrolysis 

 of starch to maltose. They differ from starch by being soluble in cold 

 water and from maltose by being insoluble in alcohol. They are also 

 found as native products in the roots, stems, and leaves of many plants. 

 Starchy seeds in the resting stage contain a small percentage of dextrin 

 and when germinating, a large amount. 



Dextrins are formed from starch in many household operations re- 

 quiring heat: baking of bread, cake, etc., and ironing of starched clothes. 

 The toasted breakfast foods, corn flakes, shredded wheat, puffed rice, and 

 so on, contain considerable quantities of dextrin produced by the heating 

 of these foods. 



Commercial dextrins are made by heating starch with or without the 

 addition of acid. If acid is used, a lower temperature is sufficient to 



