CARBOHYDRATES 



57 



of foreign substances whose starch granules are different from those of 

 the pure materials. 



Conmiercial starch is usually made from corn or wheat in the United 

 States and from jiotatoes in Europe. Other commercial starches are sago, 

 tapioca, and arrowroot. The preparation of starch in the United States 

 is associated with the manufacture of numerous other products such as 

 corn oil, gluten feed, and glucose sirup. For details concernmg the proc- 

 ess of manufacture, see the industrial chemistries listed. 



Starch is insoluble in cold water, but at higher temperatures (52° to 

 72°C., varying with the kind of starch) the starch grains absorb water, 

 swell, and finally form a sticky paste or opalescent semisolution. The 

 absorption of water and swelling of starchy material on heating is well 

 illustrated by the changes in volume and viscosity that rice undergoes 

 when it is boiled. 



Like other polysaccharides, starch is hydrolyzed by boiling in dilute 

 mineral acid solutions. If the boiling is continued long enough, the starch 

 is converted entirely into glucose, as shown in the following equation: 



(CeHioOs)^ + a;H20 -^^ xCeHijOs 

 Starch Glucose 



However, the large starch molecule does not split up all at once into 

 glucose but passes through a number of intermediate stages. At first 

 only a few of the glucosidic linkages are hydrolyzed so that large frag- 

 ments of the original molecule are formed. This renders the starch water- 

 soluble. More hydrolysis leads to smaller fragments of the starch mole- 

 cule, which are called dextrins. These in turn are broken down into 

 maltose and finally glucose. 



The manufacture of sirup from starch involves its hydrolysis by acid 

 with glucose and maltose as the principal products, together with a con- 

 siderable quantity of dextrin. The hydrolysis is commonly carried to 

 the point at which iodine no longer gives a color with the hydrolysis 

 mixture. The composition of commercial corn sirup, as calculated from 

 a number of analyses reported by Fetzer, Evans, and Longenecker, is 

 as follows: 



per cent 



Water 18.48 



Dextrins 28.11 



Maltose 36.33 



Dextrose 16.78 



Crude protein 0.05 



Ash 0.25 



Starch, dextrins, and glycogen are also hydrolyzed by various starch- 

 splitting enzymes called amylases. This process differs from acid hy- 

 drolysis in that the chief product formed is maltose rather than glucose: 



