CARBOHYDRATES 1 23 



ent in the intestinal juice is another enzyme maltase which 

 hydrolyzes maltose into glucose. This enzyme is also present 

 in yeast. The complete enzymatic hydrolysis of starch there- 

 fore yields glucose as the final product. In the acid hydrolysis 

 of starch glucose is also the final product, the intermediate 

 products being probably the same as in the case of enzymatic 

 hydrolysis. 



Source of Glucose. — The acid hydrolysis of starch is em- 

 ployed in the manufacture of commercial glucose from starch. 

 The starch is boiled with dilute sulphuric acid until a test with 

 iodine for starch and with Fehling's solution for glucose shows 

 that all starch has been converted into the sugar. The excess 

 of sulphuric acid is then removed by precipitation and the 

 sugar-containing filtrate is evaporated to a thick sirup known 

 as glucose sirup (Karo Corn Sirup). In making Karo Corn 

 Sirup the hydrolysis is not carried to completion, a good deal 

 of maltose and dextrin being present which prevents the crys- 

 tallization of the sirup. By complete hydrolysis and subse- 

 quent evaporation the glucose sugar crystallizes out as a solid. 

 Glucose so obtained is a commercial product used as described 

 under glucose sugar. 



Determination. — Starch may be determined quantitatively 

 by converting it into glucose either by means of diastase and 

 maltase, by boiling with acids or by heating with steam under 

 pressure. The resulting glucose is then determined by means 

 of FehHng's solution or by its optical rotation. 



EXPERIMENT STUDY XXIV 

 Starch 



(i) Test corn, potato and wheat or rice starch for general char- 

 acter, and under the microscope for shape of starch grains. While 

 under microscope add a drop of iodine solution. 



(2) Test solubility of starch (any variety) in cold water. 



(3) Mix a little starch (2.0 g.) with cold water to make a thin paste. 

 Heat a liter of water to boiling. WhUe boiling add the cold starch 



