CHAPTER XXIII. 



STARCH, SUGAR, ALCOHOL AND OILS. 



STARCH, C 6 H 10 5 , is a product prepared by growing plants, 

 as food, for the nourishment of succeeding plants of their kind. 

 The starch of commerce is prepared from potatoes, rice and wheat. 

 Starch has the appearance of a white powder, but when examined 

 under the microscope it is found to consist of grains which vary 

 somewhat in size and appearance in different plants. The grains 

 of potato starch are about ^-^ of an inch in diameter, those of 

 wheat about y-oVo"' and of rice about 3-^^ of an inch. 



Starch is insoluble in cold water but if heated with water to a 

 temperature of about 140 F. the grains swell up, burst and 

 yield the well-known paste used for stiffening linen and other 

 fabrics. A dry heat of 140 will also cause the grains to burst, 

 as in a roasted potato. A few drops of iodine added to a solu- 

 tion of starch gives a deep blue color, which disappears as the 

 solution is heated, reappearing as it cools. 



When starch is heated to a temperature of about 400 F. it is 

 converted into a substance, called dextrine, which is soluble in 

 water. Dextrine has all the properties of gum, and is used by calico 

 printers for thickening their colors. It is also used as a muci- 

 lage, and for immovable surgical dressings. Dextrine (C 6 H 10 5 ), 

 i&isomerie with starch. 



When starch is boiled with water containing a few per cent, 

 of sulphuric acid it is first converted into dextrine, and then into 

 glucose or grape sugar (C 6 H 12 6 ). These changes are usually 

 explained by supposing that in dextrine the starch molecule has 

 been changed in form, and in glucose has taken up one molecule 

 of water. Cellulose may be changed to grape sugar by the same 

 process. 



SUGAR (C 12 H 22 O n ), occurs in the juices of several plants, as in 

 sugar-cane, sugar-maple, and sugar-beet. In this country most 



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