ACTION OF HEAT ON STARCH. TIS 



instantaneous deep blue on the addition of iodine, showing that starch 

 has been formed. 



Woody fibre, therefore, may be changed into starch, either by the un- 

 aided action of heat, by that of sulphuric acid, or by boiling with caustic 

 potash, — and the starch thus produeed may be further transformed, first 

 into gum and then into grape sugar, by the prolonged action of dilute 

 sulphuric acid, assisted by a moderate heat. 



II. STARCH. 



1°. Action of heat. — When flour, potato, or arrow-root starch is 

 epread out upon a tray, then introduced into an oven and gradually 

 heated to a temperature not exceeding 300^ F., it slowly changes, ac- 

 quires a yellow or brownish tint according to the temperature employed, 

 and becomes entirely soluble in cold water. It is changed into gum. 

 Under the names of starch-gum, or British-gum, this substance is large- 

 ly manufactured in this country, and is successfully substituted for gum 

 arabic by the calico-printers in thickening many of their colours.* 



The gum thus prepared not unfrequently also possesses a sweet taste, 

 from the further change of a portion of -the gum into sugar. 



2°. Action of water. — When starch is dissolved in boiling water, and 

 is then allowed to stand in the cold either in a close vessel or exposed to 

 the air, it gradually changes into gum or sugar. The process, however, 

 is slow, and months must elapse before the whole of the starch is thus 

 spontaneously transformed in the presence of water (De Saussure). It 

 takes place more rapidly when starch and water are boiled together for 

 a length of tim&. 



3°. Action of sulphuric acid. — From what has been already stated in 

 regard to the action of this acid on woody fibre it will readily be supposed 

 that native starch, of any variety, is likely to undergo transformation 

 when subjected to its influence. 



In reality, if 50 parts of starch, 12 of sulphuric acid, and 139 of water 

 be taken, and if the starch be thoroughly moistened with a portion of the 

 water, and then poured into the mixture of the acid with the remainder 

 of the water, and heated to 190° F., the starch will be entirely convert- 

 ed into gum. By further and more prolonged heating this gum is 

 changed into grape sugar. The gum or sugar may be obtained in a 

 separate state by adding to the solution either chalk or lime, which will 

 combine with and carry down the acid.f One hundred pounds of starch 

 treated in this way will yield from 105 to 122 lbs. of dry grape sugar. 



The rapidity with which this transformation takes place depends 

 partly upon the temperature and partly upon the proportion of acid em- 

 ployed. Thus 100 lbs. of starch mixed with 600 of water and 10 of 

 sulphuric acid, will be converted into grape sugar by boiling for seven 

 hours. If by increasing the pressure the temperature be raised to 250° 

 F., the transformation will be eflfected in a few minutes. With only one 



• During the baking of bread this conversion of starch info gum takes place to a consider- 

 able extent. Thus Vogel found that flour which contained no gum gave, when baked, a 

 bread of which 18 per cent., or nearly one-fifth of the whole weight, consisted of gum. 

 Thus one of the effects of baking is to render the flour-starch more soluble, and therefore (?) 

 more easily digestible. 



t It forms gypaum with it (sulphate of lime) whic}i is a compound of lime and sulphuric 

 acid. 



