CARBOHYDRATES 135 



On account of their great surface development, the adsorptive 

 capacity of starches is very great. As has been said, when they 

 swell they adsorb water, dyes, etc. A very characteristic adsorption 

 compound is formed with iodin. lodin is the best known reagent for 

 starches; by it they are stained blue. It was formerly believed 

 that iodin and starch united chemically; W. BILTZ showed that it is 

 merely an adsorption. According to the degree of dispersion, iodin 

 solution is blue, red, orange or yellow, inasmuch as the starch solu- 

 tion acts as a protective colloid (W. HARRISON*). There are, in 

 addition, varieties of starch which give at once a brownish red or a 

 wine red color with iodin. Inulin and lichenin are colored yellow 

 by iodin. 



The swelling and pasting of starches, hydration, is analogous to the 

 swelling of proteins, which is a preliminary to their hydrolytic cleavage. 

 The swelling of starches is favored by electrolytes, especially alkalis, 

 so that swelling commences at a much lower temperature in their 

 presence. For this purpose the anions are especially important and 

 in fact, in a lyotropic series, similar to that of acid albumin. See 

 page 152 (M. SAMEC).* 



Starch paste increases the surface tension of water (ZLOBICKI*). 

 A solution of starch in water, as well as one of dextrin, dissolves less 

 C0 2 than pure water (according to A. FINDLAY*). (A gelatin solu- 

 tion dissolves more C02 than pure water!) 



Under the influence of dilute acids or diastatic ferments, the starch 

 molecule takes up water and, step by step, breaks into small frag- 

 ments, soluble starches, amylodextrin, various dextrins some of which 

 crystallize, and finally into grape sugar. The larger the fragments 

 the more marked is their colloidal character. 



As the result of osmometric experiments W. BILTZ * arrived at the 

 following molecular weights: 



Amylodextrin 22,200-20,500 



Higher achroodextrin 11,700- 8,200 



Erythrodextrin 6,800- 3,000 



Acid dextrin 4,000 



Lower achroodextrin 1,800- 1,200 



Dextrin (C 6 HioO B ) 6 905 



Commercial dextrin 6,200- 2,700 



"Soluble starch" (according to H. FRiEDENTHAL* 1 ) produces a 

 definite lowering of the freezing point, which is proportionate to the 

 amount of the substance that is dissolved. 



Crystallizable dextrins [amy loses (C 6 H]o0 5 )6] prepared by H 

 PRINGSHEIM* and EISSLER combine with iodin to form iodin-addition 

 compounds which dissolve like iodin starches in cold water with a 

 transitory blue color. 



