DEXTRIN OR MUCILAGINOUS STARCH. 743 



is prepared by boiling a solution of starch with a few drops of 

 sulphuric acid, or by heating to 200, 100 parts of starch, 20 of 

 sulphuric acid and 28 of water, filtering and precipitating by 

 alcohol, as a white glutinous substance becoming pulverulent by 

 repeated washings with alcohol. It may also be prepared by 

 diastase ; an infusion of malt is mixed with a solution of starch, 

 in the proportion of 6 or 8 of malt to 100 starch, and the liquid 

 kept at 150 for twenty minutes. From being milky and viscid, 

 it becomes nearly as fluid as water. It is then heated quickly 

 to 212, to stop the farther action of the diastase, filtered and 

 precipitated by alcohol. The solution of dextrin was supposed 

 not to be affected by iodine, but Jacquelain finds that dextrin 

 is coloured purple by iodine, and not blue, like fecula, or that 

 is not colourable by iodine, according to the circumstances of 

 its preparation. Thus 1 part of the granules with 5 parts of 

 water, heated to 320 for forty-five minutes, gave a dextrin 

 which was colourable purple by iodine ; while the same mate- 

 rials heated for one hour and forty-five minutes, at the same tem- 

 perature, gave a dextrin not colourable by iodine. There can 

 be little doubt, therefore, that there are two varieties of dextrin, 

 dextrin colourable by iodine, and dextrin not colourable by iodine. 

 These varieties of dextrin also appear in the succession of 

 changes which fecula undergoes under the influence of an acid. 

 Thus a mixture of 1 part of fecula with 1 part of water and 

 TTBTJ- of oxalic acid, gave when heated at 266 for twenty minutes, 

 dextrin colourable purple, for one hour dextrin not colourable, 

 for two hours sugar of starch also not colourable. 



Dextrin is not fermentable by yeast, but is readily convertible 

 by diastase and dilute acids into a sugar, which is fermentable. 

 The name dextrin was applied to it by Biot from its effect upon 

 a ray of polarised light passing through its solution, in causing 

 the plane of polarisation to deviate very considerably to the 

 right.* The composition of dextrin according to M. Payen, 



* Memoirs by M. Biot, on Circular Polarisation; Taylor's Scientific Memoirs, 

 vol. 1 pp. 584, 600, and Annales de Chimie et de Physique, tomes 69, p. 22, et 74, 

 p. 401. When light polarised by reflection from the surface of a plate of black 

 glass or from the surface of a pile of superposed plates of transparent glass reaches 

 the eye through a disc of tourmalin, a solution of dextrin being interposed in a 

 tube between the reflecting plate and tourmalin, the light does not disappear in 

 those positions of the tourmalin in which light would be completely absorbed 

 without the interposition of the solution of dextrin ; but prismatic colours are 



