52 PHYSIOLOGICAL CHEMISTRY. 



potato, in peas and beans, and in most vegetable substances used as 

 food. It constitutes almost entirely the preparations known as sago, 

 tapioca, arrow-root, and maizena, which are nothing more than varie- 

 ties of starch, extracted from different plants. 



The following list, compiled mainly from the tables of Pay en,* shows 

 the percentage of starch in various kinds of food : 



QUANTITY OF STABCH IN 100 PARTS IN 



Wheat .... 57.88 Potatoes . . . 20.00 



Eye .... 64.65 Sweet potatoes . . 16.05 



Oats .... 60.59 Peas .... 37.30 



Barley .... 66.43 Beans. . . . 33.00 



Indian corn . . . 67.55 Flaxseed . . . 23.40 



Rice .... 88.65 Chocolate nut . . 11.00 



Starch derived from all these sources has essentially the same chemi- 

 cal composition, and may be recognized by the same tests. It is in- 

 soluble in cold water, but if treated with about twenty times its weight 

 of boiling water its granules swell, become gelatinous and amorphous, 

 combine with a certain proportion of water, and fuse into an opaline 

 liquid, which is thicker or thinner according to the quantity of water 

 present, and which solidifies, on cooling, into a nearly homogeneous 

 paste, the water remaining united with the amylaceous matter. The 

 starch is then in a pasty and amorphous condition, its chemical proper- 

 ties remaining essentially unaltered. If treated with 100 or 150 

 parts of water at the boiling temperature it makes a liquid which does 

 not gelatinize on cooling ; but the imperfectly liquefied portions, con- 

 taining the insoluble cellulose, gradually subside as a turbid deposit, 

 while the soluble starch remains above, forming a clear and colorless 

 liquid. 



Starch is especially distinguished by its property of striking a blue 

 color with iodine. This reaction will take place even with raw starch, 

 and its granules may be recognized under the microscope by thig 

 means. It is still more prompt when the starch has been boiled to a 

 paste, and especially when it is in solution. A minute quantity of tinc- 

 ture of iodine, added to a starch solution, produces at once a deep blue 

 color, which may be largely diluted without losing its characteristic 

 tinge. This test, however, must be employed at a moderate tempera- 

 ture. If the solution be too hot, no visible reaction will occur ; and 

 even after it has taken place, if heat be applied the blue color will dis- 

 appear, to return again after cooling down to the proper temperature. 

 The iodine must also be used in a free state. If added in the form of 

 a soluble iodide it will produce no effect, since the starch has not suf- 

 ficient affinity to withdraw it from its union with other matters. 

 Finally, no third substance must be present which would be capable of 

 combining with the iodine and thus preventing its action on starch. 

 Many animal fluids, such as the serum of blood, saliva, mucus, and urine, 



* Substances Alimentaires. Paris, 1865. 



