172 



ALIMENTATION. 



FIG. 49. Arrowroot starch-granules ; magnified 

 370 diameters (from a photograph taken at the 

 United States Army Medical Museum). 



crackling sound. The different varieties of starch may be recognized micro- 

 scopically by the peculiar appearance of the granules. 



Starch is insoluble in cold water ; but when boiled with several times its 

 volume of water, the granules swell up, become transparent, and finally fuse 



together, mingling with the water 

 and giving it a mucilaginous con- 

 sistence. The mixture on cooling 

 forms a jelly-like mass of greater or 

 less consistence. This change in 

 starch is called hydration and is im- 

 portant as one of the transforma- 

 tions whidh take place in the process 

 of digestion, when starch is taken 

 uncooked. This change is generally 

 effected more or less completely, 

 however, in the process of cooking. 

 The most important properties 

 of starch are connected with its 

 transformation, first into dextrine, 

 and finally into glucose. This al- 

 ways takes place in digestion, before 

 starch can be absorbed. In the digestive apparatus, the change into sugar is 

 almost instantaneous, and the intermediate substance, dextrine, is not easily 

 recognized. By boiling starch for a number of hours with dilute sulphuric 

 acid, it is transformed, without any change in chemical composition, into 

 dextrine, which is soluble. If the action be continued, it appropriates one 

 atom of water and is converted into glucose. The change of starch into 

 dextrine may be effected by a dry heat of about 400 Fahr. (204 C.), a pro- 

 cess which is commonly employed in commerce. 



Vegetable Substances resembling Starch. In certain vegetables, substances 

 isomeric with starch, but presenting slight differences as regards general 

 properties and reactions, have been described, but they possess no great im- 

 portance as alimentary matters and demand only a passing mention. These 

 are inuline, lichenine, cellulose, pectose, mannite, mucilages and gums. Inu- 

 line is found in certain roots. It is convertible into sugar but does not pass 

 through the intermediate stage of dextrine. It differs from starch in being 

 very soluble in hot water. Lichenine is found in many kinds of edible mosses 

 and lichens. It differs from starch only in its solubility. Mannite is a 

 sweetish substance found in manna, mushrooms, celery, onions and asparagus. 

 It is perhaps more analogous to sugar than to starch, but it is not fermentable 

 and has no influence on polarized light. 



Gums and mucilages may enter to a certain extent into the composition 

 of food, but they can hardly be considered as. alimentary matters. Gums are 

 found exuding from certain trees, first in a fluid state, but becoming hard on 

 exposure to the air. A viscid, stringy mucilage is found surrounding many 

 grains, such as the flax-seed and quince-seeds, and exists in various roots 



