CH. xxv.] CARBOHYDRATES. 389 



with a dilute mineral acid, or by means of an inverting ferment, 

 such as occurs in the intestinal juice, it is converted into dextrose. 



C H n + H.O = 2C fl H ls O 

 [Maltose] [Dextrose] 



It undergoes readily the alcoholic fermentation. 



Phenyl hydrazine test. The three important reducing 

 sugars with which we have to deal in physiology are dextrose, 

 lactose, and maltose. They may be distinguished by their rela- 

 tive reducing powers on Fehling's solution, or by the characters 

 of their osazones. The osazone is formed in each case by adding 

 phenyl hydrazine hydrochloride, and sodium acetate, and boiling 

 the mixture for half an hour. In each case the osazoue is de- 

 posited in the form of bright canary-coloured, needle-like crystals, 

 usually in bunches, which differ in their crystalline form, melting- 

 point, and solubilities. 



Starch is widely diffused through the vegetable kingdom. It 

 occurs in nature in the form of microscopic grains, varying in 

 size and appearance, according to their 

 source. Each consists of a central 

 spot, round which more or less con- 

 centric envelopes of starch proper or 

 granulose alternate with layers of 

 cellulose. Cellulose has very little 

 digestive value, but starch is a most 

 important food. 



Starch is insoluble in cold water: it Fig. 33 6. Grains of potato 



... starch. 



forms an opalescent solution in boiling 



water, which if concentrated gelatinises on cooling. Its most 



characteristic reaction is the blue colour it gives with iodine. 



On heating starch with mineral acids, dextrose is formed. By 

 the action of diastatic ferments, maltose is the chief end product. 

 In both cases dextrin is an intermediate stage in the process. 



Before the formation of dextrin the starch solution loses its 

 opalescence, a substance called soluble starch being formed. This, 

 like native starch, gives a blue colour with iodine. Although the 

 molecular weight of starch is unknown, the formula for soluble 

 starch is probably 5(C 12 H2oO 10 )2o. Equations that represent the 

 formation of sugars and dextrins from this are very complex, and 

 are at present only hypothetical. 



Dextrin is the name given to the intermediate products in 

 the hydration of starch or glycogen, and two chief varieties are 

 distinguished : ert/thro-flejrfrin, which gives a reddish-brown colour 

 with iodine ; and achroo-dextrin, which does not. 



