36 PHYSIOLOGICAL CHEMISTRY 



at room temperature, crystallization begins and is complete in two hours. By 

 scratching the sides of the flask or by inoculation, the solution quickly congeals to 

 form a thick paste of reddish-yellow silky needles. These are the crystals of methyl- 

 phenylfructosazone. They may be recrystallized from hot 95 per cent alcohol and 

 melt at i53C. 



CH 2 OH 



I 

 GALACTOSE, (CHOH) 4 



CHO 



Galactose occurs with glucose as one of the products of the hydro- 

 lysis of lactose. It is dextro-rotatory, forms an osazone with phenyl- 

 hydrazine and ferments slowly with yeast. Upon oxidation with nitric 

 acid galactose yields mucic acid, thus differentiating this monosac- 

 charide from glucose and fructose. Lactose also yields mucic acid 

 under these conditions. The mucic acid test may be used in urine 

 examination to differentiate lactose and galactose from other reducing 

 sugars. 



EXPERIMENTS ON GALACTOSE 



1 . Phloroglucinol-Hydrochloric Acid Reaction (Tollens) . To equal volumes of 

 gatactose solution and hydrochloric acid (sp. gr. 1.09) add a little phloroglucinol, 

 and heat the mixture on a boiling water-bath. Galactose, pentose and glycuronic 

 acid will be indicated by the appearance of a red color. Galactose may be 

 differentiated from the two latter substances in that its solutions exhibit no 

 absorption bands upon spectroscopical examination. 



2. Mucic Acid Test. Treat 100 c.c. of the solution containing galactose with 

 20 c.c. of concentrated nitric acid (sp. gr. 1.4) and evaporate the mixture in a broad, 

 shallow glass vessel on a boiling water-bath until the volume of the mixture has 

 been reduced to about 20 c.c. At this point the fluid should be clear, and a fine 

 white precipitate of mucic acid should form. 



If the percentage of galactose present is low it may be necessary to 

 cool the solution and permit it to stand for some time before the 

 precipitate will form. It is impossible to differentiate between galactose 

 and lactose by this test, but the reaction serves to differentiate these 

 two sugars from all other reducing sugars. Differentiate lactose from 

 galactose by means of Barfoed's test (page 30). 



3. Phenylhydrazine Reaction. Make the test according to directions given 

 under Glucose, 3 or 4, pages 22 and 23. 



Pentoses, C 5 Hi 5 



In plants, and more particularly in certain gums, very complex car- 

 bohydrates, called pentosans (see page 50), occur. These pentosans 



