210 PRACTICAL PHYSIOLOGY 



EXPERIMENT. Seliwanoff's Test. Mix a few cubic centimeters 

 of a solution of laevulose with half its volume of concentrated HC1. 

 Add a few crystals of resorcin and heat the mixture. A deep 

 red colour develops and later a brown precipitate. The colour can 

 be extracted by shaking with amyl alcohol. The characteristic 

 red colour should appear before the solution boils. 



.Repeat this experiment with pure dextrose solution instead of 

 laevulose. A slight red colour develops but no precipitate. 



CHAPTER V 

 CARBOHYDRATES Continued 



There are, however, other reactions of carbohydrates which do not 

 depend on their being aldehydes or ketones. The most important 

 of these are : 



/. Molisch Test. This is an extremely sensitive test, being especially 

 suitable for the detection of minute traces of carbohydrate. For example, 

 most proteins (e.g. egg albumin) give it, on account of the carbohydrate groups 

 which they contain. 



EXPERIMENT I. To about 2 c.c. of a very dilute sugar solution, or of a 

 strong solution of egg albumin, add a drop of a saturated alcoholic solution of 

 et-naphthol. Then carefully pour about an equal volume of pure concentrated 

 H 2 SO 4 down the wall of the test tube so that it forms a layer at the bottom. 

 On standing a minute or so a deep violet ring forms at the line of contact of 

 the two fluids. The greenish colour which also develops is due to the reagents 

 and is no part of the test. 



//. Fermentation with Yeast. By allowing yeast to grow on a 

 solution of dextrose, the latter is split up into alcohol and carbon 

 dioxide, 



C 6 H 12 6 = 2C 2 H 5 OH + 2C0 2 



Dextrose. Ethyl alcohol + carbon dioxide. 



All carbohydrates do not give this reaction, so that it is of value as a 

 distinguishing test for the presence of dextrose in the urine. Commercially 

 it is the agency employed in the preparation of alcoholic beverages. 



To ascertain whether the addition of yeast to a solution produces fermenta- 

 tion, the process should be allowed to proceed in an inverted tube over mercury, 

 or in a Southall's ureometer so that any carbon dioxide gas which develops 

 may be collected, and if necessary tested. 



EXPERIMENT II. Shake up a 1 per cent, solution of dextrose, which has 

 been previously boiled to expel the air and then cooled, with a piece of yeast 

 the size of a split pea. Pour the opalescent solution thus obtained into a 

 Southall's ureometer so that it completely fill the vertical tube. Now place 

 the tube aside in a warm place for some time, when it will be found that 

 a certain amount of gas has collected at the top of the tube. This gas is CO 2 

 as can be shown by adding some NaOH to the tube by means of a pipette 

 and shaking ; the gas disappears. As a control, a tube filled with water and 

 yeast should also be incubated. This should yield no gas. 



EXPERIMENT. Repeat the above experiment with similar solutions of 

 maltose, lactose and cane sugar, and note that, after 24 hours, lactose has not 

 undergone any fermentation, whereas it is marked in the case of maltose; 

 cane sugar may also show a certain amount of fermentation. Yeast contains 



