ESTIMATION 141 



a known volume ; this is solution A. A portion of this 

 solution is freed from lead by sodium carbonate and made 

 up to a known volume ; this is solution B. Solution B is 

 divided into portions : (i) For the direct determination of the 

 reducing power due to dextrose, levulose, maltose, and pentose, 

 and also the combined rotation. (2) For determining sucrose 

 by inverting with invertase,* and with 10 per cent citric acid ; 

 each of these values should agree closely. (3) For the deter- 

 mination of pentoses by the Krober method ; and (4) For the 

 estimation of maltose. To do this 50 c.c. of the solution are 

 made slightly acid with hydrochloric acid, and hydrogen sul- 

 phide is bubbled through in order to remove the last traces of 

 lead. Any precipitate is filtered off, and a current of air is 

 passed through the solution to remove the sulphuretted hydro- 

 gen. The resulting solution should be absolutely free from lead, 

 else the yeast will not grow in it, and faintly acid to litmus. 

 To it are added 5 c.c. of yeast water and the mixture sterilized 

 by twenty minutes heating at 115-120° C; when cool it is 

 inoculated with a little yeast and incubated at 25° C. for three 

 or four weeks. f On the completion of fermentation, 5 c.c. of 

 alumina cream are added to clarify the solution, and the whole 

 is well boiled ; it is then filtered and the precipitate washed 

 until the filtrate and washings measure 100 c.c. An aliquot 

 portion is used for determining the reducing power. The 

 yeast must be a pure strain free from maltase, thus all sugars 

 except maltose are fermented away. 



C. POLARI METRIC METHODS. 



The presence of an asymmetric carbon atom confers upon 

 a compound the property of optical activity, by which is meant 

 the power of the substance to rotate to the right or to the 



* The invertase required for this purpose is prepared by washing fresh- 

 pressed beer yeast, to remove adherent wort, packing it into a large wide- 

 mouthed bottle and adding 30-50 c.c. of toluene, which percolates through 

 the mass. The bottle, covered with a sheet of paper, is left in a warm 

 place at a temperature of 25°-30° C. At the end of a fortnight, nearly 

 the whole is liquefied ; it is then filtered on a Buchner funnel. The 

 filtrate yields a highly active preparation of invertase, free from maltase 

 and zymase. 



f 0-2 to 0-5 gram of cane sugar are completely fermented in about 

 three weeks in these conditions. 



