530 ENZYMES DECOMPOSING SACCHARIDES. 



pressed yeast by low-fermentation yeast on the exclusive faculty 

 of the latter for fermenting melibiose. 



In the case also of Buchner's permanent yeast, it is possible to 

 detect whether the same is composed of top or bottom yeast, or of 

 & mixture of both, the presence of melibiase being sufficient to 

 demonstrate that of bottom yeast. The test is easily performed 

 by the aid of melibiose, which sugar cannot be split up 

 into its components, cZ-glucose and tl-galactose, by any yeast 

 enzyme other than melibiase. These components can be readily 

 identified by means of phenylhydrazine, the experiment being 

 carried out in the following manner : A i per cent, solution of 

 melibiose, entirely free from any other kind of sugar, is treated 

 with 2 per cent, (or a little more) of the yeast under examination, 

 in presence of i per cent, of toluene, and kept for 1-3 days at 

 about 25 G. The extract is filtered, and the nitrate is boiled 

 with a small quantity of good bone black, then refiltered until clear, 

 and the liquid tested with phenylhydrazine, 2 grins, of which, and 

 2 grms. of 50 per cent, acetic acid, are added for each gramme of 

 melibiose employed, the mixture being heated for an hour on the 

 boiling water bath. The mixture is poured out into cold water 

 about 3 vols. to each unit of melibiose solution originally taken and 

 filtered, the residue being washed once with water and then rinsed 

 into a beaker, in which it is boiled up with water. If the resulting 

 osazone dissolve completely in boiling water, no bottom yeast is 

 present (at least in detectable quantity) ; but if the osazone remain 

 undissolved, the presence of bottom yeast is demonstrated, since 

 glucosazone and galactosazone are only sparingly soluble in boiling 

 water. In addition to the ratio of solubility in hot water, the 

 microscopical examination affords further indications, inasmuch 

 as melibiosazone crystallises in fine needles, invariably arranged 

 in stellar groups, whereas the two hexosazones chiefly form coarse, 

 long, and thick needles. This difference will be sufficient for the 

 experienced analyst, whilst those who wish to determine the 

 character of the osazone by the melting-point and ultimate 

 analysis must adopt a complicated method on which point see 

 BAU (XV.), who succeeded in detecting with certainty the presence 

 of 10 per cent, of bottom yeast in top-fermentation permanent 

 yeast by this method. 



330. Lactase. 



Milk sugar, or lactose, is one of the oldest known sugars, having 

 been described by Fabricio Bartoletti as long ago as 1615. It 

 occurs in the milk of mammals, cows' milk containing 3.6-5 per 

 cent, (mean about 4.5 per cent.), goats' milk 3.26-6.65 per cent., 

 ewes' milk 3.43-6.62 per cent., mares' milk 4.72-7.32 per cent., 

 and the milk of the she-ass 5.29-7.63 per cent. 



When lactose is boiled with dilute mineral acids, it is split up 

 into equal molecules of d-glucose and d-galactose, the hydrolysis, 



