206 OUTLINES OF BACTERIOLOGY 



to milk, the latter stiffens into a clot, and out of it a watery' fluid 

 (whey) oozes. The clot is derived from the principal constituent of 

 milk, viz. casein, and is the crude cheese. 



Thrombose, the fibrin ferment. Freshlj'- drawn mammalian blood 

 on exposure rapidly forms a clot, from which after some hours a yellow 

 liquid (serum) exudes The clotting is due to the ferment thrombose, 

 which is not present in normal blood, and can be first demonstrated 

 only when coagulation takes place. 



Pectase. Ferment which forms jellies from many ripe fruits. It 

 acts on one of the constituents of the cell sap, called pectins. 



VII. Urease, which sets up ammoniacal fermentation in urine. 

 Urine on exposure changes from an acid body to an alkaline, owing to 

 the decomposition of the contained urea into ammonium carbonate. 

 This decomposition is due to the agency of urease, which is secreted by 

 the organisms which drop into the urine from the atmosphere. 



VIII. Oxidases, or oxidising ferments. These are ferments which 

 promote the oxidation of various suljstances by acting as carriers of 

 oxygen to the latter. The best known are : 



Laccase, which is concerned in the production of lacquer varnish, 

 from the sap of the lac tree of south-east Asia. 



Tyrosinase. Found in several fungi and in some roots. It oxidises 

 the tyrosin which is contained in these plants. 



(Enoxydase, which causes the disorder in wines which is called 

 "casse." The wine loses its characteristic colour, and a red-brown 

 precipitate is thrown down. 



IX. Zymase, the alcohol-producing ferment. This is the ferment 

 secreted by yeast. It effects the decomposition of sugar into alcohol 

 and carbon dioxide. Zymase has also been extracted from peas, from 

 barley, and from certain fruits, e.g. cherry. The yeast plant can set up 

 alcoholic fermentation in mare's milk, as is done in the preparation of 

 the Russian beverage known as koumiss, and in cow's milk, as in the 

 preparation of kephir. The various alcoholic fermentations will be 

 dealt with in a later chapter. 



§6. FEEMENTATIONS FROM WHICH NO SOLUBLE 

 FERMENTS HAVE BEEN EXTRACTED. 



In this group, the fermentation in each particular case is caused 

 either by the secretion of a ferment, the extraction of which has not 

 as yet been accomplished, or by the organism as a whole without the 



