ENZYMES OF YEAST. 255 



small number are known. To secure the results required in 

 practice, it is still necessary, therefore, to make use of living 

 growths. 



The Enzymes of Yeast. 



In addition to the general sketch included in the foregoing 

 chapter on the enzymes occurring in fungi, a few particulars 

 must be given regarding their special relations to yeast. 



Invertase is commonly found in all species. It is prepared 

 by treatment of the yeast with alcohol or ether, or by drying 

 and heating to 100 C. The enzyme is then extracted with 

 water or glycerine, and precipitated with alcohol ; the pre- 

 cipitate is afterwards dried. It hydrolyses cane sugar, which 

 is split into one molecule of glucose and one of Isevulose, and 

 it is only after this decomposition that yeast can ferment 

 sugar. The optimum temperature of invertase is about 

 55 C., and it is destroyed at 75 C., but in a dry state it can 

 stand much higher temperatures. It is resistant to small 

 doses of antiseptics, and to vigorous action of the proteolytic 

 enzyme of yeast. Its action is increased by very dilute acids, 

 but considerably diminished by treatment with alkali. 



Maltase also occurs in many yeast species. It decomposes 

 maltose into two molecules of glucose. It has its optimum 

 at 40 C., and is destroyed at 55 C. According to Bokorny 

 and others, it is much more sensitive to chemical reagents 

 than invertase. 



Melibiase. By careful treatment of raffinose with dilute 

 acid, it is hydrolysed into laevulose and melibiose. The latter 

 is decomposed by melibiase, which, according to Bau, is usually 

 present in bottom-fermentation yeast, but is often absent in 

 top-fermentation yeast. 



Lactase decomposes lactose into one molecule of glucose 

 and one of galactose. It is only after this enzyme has reacted 

 that lactose can be fermented. It has been found in a number 

 of yeast species. 



According to van Laer, P. Lindner and others, several 

 yeast species are capable of fermenting dextrin. It is assumed 

 that the fermentation is preceded by a similar hydrolysis to 



