184 AGEICULTUEAL AND INDUSTRIAL BACTEBIOLOGY 



of bringing about this change. It may be prepared from 

 yeast cells by grinding them with fine sand and diatomaee- 

 ous earth until the cell bodies have been ruptured, then 

 squeezing them by means of a hydraulic press to force out 

 the cell juices. Under suitable conditions these cell juices 

 containing zymase will produce transformation of sugar 

 into alcohol and carbon dioxide. Apparently zymase will 

 not act except in the presence of phosphates or phosphoric 

 acid. Probably there is first brought about a union of the 

 hexose sugar and the phosphoric acid to form hexose phos- 

 phate, and this is then decomposed by the zymase with the 

 formation of alcohol and carbon dioxide. 



Preliminary Preparation of Carbohydrates for Yeast 

 Fermentation. — It has already been emphasized that 

 yeasts are able to act directly upon monosaccharide sugars 

 only with the production of carbon dioxide and alcohol. 

 Carbohydrates in nature frequently exist in more complex 

 forms, as disaccharides and polysaccharides. These must 

 first be transformed into the monosaccharides before they 

 can be fermented by the yeast. 



The extracellular digesting or hydrolyzing powers of 

 yeasts show marked differences. None are known which are 

 able to attack the complex polysaccharides, such as starches 

 and cellulose. Most species, however, are able to produce 

 the enzyme maltase which hydrolyzes maltose to two mole- 

 cules of dextrose. Likewise most species produce the 

 enzyme sucrase which hydrolyzes sucrose or cane sugar into 

 dextrose and levulose, and a few species are also known 

 which produce the enzyme lactase, hydrolyzing lactose or 

 milk sugar into dextrose and galactose. It is apparent, 

 therefore, that no preliminary treatment is necessary for 

 the fermentation of cane sugar or malt sugar by yeast, and 

 that some yeasts can also ferment lactose or milk sugar. 



In manufacture of alcohol generally the polysaccharides, 

 particularly starch and cellulose, constitute the cheapest 



