50 



DAIRY BACTERIOLOGY 



develop comparatively few spores in each cell ; certain wild 

 yeasts, which spoil beer, grow as elongated cells and form large 

 numbers of spores in each cell. Yeasts which collect at the 

 bottom of the liquid towards the end of the fermentation are 

 known as bottom yeasts ; those which rise to the surface are known 

 as top yeasts. Bottom yeasts are employed in slow fermenta- 

 tion at low temperatures, in the making of light wines and beers 

 of the lager type. Top yeasts, on the other hand, cause more 

 vigorous fermentations, and are used in the making of heavier 

 wines and beers and spirits. Bakers' yeast is usually a top yeast. 

 The yeasts used in the production of alcoholic beverages ferment 

 maltose and sucrose, but not lactose ; the saccharomycetes met 

 with in dairy practice ferment lactose and sucrose, but not maltose. 



2. The Torulse. These are smaller than the saccharomycetes 



and play only a subordinate 

 part in the alcoholic fermen- 

 tation industries. On the 

 other hand, they are of far 

 more frequent occurrence in 

 dairy products, while the 

 formation of alcohol in Kefir 

 and similar beverages is 

 chiefly due to the action of 

 certain torulse which ferment 

 lactose either by themselves 

 or in symbiosis with cer- 

 tain lactic acid bacteria. A 

 lactose - fermenting species, 

 Torula amara, which lives in 



sycamore leaves, will, according to Harrison, turn milk bitter in 

 a few hours. The torulse which do not ferment sugar are still 

 more widely distributed ; according to the author's investiga- 

 tions, they develop freely in butter which is kept for some time. 

 Certain species which form characteristic stellate colonies in stab 

 cultures hydrolyse the fat more or less vigorously, provided that 

 the butter is sufficiently acid. A certain torula which hydrolyses 

 fat will colour butter red ; the growth of this organism is inhibited 

 by common salt. 



3. My coder mae. While most yeasts form a film on the surface 

 of the liquid after the fermentation is over, the mycodermse form 

 a dull-looking film at the very outset. As a rule they are elongated 

 cells containing a few bright granules. In the making of Emmen- 

 tal cheese, a home-made rennet is generally employed which, if 

 properly made, should be a practically pure culture of Thermo - 

 bacterium helveticum covered by a mycoderma film which excludes 



FIG. <l$.Mycodernta cerei-isice. 

 (After Holm.) 



