78 EXPERIMENTAL DAIRY BACTERIOLOGY 



forms present in fresh milk liquefy casein. Spore-bearing ba- 

 cilli are frequently the cause of these fermentations. The most 

 important representatives of the group are the spore-produc- 

 ing organisms persisting in milk after heating. Various kinds, 

 as shown by their action in litmus milk, should be isolated 

 and studied. 



Exercise. Each student will procure samples of milk and heat 

 them to 100 C. for twenty or thirty minutes. Incubate for six to 

 eight days at 20 C. From the samples showing curdling and a neu- 

 tral or alkaline reaction, isolate the causal organisms by means of 

 gelatin plates. Study in detail. 



Alcoholic fermentation. The production of alcohol is al- 

 most exclusively limited to the action of yeasts. Some bac- 

 teria produce alcohol, but in such small quantities that it can 

 be detected only by delicate chemical tests. The ordinary 

 yeasts beer, wine, and bread yeasts cannot ferment milk 

 sugar, and hence are of little importance in the dairy. Yeasts 

 that are able to ferment milk sugar are important in the 

 production of fermented drinks from milk, kefir, koumiss, 

 leben, matzoon, etc. Lactose-fermenting yeasts are widely dis- 

 tributed in milk, butter, and cheese, and especially in whey 

 from vats in which a part of the whey is allowed to remain 

 from day to day. 



In order to isolate the yeasts from a sample of milk, etc., 

 an acid medium is used. The acidity of the medium may be 

 increased to such a point as to 'exclude all bacterial growth 

 and still allow yeasts to grow. This is accomplished by 

 adding to the melted lactose agar, just before plating, about 

 1 per cent of tartaric acid, in the form of a 30 per cent 

 solution. The acid cannot be added to the agar before ster- 

 ilization as the solidifying properties of the agar would be 

 destroyed. 



