22 BULLETIN 319, U. S. DEPARTMENT OF AGEICULTURE. 



nearly 2 per cent of lactic acid, and on standing several days this 

 may become about 3 per cent. The most active of the ordinary lactic- 

 acid bacteria seldom exceed 1 per cent lactic acid. The more active 

 type of Bacterium caucasicum forms the inactive lactic acid, while 

 the levorotatory acid is produced by the type forming acid more 

 slowly. Small amounts of other organic acids and traces of alcohol 

 are formed. 



This bacterium is evidently the essential organism of yogurt, 

 matzoon, ceiddu, leben, and similar fermented milks. Other bacteria 

 are always present, some of them habitually and others only occa- 

 sionally. Some of these may have an influence on the flavor, while 

 others are inert. It is probable that there are none, with the excep- 

 tion of Bact. caucasicum, that can not be replaced by other species 

 without appreciably affecting the results. Doubtless slightly differ- 

 ent varieties of fermented milk have developed in different localities 

 owing to different combinations of bacteria or of bacteria and yeasts. 

 The Egyptian leben is reported to contain alcohol, but not in quanti- 

 ties sufficient to produce an effervescence such as is observed in 

 kefir or kumiss. One of the ordinary lactic-acid bacteria seems 

 to be always present with the Bact. caucasicum, and it is probable 

 that if it is not essential it is of some assistance in starting the lactic 

 fermentation and, especially if the temperature is low, in suppressing 

 contamination before the Bact. caucasicum has time to develop suffi- 

 cient acid to check extraneous bacteria. 



Hastings and Hammer (33) could not detect evidences of proteo- 

 lytic enzyms by the usual tests, but found in old-milk cultures a dis- 

 tinct peptonization of the casein which was not traceable to the action 

 of the acid. This change is so slow and so small that it can not be 

 considered as having any influence on the digestibility of the milk. 

 Otherwise the only changes in the milk constituents are in the con- 

 version of the sugar to lactic acid and very small amounts of volatile 

 acids and traces of alcohol. 



" Yogurt buttermilk " is now sold in several cities, and the growing 

 demand will doubtless soon extend its manufacture more generally. 

 In making yogurt in this country better results are obtained by using 

 with the Bact. caucasicum a culture of an ordinary lactic-acid organ- 

 ism such as is used in making buttermilk. Bact. caucasicum growing 

 alone in milk forms usually a rather slimy, tenacious curd which can 

 not be broken up into the smooth, creamy condition essential to a 

 good buttermilk. If this organism is grown in combination with the 

 ordinary lactic-acid organism, a more friable curd is obtained, and 

 the sliminess is not so evident. The two organisms can be carried in 

 mixed culture only with great difficulty, as the high acid soon sup- 

 presses the ordinary form. The most satisfactory results can be ob- 



