22 BULLETIN 319, U. S. DEPARTMENT OF AGRICULTURE. 
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 eaucasicum 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. eaucasicum, 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 Boot, eaucasicum, 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. eaucasicum 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 r ' 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. eaucasicum a culture of an ordinary lactic-acid organ- 
ism such as is used in making buttermilk. Bact. eaucasicum 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- 
