801 
Tlie Fermentations of Milk. 
acidi lactici, and apparently agreeing with that previously described 
by Lister. This organism produces no spores, and is consequently 
destroyed by a moderate temperature, heating the milk to 158° F. 
on five successive days being sufficient for this purpose. It grows 
best at a temperature of 95 deg. to- 108 deg. F., and normally curdles 
sterilised milk in twenty-three to twenty-four hours, producing 
lactic acid and carbonic acid. Hence this species effects more than 
a simple molecular change in the sugar. 
Hueppe himself found five other species of organisms in milk, 
all capable of rendering it acid and curdling it. Others have 
amply confirmed this, and Flugge in 1886 mentions no less than 
sixteen species which have the same power. Many additional ones 
have since been isolated, and now we regard the property as that 
of a class of microbes rather than of an individual. Even in the 
dairy it seems that it is not always the same species that sours milk. 
The B. acidi lactici of Hueppe has certainly been found in many 
specimens of milk in Europe, tliough by no means universally. 
The lactic organism common at Wiesbaden is different from the 
form common in Groningen, one being a bacillus and the other a 
coccus. In America no one has definitely found Hueppe’s bacillus, 
and the common one is quite distinct from it, whilst other different 
ones have been found. 
In some cases other acids than lactic (acetic and formic) have 
been found accompanying it, and the quantity of acid formed by 
different organisms has been ascertained by Warington to be very 
variable. In fact, many of the acid- forming species do not form 
enough acid to precipitate the casein (Conn). Nencki, only last 
year, described two bacteria appai’ently identical in every respect 
except that they produce chemically different varieties of lactic acid 
by their action on milk sugar. And Grotenfelt has proved that the 
B. acidi lactici itself loses its power of producing lactic acid if 
cultivated for a long time in a sugar-free medium, though in other 
respects it remains unaltered. 
The next most familiar milk fermentation is the curdling by the 
ACTION OF RENNET, which has been found from time immemorial to be 
the best suited to cheese-making. Ordinary dairy rennet is an acid 
liquid, and hence perhaps the common error of attributing its 
curdling power to the presence of an acid. That this is not the case 
is at once proved by the fact that rennet will curdle milk which is 
kept neutral or even slightly alkaline. Another common error is 
that rennet being derived from a stomach acts by virtue of the 
pepsin, or common digestive ferment always found in gastric juice. 
As long ago as 1870 Hammersten showed that pure pepsin has no 
power to curdle milk. The curd produced by an acid, besides being 
different in texture from that produced by rennet, can be shown by 
a striking experiment, due to Hammersten, to be different in its 
mode of production. If the curd produced by an acid be dissolved 
in weak alkali, the solution carefully neutralised, and an acid again 
added, the curd is again precipitated, and so on indefinitely. But 
