FERMENTED MILKS 399 



milk to avoid possible infection, although the high acidity has a 

 pronounced germicidal effect and is, therefore, destructive to path- 

 ogenic bacteria. 



Sour milk starters can easily be preserved, as shown by the 

 writer, by desiccating a small amount of coagulated milk and pre- 

 serving this in the dry state. This dry loppered milk keeps its 

 efhciency for several weeks, and as it contains large numbers of 

 Streptococcus lacticus will quickly start an acid fermentation when 

 inoculated into sweet milk. By pasteurization of sweet milk 

 before inoculation with a starter the majority of undesirable bac- 

 teria is destroyed, and those that remain are readily overgrown 

 by Streptococcus lacticus. Pasteurization, therefore, serves two 

 purposes: it insures a good aroma of the product and renders it 

 safe from pathogenic bacteria. 



Lactic acid, like many other acids, has considerable germicidal 

 power when in sufficient concentration. Statements appear in 

 the literatiu-e that lactic acid bacteria destroy pathogenic bac- 

 teria in milk, but this undoubtedly refers to the germicidal effect 

 of the acids produced by the lactic acid bacteria. Work published 

 by Barthel, Bassenge, and Behla in regard to the germicidal effect 

 of acid in buttermilk has led to contradictory results, while the 

 work of Northrup has shown definitely that typhoid bacilli are 

 destroyed by 0.33 per cent, lactic acid when this acid is produced 

 by Streptococcus lacticus (Bacterium lactis acidi). When, how- 

 ever, according to this author, the acid is produced by Bacillus 

 bulgaricus the surprising observation was made that it required 

 nearly twice as much acid for the same destructive effect upon 

 typhoid bacilli. Krumwiede and Noble, studying the longevity of 

 typhoid bacilli in sour cream, have reached the conclusion that 

 the bacilH are gradually destroyed in sour cream by the acids 

 produced, and that the rate of destruction is proportionate to the 

 degree of acidity and the number of bacilli present. These au- 

 thors "have stated further that with a moderate contamination 

 the typhoid bacilli are killed in about four days; with heavy 

 contamination or when initial multiplication took place a longer 

 time is required. Attention is also called to the difficulty of de- 

 termining by present bacteriologic methods whether all typhoid 

 bacilli really are destroyed, because other bacteria grow rapidly 

 in milk and are hable to obscure the presence of typhoid bacilh. 

 From this work it would appear that a cream produced under 

 sanitary conditions and which sours slowly would be more dan- 

 gerous, if infected with typhoid bacilli, than an ordinary cream 

 which sours rapidly. 



In an extensive study of the germicidal action of lactic acid 

 in milk the writer tested its effect by adding definite quantities 



