382 MILK 



ture the consistency remains normal. This applies chiefly, as 

 stated before, to lactic acid bacteria of the Streptococcus lacticus 

 type. Lactic acid bacteria of the Bacillus aerogenes type may 

 also produce a slimy condition in milk, but in this case the most 

 favorable temperature is higher than when Streptococcus lacticus 

 is the slime-forming organism. There are also lactobacilli that 

 produce a viscous condition in milk, and the best temperature 

 for these organisms is 42 to 45 C. 



Increase in acidity usually destroys the viscous condition of 

 milk, and it is found, therefore, that the viscosity which appears 

 when cultures are young disappears after several days. Shaking 

 a slimy milk culture usually causes the viscosity to disappear. 



Even before bacteriology had become an established science it 

 was assumed by some investigators that slimy milk was the result 

 of bacterial activity, and after the perfection of bacteriologic 

 technic many reports of organisms causing slimy milk were pub- 

 lished both in European countries and in the United States. In 

 this country Theobald Smith (1891), Marshall (1896), Conn (1899), 

 Ward (1901), and Buchanan and Hammer (1915) have reported on 

 findings of bacteria in slimy milk, and in Canada Harrison and 

 Barlow (1905-06) encountered a similar phenomenon. The writer 

 had occasion to isolate an organism from slimy milk that was 

 causing trouble in a large suburban dairy of Chicago in 1915. 



It is plain that slime-producing organisms are widely dis- 

 tributed. Some of these always produce slime when they gain 

 access to milk and when temperature conditions are suitable; 

 others suddenly acquire the ability to produce viscosity in milk, 

 and may just as suddenly lose this property. The reason for 

 this phenomenon is not understood. Streptococcus lacticus espe- 

 cially is liable to acquire slime-forming ability when transplanted 

 successively in milk, and this happens not infrequently when 

 starters are propagated by frequent transplants. Bacillus aerog- 

 enes also acquires the slime-forming capacity sometimes, and re- 

 tains it even in artificial cultures. 



When a starter that has become viscous is examined through 

 a microscope it may be observed that the chains of streptococci 

 are of unusual length and that the cells are frequently surrounded 

 by capsules. Buchanan and Hammer, who have recently made 

 an exhaustive study of slime-forming organisms, state that the 

 chains form a network in the casein and thus cause the viscosity 

 (Figs. 179-181). The sliminess, however, rapidly disappears 

 when the milk is shaken or when it is heated to a temperature 

 above 45 C. 



Viscosity in milk does not inhibit acid formation; on the con- 

 trary, Buchanan and Hainmer state that it was not unusual to 



