8 Dairy Bacteriology. 



time. This rapid rate cannot be maintained indefinitely, 

 for the bacteria soon limit their own development by the 

 production of by-products that are unfavorable to their 

 own growth. The sour milk bacillus thrives readily in 

 milk until the lactic acid that is formed by it exceeds a 

 certain amount, then growth ceases. If this acid is neu- 

 tralized by the addition of chalk, the lactic bacteria will 

 start again to grow, and produce acid to the point of 

 excess. There is a marked difference with different 

 forms in their rate of growth, and the conditions under 

 which development best occurs. 



15. Effect of external conditions. Bacteria, even 

 in a vegetating stage, possess a much greater resistance 

 toward external forces than other forms of animal or veg- 

 etable life. When they are in a spore stage, this resist- 

 ance is still further increased. A thorough knowledge 

 of the effect of these external forces is essential, for it is 

 by their action that w r e are often able to destroy undesir- 

 able forms of germ life. 



16. Physical forces. 1. Heat. The bacteria possess 

 a high power of resistance toward heat ; but this relation 

 varies, depending upon the condition in which it is ap- 

 plied, whether it is in a dry or moist state. At some 

 temperatures moist heat, on account of its greater pene- 

 trative power, is much more effective than dry in destroy- 

 ing germ life. The temperature at which any form is 

 killed is called its thermal death point. For the majority 

 of germs in a spore-free, developing condition, a temper- 

 ature from 130-140 F. for ten minutes in a liquid 

 medium is fatal. A shorter exposure necessitates a some- 

 what higher temperature. 



When in the more resistant spore- stage, many of them 

 are able to withstand moist heat in the form of steam 

 (212 F.) from one to three hours. To destroy spores 



