MICRO-ORGANISMS AND ENZYMS 293 



the milk. Milk begins to taste sour when its acidity 

 amounts to about 0.3 per cent ; which really means 

 when a little over 0.2 per cent of lactic acid has been 

 formed from milk-sugar; because the milk-casein 

 itself and the soluble phosphates have an acidity of 

 nearly o.io per cent (p. 153) when the milk is 

 freshly drawn and no milk-sugar has had a chance 

 to be changed into lactic acid. According to recent 

 work done at the New York experiment station, 

 milk curdles on boiling when the acidity reaches 

 0.32 to 0.46 per cent, and at ordinary room tempera- 

 ture when it reaches 0.58 to 0.72 per cent. When 

 artificial lactic acid is added directly to fresh milk, 

 curdling takes place on boiling when the acidity 

 reaches 0.36 per cent and at room temperature when 

 the acidity reaches 0.57 per cent. Bacteria continue 

 actively converting milk-sugar into lactic acid, 

 until the amount of acid reaches 0.8 to i.o per cent 

 of the milk; and then they greatly diminish or cease 

 their activity, because they cannot thrive in a solu- 

 tion showing this amount of acidity. Their activity 

 is thus stopped by the accumulation of the chief 

 product of their own activity, and not because the 

 supply of milk-sugar runs out; for, when their 

 activity ceases, about three-quarters of the milk- 

 sugar remains still unconsumed. Products besides 

 lactic acid are formed, varying according to tem- 

 perature and other conditions. In recent work at 

 the New York experiment station, we have ob- 

 tained, in the form of lactic acid, about 80 per cent 

 of the milk-sugar that was decomposed. In connec- 

 tion with cheese-making, the total acidity of the whey 

 may rise as high as 1.2 per cent. Under conditions, 

 which are not present in cheddar cheese-making, 

 some micro-organisms may produce as much as 3 



