356 MILK 



Prucha that citric acid had disappeared and had been converted 

 into acetic acid and carbon dioxid. 



The acid thus formed rendered some insoluble inorganic con- 

 stituents soluble. In fresh milk the insoluble calcium is in com- 

 bination with phosphoric acid as CaHPO 4 and with casein as Ca 4 

 caseinate. When lactic acid was produced by bacterial activity 

 the dicalcium phosphate was converted into monophosphate 

 (CaH 4 P 2 O 8 ), and the calcium caseinate changed first into a casein- 

 ate containing less calcium, and finally the calcium was com- 

 pletely eliminated from its combination with casein and the 

 latter precipitated. 



The albumin of milk is also profoundly affected according to 

 Van Slyke and Bosworth. The authors found that more albu- 

 min passes through a porous filter from sour milk than from sweet 

 milk. The authors think that this phenomenon is due to the for- 

 mation of lactic acid which decreases the absorption of albumin 

 by casein, or that it may be due to a combination of a base with 

 the albumin and a gradual separation of the base and protein in 

 much the same manner as happens with casein. 



The authors furthermore determined the rate of acid forma- 

 tion, and found that during the first period the acid increased 

 slowly, while much acid was formed from the tenth to the twenty- 

 fifth hour. After 0.7 per cent, lactic acid was present, bacterial 

 activity was much reduced. Between the forty-eighth and ninety- 

 sixth hours the sugar was changed but slightly. There was, 

 therefore, a rapid increase of acidity during the first twenty-four 

 hours, followed by a much smaller increase. In fresh milk no 

 lactic acid was found, and the acidity of fresh milk must, there- 

 fore, be due to acid phosphates. The acidity of the milk serum 

 also increased but little after the lapse of twenty-four hours. 



The following table is given by Van Slyke and Bosworth, and 

 shows the results of their work in detail : 



FORMATION OF LACTIC ACID IN THE SOURING OF MILK 



Amount of Fermented 



unchanged sugar 



Age of milk when sugar in 100 Milk-sugar changed into 



sampled. Sugar c.c. of milk. Lactic acid. changed. lactic acid. 



Hours. Grams. Grams. Grams. Per cent. Per cent. 



Fresh 5.30 0.00 0.000 0.0 



10 5.07 0.23 0.200 4.3 87 



12 4.83 0.47 0.330 8.9 70 



14 4.68 0.62 0.513 11.7 82 



19 4.58 0.72 0.671 13.6 93 



25 4.42 0.88 0.665 16.6 75.5 



48 4.30 1.00 1.052 18.9 



96 4.26 1.04 1.124 20.0 



The figures in this table indicate very clearly that the most 

 rapid acid formation occurs during the interval from the tenth to 

 the twenty-fifth hour. At this time 0.665 per cent, lactic acid 



