DAIRY FARMING — DAIRYING. 631 



method': Two liundied cubic centimeters of fresh, normal cows' milk 



is heated in a .'JOO cc. thisk to 35^ C. (not liigher), 2 cc. of rennet solution 



added and tlie heating continued in a constant water bath with frequent 



gentle agitation until the milk is curdled. After a few minutes' heating 



the milk will become thick and will appear cheesy and stringy on the 



glass. The temperature must be kept at exactly 35°. In preparing 



the rennet solution for the test 10 cc. of rennet extract or 1.25 gm. of 



rennet i)owder is made to 200 cc. with water. Representing by t the 



time required for coagulation the curdling power of rennet extracts is 



80,000 , ^ ^ , 640,000 



. , and ot rennet powders — :, — -. 



The author made many experiments to study the effect of various 

 properties of the milk and other factors on the action of rennet and 

 on the rennet test. The results of these are tabulated and discussed 

 at lengtli. 



I'sing a given rennet solution, the time of coagulation varied with 

 the milk of the same cow at different times, the maximum variation 

 being about 1 minute. On an average the morning's milk required about 

 O.o.l minute less for coagulation than that milked at noon, although the 

 specific gravity, fat content, and acidity were practically uniform. With 

 the mixed milk of several cows the differences were much smaller. The 

 morning's milk was found to be much more regular than the noon milk, 

 and is believed to contain more lime salts. It is suggested that a study 

 of breed differences in this respect would be interesting. 



Skimming milk (without allowing it to sour) had practically no effect 

 on the rennet test; and likewise the addition of a refined, nearly neu- 

 tral oil of about the same specific gravity as milk fat had only slight 

 effect. 



Goats' milk curdled in considerably less time than cows' milk. Milk 

 preservatives, as was to be expected, affected the curdling. The acidity 

 of the milk Avas found to be the principal controlling factor. The 

 author recognizes three kinds of acidity of milk, (1) the original acidity 

 attributed to the ability of casein to take up bases leaving acid phos- 

 phates, (2) that due to lactic acid i)roduced in the milk, and (3) that 

 due to carbon dioxid (very small). The effect of the first two is quite 

 different. The first is due to the presence of monopotassium phosphate 

 and casein in milk. The monophosphate is shown by experiment to 

 act principally by rendering more soluble the nearly insoluble calcium 

 l)hosphates of the milk. Trials of adding to milk different amounts of 

 a solution of monopotassium phosphate of known strength showed that 

 in spite of the increased acidity of the milk the time of coagulation of 

 the milk was only slightly decreased, while an ei[uivalent increase in the 

 acidity from adding a free acid affected the coagulation very much. 

 This is thought to account for the great instability in the solubility of 

 the calcium phosphate of milk, which is greatly affected by outside 

 inrtuences, and so affects the curdling of the milk. 



Milch Ztj,'., () (1877), p. 513. 



