€32 EXPERIMENT STATION RECORD. 



Exi)erimeut showed that the action of rennet is favored by free acids 

 in the milk, first, hy dissolving the insoluble calcium phosphates of the 

 milk, until the limit of this is reached ; then by their action and decom- 

 position of the calcium-casein compounds in the milk, whereby the 

 casein gradually changes to a form insoluble in milk, and consequently 

 a more favorable proportion for the action of rennet is brought about 

 between the soluble casein and lime salts in the milk. With the increase 

 of lactic acid the curdling power is increased until the milk curdles 

 merely on heating. Free acids do not of themselves affect the action 

 of rennet, which depends not upon the absolute but only the relative 

 amount of casein and soluble lime salts. Hence their action is only 

 indirect. The ability of milk to curdle with rennet bears no relation to 

 its natural acidity; this is clear when it is remembered that the ajipar- 

 ent acidity stands in no relation to the quantity of soluble lime salts in 

 milk. But in this connection it should not be forgotten that the dipo- 

 tassium phosphate in milk slightly increases its acidity and at the same 

 time has an unfavorable action on the solubility of lime salts, which is 

 shown by an experiment. 



The action of carbonic acid is different from that of other free acids, 

 being similar to that of monophosphates. 



The addition of water to milk diminishes ils curdling power notice- 

 ably in proportion to the quantity added. This is believed to be due 

 to a diminished solubility of the suspended lime phosphates as a result 

 of the dilution of the dissolving salts; a part of the dissolved lime 

 salts are precipitated by diluting, changing the proportion between the 

 casein and soluble lime salts, and consequently diminishing the curd- 

 ling ])ower of the milk. 



Heating milk above 35<^ C, and cooling to a low temperature, both 

 have an unfavorable eifect on the curdling of milk by rennet. Con- 

 tinued boiling of milk renders it incapable of curdling in a reasonable 

 time. The cooling of milk from the temperature when milked (about 

 3(3° C.) down to the air temperature (15 to 17° C.) increased the time 

 of curdling about 0.3 minute. The ability of milk to curdle is at the 

 optimum when it is fresh and warm from the cow; begins to diminish 

 at once, and only becomes constant when the temperature of the sur- 

 rounding air has been reached, after which it increases as a result of 

 the development of lactic acid. In a number of experiments in which 

 milk was cooled to 2° or 0° C. the time afterwards required for curd- 

 ling Avith rennet was increased 0.11 to 0.79 minute. A samxde of milk 

 was kept at 0° 0. for 4 days. During this time the acidity showed no 

 increase, but the time of curdling with rennet gradually increased from 

 day to day. Hence, although the action of bacteria was checked, there 

 was a gradual separation of lime salts, and possibly also a change in 

 the solubility of the casein salts, both of which affect the curdling. 



Vigorous shaking of milk free from carbon dioxid slightly prolonged 

 the time required for curdling, as did also treatment of partially sour 

 milk with an electric current. 



