204 SCIENCE AND PRACTICE OF DAIRYING. 



rennet. When, however, free acid develops in the milk, which is 

 able to attack the caseous matter, the coagulation which is formed 

 no longer exhibits those properties which belong to a coagulation 

 exclusively formed by rennet. It is worthy of note that the 

 reaction of milk is not altered to the slightest extent by the action 

 of rennet. The action of the rennet ferment is largely influenced 

 by the temperature and by heat. 



By boiling, or by the addition of an alkali, milk loses the power, either 

 entirely or partially, of being precipitated by rennet. In milk which has 

 been heated for a long time, or in milk which has been boiled, after the 

 addition of rennet, a precipitate is formed, it is true, but it consists of a 

 coagulum which is highly flocculent, and never forms a firm united mass. 

 The reason of this, as Soldner has shown, is due chiefly to the fact that 

 in this action a precipitation of calcium phosphate is effected, which causes 

 the entire removal or diminution of the soluble lime salts. Probably 

 other changes unfavourable to the action of rennet may also take place 

 in the milk. Milk which has been boiled, or to which an alkali has 

 been added, and which has thus lost, either entirely or partially, its 

 susceptibility to the action of rennet, regains this susceptibility if it be 

 treated with calcium chloride or other soluble lime salt, or if a small 

 portion of the precipitated lime salts be dissolved again by the addition 

 of carbonic acid or dilute acid. If fresh milk be not coagulated by the 

 action of rennet a fact which has been very rarely noticed, this may 

 be accounted for by some disturbance in the milk-gland, through which 

 the milk exhibits a slightly alkaline reaction, and does not contain soluble 

 lime salts. 



Fresh milk of ordinary quality exhibits to litmus colouring matter an 

 amphoteric that is, a faintly alkaline, and, at the same time, a faintly acid 

 reaction. The degree of acidity caused by the presence of acid phosphates, 

 which varies within narrow limits, may be easily determined by titration. 

 This is carried out, according to the directions of Soxhlet and Henkel, 

 as follows : 50 c.c. of milk is titrated after the addition of 2 c.c. of a 

 2-per-cent phenol-pthalein solution and J normal soda solution. The end 

 of the reaction is denoted by the formation of a faint red colour in 

 the fluid. The number of c.c. used, when calculated on 100 c.c. of milk, 

 represents the measure of the acidity of the milk. This, as a rule, amounts 

 to 7. The greater the acidity of the milk, the more powerful is the action 

 of rennet when the conditions are otherwise similar. By the addition of 

 normal soda solution, or, J normal hydrochloric acid solution, we can 

 impart to the milk at any time a quite definite acidity. In using the 

 numbers denoting the acidity of milk found by Soxhlet and Henkel, it 



