MILK 



Then the test-tube is inclined so that the mixed liquid covers 

 a portion of the side. On slowly raising the tube to the 

 vertical position it should be noted whether any white flakes 

 adhere to the glass, for if so the milk is strongly acid. A slight 

 film indicates a small degree of acidity, whereas with fresh 

 milk the side of the tube which has been moistened with the 

 liquid remains clear. 



The three methods given above only allow of a rough 

 estimate of the degree of acidity being made. When an 

 accurate determination is re- 

 quired, titration against an 

 alkali must be made. This may 

 be done in several ways, and 

 the most important methods 

 are the following : 



4. Soxhlet- ffcnkel 1 Method of 

 Titration. 



This is the oldest method 

 and the one generally used in 

 practice. Fifty c.c. of milk, to 

 which 2 c.c. of a 2 per cent, 

 alcoholic solution of phenol- 

 phthalein have been added, are 

 titrated with A 7 /4 NaOH until 

 a faint red colour is perceptible. 

 The number of cubic centi- 

 metres of caustic soda which are 

 necessary is termed the degree 

 of acidity. In this method no 

 water is added to the milk. 

 According to Soxhlet, fresh cows' milk should have, on an 

 average, 3 '5 degrees of acidity. Milk which coagulates on 

 boiling has, as already mentioned, about 6 degrees, whilst 

 milk which has become sour and coagulated of itself usually 

 has about 15 degrees. Good ripened cream for butter-making 

 should have an acidity of 12-16 degrees. The figure given 

 above shows an apparatus designed for the titration, the burette 

 being automatically regulated to fill to the zero point. 

 1 Chemisches Centralblatt, 1887, p. 229. 



FIG. 10. Soxhlet- Henkel Titration 

 Apparatus. 



