24 MODERN METHODS OF TESTING MILK 



small portions at a time, finely powdered caustic soda 

 or potash, allowing the milk to stand some time after 

 each ad(litit)n of pow'dered alkali and shaking vigor- 

 ously, the additions of alkali and the agitation being 

 continued only until the milk becomes completely li(juid. 

 In using the alkali in solid form, no correction of re- 

 sults needs to be made. The alkali solution or tab- 

 lets described on page 96 may be used. A darkening 

 of the milk by alkali may occur without afifecting the 

 results of the test. In testing, caution must be ob- 

 served when adding sulphuric acid (see p. 56) to milk 

 in which an alkali has been used, since an unusual de- 

 gree of heat is produced and the contents of the test- 

 bottle may spurt out. The acid must be added slowly 

 and mixed with the milk much more deliberately than 

 usual. 



COMPOSITE SAMPLING OF MILK 



Composite samples of milk. — A mixture of daily 

 samples of milk, taken from day to day for several 

 days in succession, is known as a composite sample. 

 In commercial work at creameries, cheese-factories, 

 milk-shipping stations, etc., where the number of pa- 

 trons is large, a daily test of the milk for its fat con- 

 tent is impracticable. To obviate the great amount of 

 work involved in making daily tests, a jar is provided 

 for the milk of each patron and in this jar is placed 

 a sample of each day's milk, when it is delivered, these 

 daily samples being mixed and allowed to accumulate 

 for a period of one or two weeks. A determination 

 of fat in such a composite sample gives the average 

 percentage of fat in the milk for the period covered 



