112 MANUAL OF MILK PRODUCTS 



show that the results average practically the same as where 

 aliquot portions are taken. 

 Objection to composites. 



The chief objection to composite samples of milk is that the 

 composite samples are usually held too long before testing. 

 This causes a more or less complete separation of the butter- 

 fat in the form of a thick and tough layer of cream. This cream 

 mixes with difficulty back into the remainder of the sample so 

 that the portion transferred to the test bottle is often not 

 representative of the true richness of the milk. This defect is 

 especially pronounced where the samples are not protected 

 against high temperature (summer heat). 



Composite samples of milk should be held for not longer 

 than one week and tested at the end of that period. 



Care of the milk sample 



The milk sample should be in as normal condition as possible 

 at the time of testing, otherwise it will be difficult to pipette a 

 representative portion into the test bottle. This difficulty is 

 entirely avoided where the sample is transferred with the 17.6 

 c.c. pipette from the weigh can direct to the test bottle. The 

 changes which the improperly kept sample undergoes are 

 caused by evaporation and by fermentation. 



Evaporation causes the percentage of fat and other solids to 

 increase, yielding misleading tests. It also tends to dry the 

 milk on the surface, causing the formation of a tough, leathery 

 layer. In this condition it is difficult to secure a representative 

 portion for the test. This can be prevented by giving the sample 

 jar a gentle rotary motion after each addition of milk, by 

 properly replacing the cap or stopper after each addition of the 

 milk, by protecting the sample from excessive heat, and by test- 

 ing at least once a week. 



Fermentation causes the milk to spoil, and the milk usually 



