Calculation of Butter- and Cheese Yield. 179 



214. ** Overrun " of churn over test. The yield of 

 butter is not, however, as a rule compared with the 

 amount of butter fat contained in the butter, but with 

 the total butter fat of the whole milk from which it was 

 made. This " increase of the churn over the test" is 

 what is generally called overrun in creameries. 



The overrun obtained in different creameries, or even 

 in the same creameries at different times, will be found 

 to vary considerably. When the milk is accurately 

 tested and the butter well worked, this overrun will vary 

 from 10 to 16 per cent. ; that is if a quantity of milk 

 contains exactly 100 Ibs. of butter fat, as found by the 

 Babcock test or any other accurate method of milk test- 

 ing, from 110 to 116 Ibs. of butter ready for market may 

 be made from it. 



215. Factors influencing the overrun. Even under the 

 very best of care and attention to details, variations will 

 occur in the speed of the separator, in the conduct of the 

 ripening and churning processes, and in the condition of 

 the butter when the churn is stopped; hence absolutely 

 uniform losses of fat in skim milk and butter milk, or 

 the same water- and salt contents of the butter, cannot 

 be expected. 



The overrun is influenced by two factors: the losses of 

 butter fat sustained in separating the milk and churning 

 the cream, and the gain due to the admixture of water, 

 salt, etc., in the manufacture of butter. Considering 

 first the losses of fat in skim milk and butter milk, the 

 separator will usually, when run at normal speed and 

 capacity, leave the same per cent, of fat in skim milk, 

 whether rich or poor milk is skimmed. An exception 



