404 



THE KEEPING OF MILK. 



Table CXLII. gives the average amount of acidity above the 

 normal acidity of milk developed in the times at various stated 

 temperatures (see also Fig. 45). 



The rate of souring increases 1-5 times for a rise of 10 F. in 

 temperature, or 2-075 times for a rise of 10 C. 



When milk reaches an acidity of 13 above the normal it 

 curdles on boiling, and at 65 it curdles spontaneously ; the 

 times taken to reach these points are : 



Time in Hours at 



Acidity. 

 13, 

 65, 



47 



68 



70 F. 

 31 

 44 



80 F. 

 21 

 30 



90 F. 

 14 

 20 



It is seen that lowering of temperature has an immense influ- 

 ence on the life of milk. 



Fig. 45. Curve of Development of Acidity. 



Boric acid preservatives are used in quantities varying from 

 0-01 up to 0-3 per cent. 



The following figures show the amounts used : 



Up to 



From 0-03 to 0-06 



Above 0-06 



0-03% in 45 0/ 



of samples containing boric preservative. 



In the bulk of the samples the quantity added was insufficient 

 to have any really useful effect in hot weather. 



Formaldehyde appears to be added in proportions varying 

 from 0-002 to 0-005 per cent., and only the larger quantities 

 would be really efficient as preservatives. 



To sum up, it appears that to be of any real use in hot weather 

 at least 0-1 per cent, of boric preservative, or 0-004 per cent, of 



