198 PASTEURIZATION 



cases the heating and cooling surface of a 300-gallon vat is 

 practically the same as that of a 600-gallon vat, because the same 

 size coil is used in different size vats. Generally speaking, large 

 vats have less heating surface per gallon of cream than small 

 vats. For efficient and reasonably rapid heating and cooling a 

 vat should have not less than about twenty square inches of 

 heating surface per gallon of cream. 



The great majority of the coils and outer shafts are con- 

 structed of copper, tinned over and the lining of the vats is of 

 the same construction. In some rare cases vat pasteurizers have 

 been equipped with German silver coils. 



The inner shafts in the older vats were made of iron. This 

 caused them to corrode rapidly and give much annoyance due to 

 electrolytic action of two metals in brine. This objection has 

 now been largely removed by either doing away with the inner 

 shaft entirely or lining it with copper. 



The exposure of the coils and vat linings to heat, cold, acid, 

 and neutralizers is exceedingly hard on the tin coating. The 

 tin coating soon yields to these corrosive agents and wears off. 

 This is especially true where the vats are not thoroughly cleaned 

 and freed from lime, cream and alkali washing powder, or when 

 wire dish cloths or other similar scouring equipment is used 

 for cleaning. Caustic alkalies should not be used for washing 

 the vats and the vats should be thoroughly rinsed out with 

 water after cleaning, and drained and steamed so that they dry 

 quickly. If alkaline washing powder is used, it should not be 

 sprinkled over coil, vat lining and cover lining and allowed to 

 remain there dry. Even the smallest specks of dry and moist 

 washing powders quickly attack the tinned copper surface, caus- 

 ing the surface to become covered with black spots and blotches. 

 And any alkali deposited in the bottom of the vats will inevit- 

 ably tarnish the lining, and expose the copper. The washing 

 powder should be placed into the water in the vat where it dis- 

 solves quickly and completely and in which form it can be re- 

 moved completely at the conclusion of the cleansing operation. 



The copper lining of the pasteurizing vats and covers is ob- 

 jectionable at best, for the reasons discussed under construction 

 of cream ripening vats ? and here, too, the use of glass- 



