UNSWEETENED CONDENSED MILK DEFECTS 227 



temperatures, so that their contents melt again, the milk contracts 

 and the cans resume their normal shape. 



While the wholesomeness and flavor of the product are not 

 affected by, the freezing process, the remelted evaporated milk 

 is usually less smooth and often slightly grainy. This is due to 

 the fact that, during the process of freezing, there is a partial 

 separation of the watery portion from the caseous material. The 

 casein contracts and the watery portion freezes. When melted, 

 the emulsion is less complete than it was before freezing. The 

 casein remains in its contracted form and robs the product of its 

 original smoothness. 



Blown Evaporated Milk Due to Chemical Action. While 

 properly processed evaporated milk is perfectly sterile, and from 

 the biological point of view, keeps indefinitely, the cans of very 

 old evaporated milk may bulge, as the result of the action of the 

 acid in the milk on the container. Evaporated milk contains 

 from .35 to .50 per cent, acid (calculated as lactic acid). When 

 the tin cans are filled with the evaporated milk, the tinplate is 

 bright and untarnished, both, inside and out. After the sterilizing 

 process, the inside surface of the cans is dark and dull. This is 

 caused by the combined action of acid and heat, which seems to 

 weaken the tinplate. This phenomenon is further illustrated by 

 the fact that where creameries pasteurize their skimmilk and 

 return it to the patrons in the milk cans hot, the milk cans are 

 short-lived ; they soon corrode and begin to leak. 



The acid in the evaporated milk continues to act on the tin- 

 plate of the can after manufacture and in the case of very old 

 evaporated milk, the acid may decompose a considerable part of 

 the iron. This action is accompanied by the evolution of hydro- 

 gen gas, which causes the cans to bulge. This action is hastened 

 by continued exposure of the goods to high temperatures (sum- 

 mer heat). This fact was -experimentally demonstrated, 1 also, 

 by scratching the bottom of tin cans on the inside with a file, 

 then filling the cans with a .4 per cent, solution of lactic acid and 

 acetic acid, respectively. After sealing, the cans were sterilized 

 in the autoclave, so as to avoid any possibility of bacterial action. 

 After cooling, these sterilized cans were incubated for some time 

 at 90 degrees F. The cans containing the dilute acid began to 



1 Hunziker and Wright, Indiana Agricultural Experiment Station. Results 

 not published. 



