PASTEURIZATION 1 53 



the loss of heat is comparatively slight, but the time required 

 for doing the work is greatly increased. It is already under- 

 stood that the area of the heating surface, the thermal conduc- 

 tivity of the metal, the thickness of the metal, and the tempera- 

 tures applied are all factors which influence the efficiency of the 

 pasteurizer. Factors greatly affecting the conductivity of the 

 pasteurizer wall are the formation of a layer of water on the 

 outside of the heating cylinder and the burning on of milk 

 solids on the heating surface. 



1. Layer of Water on Outside of Heating Surface. The 

 layer of water on the outside of the heating surface is formed by 

 condensation of steam during the process of pasteurization. As 

 the thermal conductivity of water is only .00124 the thermal 

 conductivity of copper is 581 times greater than that of water; 

 it is therefore economy to have the thickness of the layer of 

 water reduced to the minimum. 



The Danish Experiment Station succeeded in increasing the 

 efficiency of the continuous pasteurizer materially by soldering 

 on the outside of the heating cylinder a series of rings at a 

 downward angle of 45. These rings remove the water quickly, 

 the thickness of the layer of water was reduced, and the eco- 

 nomic efficiency of the machine increased about 50 per cent 

 (Fig. 28.) 1 



2. Burning of Milk Solids on Heating Surface. Burned-on 

 milk is extremely poor in conductivity. It is therefore of much 

 importance that all particles of burned-on milk are properly 

 removed after each time the machine has been used. This is 

 done most readily by filling the machine with water and washing 

 powder immediately after pasteurization. By leaving the 

 dasher revolving from ten to fifteen minutes it is quite easy to 

 remove with a brush all milk particles adhering to the heating 

 surface. A knife or any kind of metal should never be used for 

 scraping the surface, as such tools will cause rough places and 

 when the milk strikes such it will readily burn on. If any 

 amount of milk has burned on to the pasteurizer slacked lime 

 and sal soda mixed in equal proportions will prove of greater 



1 Bui. 43, Danish Experiment Station, 1899. 



