PASTEURIZATION. 177 
an uncommon occurrence to have the heating wall of a pasteur- 
izer burst. This could be prevented by connecting a safety, or 
pressure, valve, to the pasteurizer. 
(2) Degree of Adhesiveness.— Roughness, due to either 
defects in the metal itself, or to milk or cream being burned 
on the heating-surface, is a serious defect. Such a condition 
Fic. 117.—The Simplex regenerative pasteurizer (apart). 
causes particles of milk or cream to move very slowly over the 
heating-surface; it tends to roll in much the same way as 
drops of liquids do when caused to flow over a slanting dry 
rough surface. As a consequence more and more casein will 
adhere. The thicker the layer of foreign matter is on the 
heating-surface, the greater the difficulty in getting the greatest 
efficiency from the pasteurizer. 
It is important that the milk or cream be forced over the 
heating-surface with greater rapidity than could result from 
its own gravity. On heaters or pasteurizers, where milk flows 
