14 BULLETIN 890, U. S. DEPARTMENT OF AGRICULTURE. 
surface is so small that the heating medium must be of a considerably 
higher temperature than that to which the milk is to be heated, and 
steam is introduced directly into the heating chamber. As the heating 
medium is sometimes allowed to run as high as from 190° to 200° F., 
certain parts of the milk may be overheated, which gives a scorched 
or burnt taste to the milk. An automatic temperature control is very 
essential with this type of heater. The flow of the milk to the ma- 
chine, as well as its temperature, will fluctuate more or less, and unless 
there is an accurate automatic temperature control attached there is 
almost sure to be an unevenness of temperature in the milk as it comes 
from the heater, even though a man spends his entire time in operat- 
ing the machine. For example, at a plant using this type of heater 
with no temperature control, temperatures were taken in each com- 
partment of a positive holder of the milk as it came from the heater 
and these temperatures varied from 14A C to 156° F. 
Good results are obtained by using a preheater or a regenerator in 
conjunction with this type of heater. In this way the capacity of the 
heater is increased, thus lessening the tax on the machine, and less 
extreme temperatures can be used. At some plants using this system 
of pasteurization the milk is preheated to from 110 c to 120° F. before 
it enters the pasteurizer proper : thus the milk has to be heated only 
25 or 30 degrees more in the machine. Much more satisfactory and 
uniform results are obtained in this way. Temperatures obtained 
with this preheating system are shown in Table -i. plant Xo. 7. 
A combination of this type of heater and a series of vats will pro- 
duce good results. The milk can be heated to 120° or 135° F. before 
it goes to the vats, and thus the dangers of overheating can be 
eliminated. The remainder of the heating will be accomplished in 
the vats, and the milk will be held in them until it is to be cooled. 
Drum or film heater. — With some machines of the drum or film 
type (fig. 7) the milk flows between two or more heating surfaces in a 
thin film, while in others it flows in a thin film over a revolving drum 
inside of which is the heating medium. The heating water is kept 
at the desired temperature either by heating it in a tank outside the 
machine or by the introduction of steam directly into the machine. 
The former method is preferable, as there is less chance of overheat- 
ing the milk. The milk must be kept in motion, and this is usually 
done by means of a revolving drum. The regenerative principle is 
used with some machines of this type. 
Some of the advantages of this type of heater are : 
1. The area of the heating surface is considerably greater than with 
the " kettle " type, and there is less danger of overheating some of the milk. 
2. It is less expensive than some other types of heaters. 
3. The machines are durable. 
