PRESENT STATUS OF THE PASTEURIZATION OF MILK 11 
Thornton (57), who studied this question in England, came to the 
conclusion that the killing of bacteria must be regarded as due largely 
to thermal rather than to electrical changes, but thought his results 
indicated some electrical action on the molecular structure of the 
bacteria. Beattie (9, 10), also working in England on the same prob- 
lem, came to the conclusion that heat was not the principal factor in 
killing bacteria by electricity, but found that to obtain satisfactory 
results the temperature should not be below 145° F. In the United 
States an electric process was investigated about 1918 by Anderson 
and Finkelstein (2, p. 405). Their conclusion was: 
The destruction of bacteria in the Electro-pure process is apparently due to the 
heat produced by the electric current rather than to the electric current itself. 
The Electro-pure process furnishes a method for producing a very sudden high 
temperature for a brief period of time. 
Gelpi and Devereux (27) concluded that in laboratory tests for 
spore destruction the electro pure process at 71° C. (159.8° F.) with 
momentary holding was superior to pasteurization at 62.8° C. (145° 
F.) for 30 minutes. 
It seems evident from a review of the literature that in the use of 
electricity, as it has been applied, sufficient heat is generated by elec- 
tricity, or by a combination of steam and electricity, to raise the milk 
to a high temperature. Since the temperatures reached are in them- 
selves destructive to most bacteria, the problem of determining 
whether the effect of electricity is due to heat or to electric action is 
difficult. 
The use of ultra-violet rays for the destruction of bacteria in milk 
has not proved to be of commercial value. Experiments with these 
rays carried on by Ayers and Johnson (6) showed that, while the rays 
cause great destruction of bacteria in milk when exposed under suit- 
able conditions, the process in its then state of development could not 
replace pasteurization. It was difficult to obtain the proper exposure 
of milk to the rays on a scale sufficient to permit of practical operation, 
and it was impracticable to secure suitable bacterial reductions with- 
out seriously injuring the flavor of the milk. 
METHODS OF HEATING MILK 
At present three processes of heating milk are practiced in the 
United States. The first is known as the flash process, the second as 
the holder or holding process, and the third as pasteurization in the 
bottle. 
The flash process consists in heating the milk rapidly, then cooling 
it quickly. In this process the milk is heated for 30 seconds to 1 min- 
ute only, usually at a temperature of 160° F. or above. This method 
does not comply with the usual time and temperature definition of 
"pasteurization." Most cities prohibit its use in the pasteurization 
of milk. 
By the holder process the milk is heated to temperatures of from 
142° to 145° F. and held for approximately 30 minutes, after which 
it is cooled rapidly. Sometimes the milk, instead of being held at a 
certain temperature in one tank for 30 minutes, is merely retarded in 
its passage through several tanks, or other retarding device, so that 
the theoretical length of time required for the milk to pass through is 
about 30 minutes. In such cases, however, there is not always assur- 
ance that all the milk is held for the desired time, 
