649 
So far as we are able to judge from our present knowledge the best 
temperature is 60° C. continued twenty minutes. A higher degree of 
heat for a shorter time is just as effective so far as the destruction of 
the bacteria is concerned. 
It may be confidently stated that the tubercle bacillus and the 
specific micro-organisms causing typhoid fever, diphtheria, dys- 
entery, cholera, etc., are rendered harmless by heating to 60° C. for 
twenty minutes. This opinion is based not only upon experimental 
data which have been obtained in the Hygienic Laboratory and re- 
cently published a but upon the experience and experiments of many 
others who have investigated this subject. It is fortunate that the 
thermal death points of the pathogenic bacteria that most concern us 
are below those of the ferments in milk, for in this way all infectious- 
ness may be destroyed without “ devitalizing ” the milk. 
So far as the true bacterial toxines are concerned, our knowledge is 
not so precise. We know that the true bacterial toxines are thermo- 
labile ; that is, readily affected by heat. Kitasato 6 showed that 65° C. 
and above is sufficient to destroy tetanus toxine in five miutes or less. 
It will sometimes withstand 60° C. for ten minutes, but is destroyed 
at 60° C. for twenty minutes. 
Diphtheria toxine is also rendered almost inert at about 60°, and 
botulism toxine is almost equally sensitive to heat. 
There is a group of bacterial poisons, however, which resists high 
temperatures. For instance, Marshal and Gallston found that heat- 
ing the cell substance of B. coli commune to 134° C. for fifteen min- 
utes did not appreciably lessen its toxicity. Cooley and Yaughan 
heated the same substance in a sealed tube to 164° C. without ren- 
dering it inert. Yaughan states further in his recent Shattuck lec- 
ture that many crude bacterial poisons withstand the boiling tem- 
perature. The nature of these poisons is not known. They are ob- 
tained by laboratory devices, and similar substances have never been 
found in market milk. 
It must further be observed that the poisonous properties of all 
these substances have mostly been determined by inoculation experi- 
ments and not by feeding. It can not be denied that milk may at 
times contain heat-resisting poisons, but their existence has been in- 
ferred, not demonstrated. Emphasis is placed upon this seemingly 
inconsequential point, for the reason that one of the principal objec- 
tions to pasteurization has always been that the heat does not neces- 
sarily destroy the poisonous products of bacterial activity. If such 
stable poisons are present in milk, and heat does not render them 
a Hygienic Laboratory Bulletin 42, “ The thermal death points of pathogenic 
micro-organisms in milk,” by J. M. Rosenau, Washington, 1908. 
1 Kitasato, S. : Experimentelle TJntersuchungen liber das Tetanusgift. Zeit. f. 
Hyg., vol. 10, 1891, p. 267. 
