ALTERATION OF MILK BY HEAT. 115 



Conditions required for killing bacteria. Temperature and 

 time of exposure are important factors with reference to meth- 

 ods of killing bacteria by heat. A high temperature foi^a 

 short time is equally as efficient as a lower temperature for a 

 longer time. 



Bacterium tuberculosis. This is the most common of the 

 pathogenic organisms found in milk, and at the same time the 

 one of them most resistant to heat. Therefore it has been used 

 as a basis for determining methods of pasteurizing, with proper 

 regard to other considerations not concerning bacteria. The 

 fact that 140F. (60C.) for twenty minutes is fatal to Bacterium 

 tuberculosis is abundantly confirmed (13, 16, 20). This is 

 equally true of the other pathogenic bacteria like B. diphtheria' 

 and B. typhosus, occasionally disseminated in milk (13). 



To kill the tubercle bacterium, when the time of exposure 

 of heat is shortened to one minute or less, the temperature 

 must be 160F. (71C.) or more (6, 18). B. diphtheria (13) 

 and B. typhosus (9) are destroyed in like manner. 



.Alteration of milk by heat. The bacteriological requirements 

 are by no means the only factors in the problem, for the heating 

 of milk presents a formidable array of obstacles that must be 

 avoided in formulating pasteurization processes. These in- 

 volve all sorts of considerations, from those concerning the 

 chemical constitution of the heated milk with respect to nutri- 

 tion, to the practical requirements of the milk trade. 



Cooked taste. This is left permanently in milk by exposure 

 to about 158F. (70C.) for fifteen minutes. Exposure to 

 170F. (76.6C.) in a closed vessel, followed by immediate 

 cooling, is the highest temperature that may be used without 

 leaving the cooked taste. Ten minutes at 160F. (7lC.) is 

 safe. 



Alterations of milk constituents. The chemical constitution 

 of milk is not appreciably altered by exposure to 140F. (60C.) 

 for a short time. At about 140F., milk heated with exposure 

 to air forms a thin pellicle on the surface, consisting of casein 

 and albuminoid matter. This is probably due to the drying 

 of the upper layer of the milk and does not occur in closed 

 vessels. Higher temperatures, especially near the boiling 



