Pasteurization of Milk 



19 



without seriously affecting those al- 

 kali producing species which develop 

 spores, will be liable to cause serious 

 conditions in the raw milks presented 

 for pasteurization. 



Action of Heat on Different Bac- 

 teria. It is well known that exceed- 

 ingly wide variations in susceptibility 

 to the destructive action of heat exists 

 among the different species, groups 

 and strains of bacteria and between 

 individual numbers of the same. It is 

 also well known that different products 

 resulting from the growth of these 

 variable biological agents have more 

 or less selective destructive action upon 

 each other, especially when tempera- 

 tures approximating their thermal 

 death points are applied. Under such 

 conditions the utilization for estima- 

 tions of pasteurization efficiency of any 

 numerical system, such as a percent- 

 age of the bacteria in the raw milk 

 killed by the process, gives results in 

 which the implied accuracy of the 

 means of expression of the results is 

 apt to be entirely fictitious. The use 

 of percentage reductions in numbers 

 of living bacteria as a measure of effi- 

 ciency of pasteurization is not to be 

 recommended. 



Interpretation of Results. It is evi- 

 dent that the results of the operations 

 of all of those very variable factors in 

 the production, handling and transpor- 

 tation of milk, up to the time of its in- 

 troduction into the pasteurization sys- 

 tem are impossible of adequate detec- 

 tion by any simple laboratory or other 

 technical procedure and that they are 

 not easy of estimation even if one has 

 a reasonably correct and detailed his- 

 tory, of the product. ' 



To those very familiar with the re- 

 sults of bacterial investigations of pas- 

 teurization apparatus and who also are 

 experienced in biological problems in 

 milk production and handling, in pas- 

 teurization processes, the results of 

 bacterial counts of the raw and pas- 



teurized products and of sample col- 

 lected at important intermediary stages 

 in the process, are of considerable 

 value, although they are of more sub- 

 stantial utility when they can be corre- 

 lated with the more or less detailed 

 history of the product including the 

 conditions to which it has been sub- 

 jected prior to pasteurization. There- 

 fore, in every investigation, having as 

 its object the determination 'of the effi- 

 ciency of a pasteurization system, one 

 of the most essential features consists 

 in the collection of a well-planned ser- 

 ies of samples for the determinations 

 of total bacterial counts, by use of 

 the methods recommended by the 

 Committee on Standard Methods of 

 the Laboratory Section of this Asso- 

 ciation for the bacteriological examina- 

 tion of milk for legal purposes, i. e., 

 those of the greatest degree of accu- 

 racy. However, in the interpretation 

 of the results of such studies, those of 

 adequate training and experience in 

 this and allied fields will always give 

 due consideration to the information 

 available regarding the history of the 

 product subjected to the pasteuriza- 

 tion process. 



(b) Biological Demonstrations of 

 Efficiency, Procedure B. 



B. Another procedure considers the 

 final total numbers of living bacteria 

 at some stage in the history of the 

 milk subsequent to its pasteurization, 

 as a measure of efficiency of the pas- 

 teurization system. 



This plan is obviously better adapted 

 to the estimation of the general bac- 

 terial condition or quality of the prod- 

 uct itself than it is useful in the de- 

 termination of the pasteurization effi- 

 ciency. It consists essentially in ac- 

 cepting the results of one, i. e., the final 

 step in the performance of a rather 

 elaborate study, as adequately repre- 

 senting the results of the entire inves- 

 tigation. As generally used it compli- 

 cates the study of the vital question of 



