STANDARD METHODS FOR THE SANITARY ANALYSIS 



OF MILK 



AMERICAN PUBLIC HEALTH ASSOCIATION 



LABORATORY SECTION 



The rapid development in the use of 

 laboratory methods for controlling the 

 quality of milk, makes a third revision 

 of the Standard Methods necessary 

 In the second report^ of this committee, 

 it was pointed out that two types of 

 bacteriological analysis of milk were 

 required. These were distinguished as 

 routine and research methods. In the 

 years that have passed there has been 

 an increasing appreciation of the need 

 for such a distinction, and less tendency 

 on the part of research workers to use 

 the routine type of analysis for inves- 

 tigations in which the greatest possible 

 accuracy of counts is more important 

 than moderate accuracy, combined with 

 a rapid and relatively inexpensive 

 technique. 



I. Routine Milk Analyses. This 

 type of analysis is designed for general 

 control of the public milk supply, and 

 for the purpose of grading milk. For 

 routine analysis the work must be 

 capable of being done quickly and 

 cheaply with fair uniformity of results. 

 Some opposition to the methods out- 

 lined in the previous report has existed 

 because of the willingness of the com- 

 mittee to meet this need. This ob- 

 jection has been considered but it is 

 still felt that the necessity for such 

 quick and inexpensive methods must 

 be recognized. No such rapid progress 

 in milk control work could have been 

 made as has been made since 1916, if a 

 more time consuming procedure had 



been outlined as the standard pro- 

 cedure. 



Numerous investigations^ to be sure 

 have shown that higher counts may be 

 obtained from agar counts where lac- 

 tose, or meat infusion, or both are 

 added, or the count may be increased 

 by prolonged incubation. The expense 

 of such procedures in time or in 

 materials would greatly reduce the 

 number of analyses that a routine 

 laboratory could make, and the quick- 

 ness with which results could be 

 utilized. Hence they limit its useful- 

 ness. Long experience in some of our 

 most important control laboratories 

 shows that the results secured by the 

 agar plate technique described in this 

 report give a sufficiently accurate pro- 

 portional fraction of the true count to 

 be of value in milk control. Therefore 

 it seems unnecessary to require the 

 more refined technique for routine 

 work; but the methods used should be 

 uniform in different laboratories. 



It is not necessary in public health 

 work that the counts used should 

 represent the actual number of bacteria 

 present or even that they should 

 represent the greatest number of col- 

 onies that could be developed on 

 agar media, if there is a fairly accurate 

 knowledge of the percentage devel- 

 oping. The standards set for various 

 grades of milk in the usual ordinances 

 recognize the fact that the counts 

 obtained by the standard procedure 



