A. P. H. A. MILK ANALYSIS 



get a more correct conception of the size 

 and the causes of the errors in counts as 

 made with the ordinary technique. As 

 these investigations have produced data 

 showing that agar plate counts are 

 sufficiently accurate amply to justify 

 their use in grading milk, much uncer- 

 tainty regarding the justification for 

 their use has disappeared. 



The development of methods for 

 counting bacteria in milk by direct 

 observation under the microscope has 

 also served to differentiate more sharply 

 the various purposes for which bacteria 

 counts are useful. Thus the micro- 

 scopic method has found a large field of 

 usefulness as a method of judging the 

 quality of fresh milk as delivered at 

 milk receiving or pasteurizing stations. 



It serves this purpose so satisfactor- 

 ily that this method is recognized as an 

 official procedure in this report. On 

 the other hand there is no method for 

 judging the efficiency of pasteurization 

 by examination of a series of process 

 samples that can replace the generally 

 used agar plate technique. Likewise 

 because of the extensive use of pasteur- 

 ization, the agar plate count remains the 

 standard method for judging the number 

 of living organisms present in milk as 

 delivered to the consumer. In small 

 places where pasteurization has not been 

 introduced, the microscopic examina- 

 tion may be used, if preferred, as a means 

 of judging the quality of milk as deliv- 

 ered to the consumer. 



Collection of Samples foe 

 Bacteriological Counts 



Although the technique of the plating 

 method is fundamentally different from 

 that involved in microscopic counting, 

 microscopic counts are readily made 

 from the same samples as those used in 

 making agar plates. As the precau- 

 tions necessary for securing a fair sample 



are identical, the method of collecting 

 samples for both methods are described 

 under a single heading. 



All collecting apparatus, glassware, 

 pipettes, collecting tubes, bottles, etc., 

 shall be sterilized at a temperature of 

 at least 175° C. for one hour. 



Each sample shall consist of at least 

 10 cc. of milk. Before taking the 

 sample the milk shall be mixed as thor- 

 oughly as possible. If the original 

 container can be inverted the mixing of 

 the milk should be done by inverting 

 it several times. If this is impossible, 

 the milk should be stirred with some 

 sterile stirrer. Any stirrer already in 

 the container may be used. If there is 

 none in the container, the sampling 

 pipette (or any other sterile article) 

 may be used; but it shall be used for one 

 container only until it is again sterilized. 



A sample merely poured from a 

 large can is not a fair sample unless the 

 milk in the can is thoroughly stirred. 

 Neither is a sample of mixed milk, 

 taken after it is poured into an un- 

 sterilized weighing vat, a fair sample 

 from which to judge the quality of the 

 milk before it was poured into the 

 vat. The sample shall be taken from 

 cans by means of a glass or aluminum 

 tube with straight sides, long enough 

 to reach the bottom of the original 

 container and inserted, not too rapidly, 

 with the top of the tube left open. 

 This will result in the tubes containing 

 a cylindrical section of the milk from 

 top to bottom of the can. The finger 

 then placed on the top of the tube will 

 make it possible to withdraw the tube 

 full of milk and transfer it to the 

 sampling bottle. The sampling bottle 

 should be large enough to hold the 

 entire contents of the tube, all of which 

 must be reserved as the sample. Each 

 tube shall be used for collecting a 

 single sample only, and must be washed 



