16 



A. P. H. A. MILK ANALYSIS 



giving approximately, the field desired 

 (for example a 6.4 x ocular). It should 

 also be fitted with a mechanical stage. 

 If the large slides described above are 

 used, this must be a special stage 

 allowing a larger area of the slide to be 

 examined than can be examined with 

 the usual mechanical stage.* 



To standardize the microscope, place 

 the stage micrometer on the stage of 

 the microscope, and by selection of 

 oculars or by adjustment of the draw 

 tube, or both, bring the diameter of 

 the whole microscopic field to .205 mm. 

 When so adjusted, each field of the 

 microscope covers an area of approx- 

 imately 1/3000 cm2. (actually 1/3028 

 cm^). This means that the dried milk 

 solids from 1/300,000 part of a cc. of 

 milk are visible in each field of the 

 microscope. Therefore if the bacteria 

 in one field only are counted, the 

 number found should be multiplied by 

 300,000 to give the estimated number 

 of bacteria per cc. In practice, how- 

 ever, more than a single field is exam- 

 ined so that the number used for 

 multiplication is smaller than this. 



As the microscopic examinations 

 must be made with greater care where 

 the bacteria are relatively few in 

 number, it is required that, in grading 

 low count milk, a special ocular 

 micrometer* with a circular ruling 

 divided into quadrants shall be used. 

 In using this micrometer, the micro- 

 scope shall be so adjusted that the 

 diameter of the circle on the eye 

 piece micrometer shall be .146 mm. 

 In this case the amount of dried milk 

 solids examined in each field of the 

 microscope is 1/600,000 part of a cc. 

 of milk. The limitation of the exam- 

 ination of the slide to the central por- 

 tion of each field, avoids using the 

 margins of the field where definition 



* May be secured from the Bausch and Lomb Optical Co., 

 Rochester, N. Y. 



is hazy, and lessens the danger of 

 overlooking bacteria. Likewise the 

 magnification used is greater than that 

 used where the whole field is examined. 



Counting and Grading Milk. 



The number of fields of the microscope 

 to be examined varies with the character 

 of the milk, and with the character of 

 the data desired. Experience has 

 shown that where the purpose is 

 primarily to detect and eliminate the 

 worst milk from ordinary market milk 

 supplies, it is entirely permissible to 

 use the entire field of the microscope 

 for examination. At least thirty rep- 

 resentative fields of the microscope 

 should be examined for each sample of 

 milk. Where the average number of 

 individual bacteria (not groups of 

 bacteria) is less than one per field, it 

 may be assumed that the milk will 

 ordinarily give an official plate count 

 of less than 60,000 per cc. Where the 

 number is less than 100 in 30 fields 

 (average of less than 3 1/3 bacteria 

 per field) it may be assumed that the 

 official plate count will be less than 

 200,000 per cc. Where less than 1000 

 per 30 fields (average of less than 

 33 1/3 per field) is may be assumed 

 that the official plate count will not 

 exceed one to two million per cc. 



Where counts are made in order to 

 enforce more stringent standards, as at 

 Grade A plants'' or as a basis for 

 premiums on milk giving an official 

 plate count of less than 10,000 per cc, 

 the special eyepiece micrometer de- 

 scribed above shall be used and the 

 microscope so adjusted that only the 

 central portion of each field is exam- 

 ined for counting. Where less than 

 5 bacteria are found in 60 fields (average 

 of less than 1/12 of a bacterium per 

 field) it may be assumed that the milk 



* Listed by the Bausch and Lomb Optical Co. 



