14 



A. P. H. A. MILK ANALYSIS 



vidual samples as showing the quality 

 of a given milk supply. 



All laboratories conforming to stand- 

 ard procedure will keep a record of the 

 exact number of colonies developed on 

 the plates that are counted; but will 

 render their reports in round numbers 

 only. Never use more than two sig- 

 nificant left hand digits in any report, 

 raising the number to the next highest 

 round number in any case; but never 

 lowering it. Those wishing to be still 

 more conservative may use a form of 

 repiort such as "official plate count less 

 than 10,000," "official plate count 

 between 10,000 and 30,000," and the 

 like. 



(b) microscopic count or bacteria 

 (breed method) 



Various methods for counting bac- 

 teria in milk by microscopic examina- 

 tion have been described, but the 

 method that is commonly described 

 as a direct microscopic examination^' 

 of a dried film of milk has been found 

 to be the simplest and most reliable 

 method of counting the bacteria as 

 they exist in the milk itself. It is 

 recognized in this report as a standard 

 or official technique of equal standing 

 with the colony count from agar plates 

 where used for judging the quality of 

 unpasteurized milk. 



Apparatus required. 



In addition to a microscope, mi- 

 croscopic slides, stains, etc., the only 

 special apparatus required is a capillary 

 pipette which discharges 1/100 cc. 

 of milk. The most satisfactory form 

 of pipette is made from a straight 

 piece of thick walled capillary tubing 

 with a bore of such a size that the 

 single graduation mark is from \}4 

 to 2>^ inches frojn the tip. The tip 

 shall be blunt and of such a form that 

 it will discharge the milk cleanly with- 



out running back on the side of the 

 tip. Pipettes of this type are now 

 listed by all of the usual supply houses. 

 The pipettes shall be calibrated so as to 

 deliver 1/100 cc, not to contain 1/100 

 cc. Because there are many inaccur- 

 ately calibrated pipettes on the mar- 

 ket, the calibration of all pipettes shall 

 be tested by weighing the amount of 

 milk discharged on chemical balances. 

 The weight for milk should be .0103 

 grams. 



Only a single pipette is needed in 

 making a series of tests, provided this 

 is kept clean while in use. In this 

 kind of work cleanliness of glassware 

 is more important than sterilization. 

 Clean towels may be used for wiping 

 the exterior of the pipettes while mak- 

 ing the microscopic preparations, and 

 their bores may be kept clean by 

 rinsing them in clean water between 

 each sample. The small amount of 

 .water left in the bore may be rinsed 

 out in the milk sample under examina- 

 tion. This method of procedure, while 

 adding a small number of bacteria to 

 each sample, introduces only a the- 

 oretical error, tests showing that such 

 bacteria cannot subsequently be de- 

 tected, and make no difierence in the 

 final result. After use, the pipettes 

 should be kept in a glass-cleaning 

 solution, such as the commonly used 

 mixture of sulphuric acid and potassium 

 bichromate. 



Routine laboratories will find it 

 convenient to use larger microscopic 

 slides than the ordinary 1 by 3 inch 

 slide. The largest slides that have 

 been found to be conveniently exam- 

 ined with the use of a mechanical stage 

 are cut 2 by 4>^ inches. Such slides 

 may be stored in ordinary card cat- 

 alogue cases and may be very cheaply 

 prepared from thin window glass or 

 old photographic negatives. A margin 



