420 CITY MILK SUPPLY 



have stood a few days and its potency have been determined. Thereafter it can be 

 used indefinitely. These tests for heated milk are only active in the case of milks 

 which have been heated to 176F. or 80C. (Jensen's "Milk Hygiene," Pearson's 

 translation, p. 192.) 



Microscopic Test for Heated (pasteurized) Milk Frost and Ravenel. About 15 c.c. 

 of milk are centrifuged for 5 min., or long enough to throw down the leucocytes. 

 The cream layer is then completely removed with absorbent cotton and the milk drawn 

 off with a pipette, or a fine-pointed tube attached to a Chapman air pump. Only 

 about 2 mm. of milk are left above the sediment which is in the bottom of the sedi- 

 mentation tube. 



The stain, which is an aqueous solution of safranin 0, soluble in water, is then 

 added very slowly from an opsonizing pipette. The important thing is to mix stain 

 and milk so slowly that clotting does not take place. The stain is added until a deep 

 opaque rose color is obtained. After standing 3 min., by means of the opson- 

 izing pipette, which has been washed out in hot water, the stained sediment is then 

 transferred to slides. A small drop is placed at the end of each of several slides and 

 spread by means of a glass spreader, as in Wright's method of opsonic index determi- 

 nations (see author's note at end of the methods). 



In an unheated milk the polymorphonuclear leucocytes have their protoplasm 

 slightly tinged or are unstained. 



In heated milk the polymorphonuclear leucocytes have their nuclei stained. In 

 milk heated to 63C. or above, practically all of the leucocytes have their nuclei 

 definitely stained. When milk is heated at a lower temperature the nuclei are not 

 all stained above 60C. The majority, however, are stained. 



87. Specific Gravity. The specific gravity of certified milk shall range from 

 1.029 to 1.034. 



88. The specific gravity shall be determined at least each month. 



The Quevenne lactodensimeter is recommended for the determination of the spe- 

 cific gravity. It is made like an ordinary aerometer and divided into degrees which 

 correspond to a specific gravity from 1.014 to 1.040, or only from 1.022 to 1.038, 

 since, by the latter division, a greater space is gained between the different degrees 

 without unduly lengthening the instrument. From such a lactodensimeter one can 

 easily read of? four decimal places. 



The milk the specific gravity of which is to be determined is well shaken and poured 

 into a high glass cylinder of suitable diameter; the aerometer is dropped in slowly, 

 in order to prevent its bobbing up and down. (The bulb should be free from adhering 

 air bubbles.) The figures on the stem are the second and third decimals of the num- 

 bers of the specific gravity, so that 34 is to be read 1.034. For this examination, the 

 temperature of the milk must be 15C. (60F.); if it is not, the specific gravity of the 

 milk at 15C. must be calculated from the specific gravity found and from the tempera- 

 ture, for in milk inspection and analysis this is the standard. 



METHODS AND REGULATIONS FOR THE MEDICAL EXAMINATION OF EMPLOYEES, 

 THEIR HEALTH AND PERSONAL HYGIENE 



89. A medical officer, known as the attending dairy physician, shall be selected 

 by the commission who should reside near the dairy producing certified milk. 

 He shall be a physician in good standing and authorized by law to practice medi- 

 cine; he shall be responsible to the commission and subject to its direction. In 

 case more than one dairy is under the control of the commission and they are in 

 different localities, a separate physician should be designated for employment 

 for the supervision of each dairy. 



