44 THE BACTERIOLOGICAL EXAMINATION OF MILK 



the pinchcock. When one quarter of the udder is exhausted, the 

 operation, if a sufficient quantity has not been drawn, can be 

 repeated with a second quarter. 



The screw pinchcock on the vacuum tubing is then screwed down 

 permanently, the milking tubes are withdrawn, and the milk stored 

 ready for use. The cream will rise to the top, and the delivery 

 tiibe will draw from the lower layer of milk free from fat. When 

 the cream has fully risen, charge the syphon by blowing through 

 the lateral orifice of the flask, and the milk can then, by releasing 

 the pinchcock, be drawn off at any time and in any quantity required. 

 The authors have kept milk in this manner without contamination 

 for more than two years, drawing off the quantity required at 

 intervals. 



{!)) Sterilised milk. — Milk sterilised by heating cannot, owing 

 to certain organic or chemical changes which take place in it at or 

 near the boiling point, be looked upon as so favourable a medium 

 for the growth of micro-organisms as that which is drawn sterile 

 from the cow in the manner referred to above. Under ordinary 

 circumstances, however, it may be necessary to employ it, and in 

 this case it is well, if possible, to procure freshly separated milk and 

 allow it to stand in a cold place for twelve hours in order that any 

 fat left by imperfect separation may rise to the surface. The fat- 

 free milk is then syphoned off from the bottom, run into sterile 

 tubes or flasks, and steamed in the Koch steriliser for one hour on 

 three successive days. As, however, this may be insufficient in some 

 cases to destroy the spores of the bacillus subtilis if they exist in 

 quantity in the milk, the containing tubes should be tested after 

 sterilisation by being placed in the incubator at 37° for twenty- 

 four hours. 



24. Litmus milk — 

 Sterilise neutral litmus solution by boiling "in a test tube, and 



when litmus milk is required, introduce into a tube of sterile 

 milk by means of a finely drawn flamed pipette, and without 

 withdrawing the cotton-wool plug, a sufficient quantity of the 

 solution to give the milk a distinct lavender tint. 



25. Gelatinised milk — 



Prepare a 15 per cent, solution of gelatine in distilled water. 

 Neutralise or make feebly alkaline to litmus^ with caustic soda 

 solution. Run into test tubes in the proportion of 5 c.c. per 



^ Neutralisation to phenolphthalein would, in this case, make the resulting 

 medium of much greater alkalinity than the normal milk itself. 



