MILK 95 



may be met with, and a comparative sensitive test is in this way 

 available. 



For milk work it is not usual to confirm the diagnosis by a 

 pathogenicity test. In certain cases this may be necessary and 

 may be carried out as described in Chapter I. 



One great value of this test is that it is a non-multiplying 

 one, so that it is especially useful for vended milk. A draw- 

 back to its utility is the fact that the spores are not always uni- 

 formly distributed in the milk. 



D. Estimation of the Sediment or Dirt in Milk. 



Numerous investigators have attempted to judge the cleanli- 

 ness of milk samples from a determination of the amount of dirt 

 or sediment obtainable from them. The methods used are either 

 by slow sedimentation or by centrifugalisation. 



Of slow sedimentation methods, Orr's modification of Hous- 

 ton's method is perhaps the best. The apparatus used is shown 

 in Fig. II. The top of the centrifugal tube fits outside the 

 lower part of the large glass cylinder, the two being connected 

 by rubber tubing. The centrifuge tube is graduated in tenths 

 and hundredths of a c.c. A litre of milk (after addition of I c.c. 

 formalin) is allowed to stand for twelve hours in the large 

 cylinder. At the end of twelve hours, a brass rod fitted with 

 a rubber stopper at the lower end is passed through the milk, 

 and fitting into the outlet of the cylinder prevents escape of the 

 fluid when the centrifuge tube is detached. The tube is centri- 

 fugalised (1500 to 2000 revolutions per minute) for three to 

 five minutes, and the milk poured off. Sodium carbonate solution 

 (i per cent.) is then added up to the 10 c.c. mark and the centri- 

 fugalising repeated. The deposit is then read off. 



A litre of milk is a large amount to use for a single test and 

 for routine work a rough but sufficiently reliable estimation of 

 the volume of the sediment may be obtained by direct centri- 

 fugalisation. Tubes, which are of narrow calibre at the lower 

 closed ends (similar in shape to those used by Orr, Fig. n), are 

 used, preferably with a capacity of 50 c.c. The milk is filled in 

 to a definite volume (50 c.c.), centrifugalised for a definite period 



