46 THE BACTERIOLOGICAL EXAMINATION OF MILK 



neutral litmus solution to produce a deep lavender colour. Run 

 into flasks or test tubes in measured quantities, and sterilise at 

 ioo° C. in the Koch for half an hour on three successive days. 

 Sloped tubes will be found preferable, as there is a tendency to 

 unequal coloration of the medium when the tubes are allowed 

 to cool in a vertical position. 



28. P etruscJiky s litmus whey ^ — 



Fresh milk is warmed up to 40°, and sufficient very dilute HCl 

 is added to cause precipitation of the casein. This latter is then 

 filtered off, and the clear whey neutralised to litmus with dilute 

 caustic soda solution. The fluid is then steamed in the Koch for 

 two hours, by which procedure any casein converted into acid 

 albumen by the hydrochloric acid is precipitated. This is filtered 

 off, and the filtrate should now be clear, colourless, and neutral in 

 reaction, its chief constituent being lactose. To this 5 per cent, of 

 a saturated solution of litmus is added, and the medium run in 

 equal quantities into test tubes and then sterilised. When growth 

 has taken place in an inoculated tube the amount of acid can be 

 estimated by dropping in from a burette standardised soda solution 

 until the tint of an uninoculated tube is reached. 



29. Litmus lactose solution — 



Dissolve 2 grammes of pure lactose in 40 c.c. of nutrient bouillon, 

 and make up to 100 c.c. with distilled water. Neutralise to litmus^ 

 steam in Koch steriliser for half an hour and filter. Add a sufficient 

 quantity of neutral litmus to produce a deep lavender colour. Run 

 into test tubes and sterilise in the Koch steriliser for half an 

 hour on three successive days. 



30. Artificial lacto-serum {Bordas and Joulin) ^ — 



This medium, which it is claimed has the same cultural pro- 

 perties as milk, is prepared as follows : — 



Lactose . . . . . -55 grammes. 



Pulverised egg albumen . . . . i8 „ 



Sodium chloride ..... 0-60 „ 



Distilled water ..... 1000 cc. 



^ The recognised formula of this medium, which was extensively used by 

 Petruschky in his researches upon the Bacillus coli, is given, although the 

 authors have found that the precipitation of the casein by HCl is not so reliable 

 or satisfactory in its final results as that by rennet in the method described 

 above. 



^ Society de Biologie, 9th Jan. 1897. 



