REACTION OF MEAT EXTRACT 149 



ten minutes. (This precipitates some of the albumins, 

 the haemoglobin, etc., from the solution.) 



4. Strain the mixture through sterile butter muslin 

 or a perforated porcelain funnel, then filter the liquid 

 through Swedish filter paper into a sterile "normal" 

 litre flask, and when cold make up to 1000 c.c. by 

 the addition of distilled water to replace the loss from 

 evaporation. 



5. If not needed at once, sterilise the meat extract in 

 bulk in the steam steriliser for twenty minutes on each 

 of three consecutive days. 



Calf, sheep, or chicken flesh is occasionally substi- 

 tuted for the beef; or the meat extract may be pre- 

 pared from animal viscera, such as brain, spleen, liver, 

 or kidneys. 



NOTE. As an alternative method, 5 c.c. of Brand's meat juice 

 or 3 grammes of Wyeth's beef juice, or 10 grammes Liebig's 

 extract of meat (Lemco) may be dissolved in 1000 c.c. distilled 

 water, and heated and filtered as above to form ordinary or 

 single strength meat extract. 



Media, prepared from such meat extracts are, however, 

 eminently unsatisfactory when used for the cultivation of the 

 more highly parasitic bacteria; although when working in tropical 

 and subtropical regions their use is well-nigh compulsory. 



Reaction of Meat Extract. Meat extract thus pre- 

 pared is acid in its reaction, owing to the presence of 

 acid phosphates of potassium and sodium, weak acids 

 of the gly colic series, and organic compounds in which 

 the acid character predominates. Owing to the nature 

 of the substances from which it derives its reaction, 

 the total acidity of meat extract can only be estimated 

 accurately when the solution is at the boiling-point. 



Moreover, it has been observed that prolonged boiling 

 (such as is involved in the preparation of nutrient 

 media) causes it to undergo hydrolytic changes which 

 increase its acidity, and the meat extract only becomes 

 stable in this respect after it has been maintained at 

 the boiling =point for forty=five minutes. 



