PREPARATION OF MEAT EXTRACt 



31 



become evident. Many organisms, however, grow best at a 

 temperature at which this nutrient gelatin is fluid, and there- 

 fore another gelatinous substance called agar, which does not 

 melt below 98 C., was substituted. Bouillon made from meat 

 extract, gelatin, and agar media, and the modifications of 

 these, constitute the chief materials in which bacteria are 

 grown. 



Preparation of Meat Extract. 



The flesh of the ox, calf, or horse is usually employed. 

 Horse-flesh has the advantage of being cheaper and containing 

 less fat than the others ; though generally quite suitable, it has 

 the disadvantage for certain purposes of containing a larger 

 proportion of fermentable sugar. The 

 flesh must be freed from fat, and finely 

 minced. To a pound of mince add 1000 

 c.c. distilled water, and mix thoroughly 

 in a shallow dish. Set aside in a cool 

 place for twenty-four hours. Skim off 

 any fat present, removing the last traces 

 by stroking the surface of the fluid with 

 pieces of filter paper. Place a clean linen 

 cloth over the mouth of a large filter 

 funnel, and strain the fluid through it 

 into a flask. Pour the minced meat into 

 the cloth, and gathering up the edges 

 of the latter in the left hand, squeeze 

 out the juice still held back in the con- 

 tained meat. Finish this expression by putting the cloth and its 

 contents into a meat press (Fig. 6), similar to that used by 

 pharmacists in preparing extracts ; thus squeeze out the last drops. 

 The resulting sanguineous fluid contains the soluble albumins of 

 the meat, the soluble salts, extractives, and colouring matter, 

 chiefly haemoglobin. It is now boiled thoroughly for two hours, 

 by which process the albumins coagulable by heat are coagulated. 

 Strain now through a clean cloth, boil for another half hour, and 

 filter through white Swedish filter paper (best, C. Schleicher u. 

 Schull, No. 595). Make up to 1000 c.c. with distilled water. 

 The resulting fluid ought to be quite transparent, of a yellowish 

 colour without any red tint. If there is any redness, the fluid 

 must be reboiled and filtered till this colour disappears, other- 

 wise in the later stages it will become opalescent. A large 

 quantity of the extract may be made at a time, and what is not 



FIG. 6. Meat press. 



