148 NUTRIENT MEDIA 



the preparation of most media absolute sterility of the 

 apparatus used is not essential. 



MEAT EXTRACT. 



A watery solution of the extractives, etc., of lean 

 meat (usually beef) forms the basis of several nutrient 

 media. This solution is termed " meat extract " and it 

 has been determined empirically that its preparation 

 shall be carried out by extracting half a kilo of moist 

 meat with one litre of water. For many purposes, 

 however, it is more convenient to have a more concen- 

 trated extract; one kilo of meat should therefore be 

 extracted with one litre of water, to form " Double 

 Strength" meat extract. 



It was customary at one time, and is even now in 

 some laboratories to use either " shin of beef" or "beef- 

 steak" both contain muscle sugar which often needs 

 to be removed before the nutrient medium can be 

 completed. Heart muscle (bullock's heart or sheep's 

 heart) is much to be preferred and from the point of 

 economy, ease and cleanliness of manipulation, and 

 extractive value, the imported frozen bullock's hearts 

 provide the best extract. 



Meat extract (Fleischwasser) is prepared as follows: 



1. Measure 1000 c.c. of distilled water into a large 

 flask (or glass beaker, or enamelled iron pot) and add 

 1000 grammes (roughly, z\ pounds) of fresh lean 

 meat e. g., bullock's heart finely minced in a 

 mincing machine. 



2. Heat the mixture gently in a water-bath, taking 

 care that the temperature of the contents of the flask 

 does not exceed 40 C. for the first twenty minutes. 

 (This dissolves out the soluble proteids, extractives, 

 salts, etc.) 



3 . Now raise the temperature of the mixture to the 

 boiling-point, and maintain at this temperature for 



