VEGETABLE INFUSIONS 37 



2. Media made from vegetable tissues. 



Vegetable infusions are but seldom used in practical bacteriology. The 

 most important of them, however, may be mentioned. 



Malt extract. 



1. Grind up 100 grams of germinated barley (malt), and add 1000 grams 

 of water. 



2. Heat the mixture to 55-58 C. for one hour : the starch is converted 

 into maltose by the diastase and a true beer wort obtained ; the tempera- 

 ture must not exceed 58 C., otherwise the diastase will be destroyed. 



3. Boil. Filter through Chardin paper. 



4. Tube. Sterilize at 115 C. 



Yeast extract. 



Mix 100 grams of yeast with 1000 grams of water. Boil and filter through 

 Chardin paper. 



Tube the slightly acid filtrate or pour it into a flask, and sterilize at 

 115 C. 



The filtrate may be neutralized or made slightly alkaline by the careful addition 

 of normal soda solution before filtering. The addition of 5 per cent, cane sugar 

 or glucose before filtration increases the nutritive value of the extract. If the 

 extract is not clear when filtered, a little phosphoric acid 1 may be added, and the 

 reaction brought back with lime water. Heat to 116-117 C. for 5 minutes. Filter. 

 Tube. Sterilize at 115 C. 



Spronck's peptone yeast extract. 



1. To 5 litres of water add 1000 grams of commercial yeast (not brewers* 

 yeast). 



2. Boil the mixture for 20 minutes, stirring frequently, pour into cylindrical 

 vessels and leave for 24 hours. 



3. Decant the cloudy liquid and add 5 grams of salt and 10 grams of 

 Witte's peptone for each litre. 



4. Neutralize exactly, and then make alkaline to the extent of 7 c.c. of normal 

 soda per litre. Boil. Filter through Chardin paper. 2 



5. Pour into flasks. Sterilize at 115-120 C. 



Hay infusion. Straw infusion. 



Macerate 15-20 grams of finely chopped hay or straw in 1000 grams of 

 water for 1 or 2 hours. Boil for a few minutes, filter, tube and sterilize 

 at 115 C. 



The infusion which is sometimes a little acid may be neutralized in the 

 ordinary way. 



Potato infusion. 



Clean and scrape a few potatoes ; add a litre of water to each 20-30 

 grams of pulp. Leave to stand for 3 or 4 hours. Decant. Boil the 

 supernatant fluid. Filter. Tube. Sterilize. The infusion is often acid 

 and can be neutralized before filtration. 



Infusion of carrot is prepared in a similar manner. 



Haricot decoction. 



1. Macerate 50-60 grams of white haricot beans in a litre of water for 

 several hours in the cold. 



1 Specific gravity 1-349, and containing 39 '4 grams of anhydrous acid per cent. 



2 If the yeast contain meal the filtered liquid remains slightly cloudy, but this is of 

 no consequence. 



