NUTRIENT SOLUTIONS 95 



one litre of water over the fire, and boiled for a quarter of an hour, and are then 

 passed through a folded filter. If the liquid passing through is turbid it is 

 returned to the filter, and in this way a clear, pale yellow filtrate is obtained, 

 which is made up to one litre by the addition of distilled water, and is then 

 sterilised (either in bulk or in portions) by exposure to 100 C. in a steamer on 

 three consecutive days, or by a single operation of twenty minutes at 120 C. 

 under pressure. By a preliminary addition of 5 to 10 per cent, of sugar a very 

 useful nutrient medium for yeast is obtained. When acidified with acetic acid 

 and qualified with alcohol, yeast- water rendered good service in Pasteur's studies 

 in acetic fermentation. 



For the cultivation of beer-yeasts the most suitable medium is hopped beer- 

 wort, sterilised in the Pasteur flask as already described. The hop-resin in this 

 liquid exerts a toxic action on many organisms, and among them the lactic acid 

 bacteria, which play an important part in distillery work ; so that hopped wort 

 must not be employed to cultivate these organisms in the laboratory, unhopped 

 wort being advisable for this and sundry similar purposes. Unhopped wort is 

 an advantageous medium for numerous fermentative organisms, and therefore 

 requires special care in sterilising. 



Wine-must serves for the artificial multiplication of wine-yeasts and fruit- 

 yeasts, and a concentrated form of it is kept in stock in the laboratory. On this 

 point fuller particulars will be found in chapter xx. 



Saprogenic and most pathogenic bacteria thrive particularly well in meat- 

 juice. This is used in the form of so-called bouillon, and, following the lines 

 indicated by the researches of PETRI and MAASSEN (I.), is prepared as follows : 

 Half a kilogram (i.i Ib.) of finely minced beef, free from fat, is placed in a tin 

 pan or earthen crock along with one litre (if pints) of welUwater, and, after 

 standing for an hour at the ordinary temperature, is heated to about 60 C. 

 during three hours, with frequent stirrings. At the conclusion of this period 

 of extraction the mixture is boiled for half an hour and then filtered through a 

 folded filter. The pale yellow effluent liquid is made up to one litre when cold, 

 and exhibits an amphoteric reaction. Its primary salts of orthophosphoric acid 

 (e.g. KH 2 P0 4 ) redden blue litmus paper, whilst on the other hand the secondary 

 phosphates (e.g. K^HP0 4 ) also present behave in the contrary manner. In 

 presence of phenolphthalein, however, only the tertiary phosphate (K 3 PO 4 ) acts 

 as a base, and consequently meat extract behaves as an acid towards both blue 

 litmus and phenolphthalein. As a general rule, 10 c.c. of this broth require 

 an addition of 1.8 c.c. of deci-normal alkali to prevent the colour change from 

 taking place with blue litmus, and an addition of 3 c.c. to enable it just to 

 redden phenolphthalein. This acid reaction of meat-broth being a hindrance to 

 the development of many bacteria, it is on that account rendered very slightly 

 alkaline, the resulting liquid containing a smaller or larger percentage of alkali 

 according to the indicator used, and which should be selected in accordance with 

 the requirements exacted of the medium in each case. After neutralisation, 

 i per cent, of dry peptone and a ^ per cent, of common salt are added to the 

 liquid, which is then boiled again for a quarter of an hour (but not longer), 

 and filtered hot ; the resulting liquid, generally known as nutrient bouillon, is 

 filled into small bottles (e.g. 5-10 c.c.) and sterilised by either a thrice-repeated 

 treatment in the steamer or once under pressure. 



When, under particular circumstances, suitable meat cannot be obtained, 

 meat extract is used instead. Hueppe's formula for making meat-extract 

 bouillon is : 30 grms. dry peptone, 5 grms. grape-sugar, and 5 grms. meat 

 extract, dissolved in i litre of water, and boiled, filtered, and neutralised as 

 previously described. The sterilisation of the (once more boiled and filtered) 

 bouillon must be performed with scrupulous care, the meat extract being rich 



