LIQUEFIABLE SOLID MEDIA. 129 



tubes" are ordinary test-tubes containing 5 to 10 c.c. of nutrient 

 gelatin. The preparation of wort gelatin is very simple, the un- 

 hopped or hopped wort (according to the purpose it is intended 

 for) being mixed with 10 per cent, of gelatin, melted, boiled for 

 half-an-hour in the steamer, filtered hot, filled into vessels, and 

 sterilised by the intermittent process. Must gelatin requires a 

 little care in preparation, the high acidity ( = 0.7 to i.o per 

 cent, of tartaric acid) of the must having to be previously almost 

 exactly neutralised by caustic potash, since otherwise the setting 

 power of the gelatin is impaired. After the acid has been neut- 

 ralised, 10 per cent, of gelatin is added, liquefied, cooled down to 

 between 30 and 40 C. and mixed with the white of an egg 

 beaten up to a froth ; then boiled for half-an-hour in the steamer, 

 filtered off, filled into the recipients and sterilised as prescribed. 

 During storage, numerous crystalline concretions (up to the size 

 of millet seed) of potassium tartrate separate out in the solid 

 medium, and. by presenting the appearance of colonies, give rise 

 to the supposition that the medium has been imperfectly sterilised. 

 Attention is therefore now called to this phenomenon. 



As already stated, nutrient gelatin liquefies above 30 C., and 

 therefore also at the usual temperature prevalent in the incubator, 

 viz., 38-39 C., on which account it is unsuitable for use in the 

 separation of such organisms as require higher temperatures for 

 their development. In such cases a medium tempered not with 

 gelatin, but with agar-agar, is used. This substance, obtained from 

 Eastern Asia, and fully described in a treatise by N. K. SCHULTZ 

 (I.), is a dried vegetable jelly prepared from various marine algae 

 and put on the market in the form of thin strips or as a powder. 

 Its manipulation being more conveniently effected in the latter 

 condition, the use of agar-agar powder is recommended as pre- 

 ferable. In French literature this gelatin is generally known 

 as " gelose." For use, not more than 2 parts of agar-agar per 

 100 of nutrient solution should be taken. It dissolves very 

 slowly and with great difficulty. For the preparation of pep- 

 tonised bouillon agar-agar Hans Buchner recommends the fol- 

 lowing process : The meat-broth (bouillon), prepared in the usual 

 manner, is qualified with peptone, ]S T aCl, and i or 2 per cent, of 

 agar-agar, and boiled under pressure at about 105 C. ; neutralised 

 after cooling down to 100 C. ; then boiled up again, filtered hot, 

 and filled into vessels for use. It is sterilised by exposure to 

 120 C., under pressure, for a quarter of an hour. The agar-agar 

 media do not readily adhere to the glass walls of the vessels, a cir- 

 cumstance which in many operations may be very troublesome, but 

 may be obviated if the adherent properties be increased by adding 

 to the agar-agar employed (i to 2 per cent.) the same amount of 

 gelatin or gum. For the study of the lactic acid bacteria of the 

 distillery, which thrive best at about 48 to 50 C., a i per cent, 

 unhopped wort agar-agar medium containing 2 per cent, of gelatin 



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