DEHYDRATED CULTURE MEDIA 249 



their nutrient medium in the absence of the selective agent, as on the conven- 

 tional hopped wort agar. 



Green and Gray^ reported that counts of viable baker's yeast may be made on 

 the Nutrient Medium at pH 5.5. If the reaction is adjusted to pH 6.5, the count 

 of baker's and distiller's yeast may be made. The differential medium at pH 5.5 

 permits a reliable count of the bacteria generally encountered in brewing. If the 

 plates of this medium be incubated under anaerobic conditions, then estimations 

 of beer cocci and lactic rods will be obtained. Aerobic incubation gives an esti- 

 mation of the acetic acid rods and termobacteria (very small rods occurring in 

 wort as described by Linder in about 1900 as Termobacterium lutescens, 

 iridescens and erythrinum) etc., present in the sample. 



For the analysis of baker's yeast and alcohol fermentation mashes the reaction 

 of the Differential Medium is adjusted to pH 6.5. Plates containing dilutions of 

 baker's yeast are incubated aerobically, whereas those from alcoholic fermenta- 

 tion mashes are incubated anaerobically under an atmosphere of carbon dioxide. 

 With the alcoholic fermentation mashes, roll tubes containing 15 ml. of medium 

 may be used and, unlike the plates, the roll tubes may be incubated aerobically 

 instead of anaerobically. In preparing media for the estimation of the number of 

 bacteria from alcohol mashes, markedly increased growth will be obtained if the 

 medium be rehydrated in a diluted clarified canned tomato juice, using 400 ml. 

 of the juice with 600 ml. of distilled water per liter. 



In making microbial counts using these media the temperature and time of 

 incubation will vary depending on the various materials under investigation. 

 Temperatures of 25 °C. are generally employed with brewing materials, and 

 30°C. for baker's yeast and alcohol fermentation mash analyses. Incubating 

 periods run from 2 to 7 days, depending on the flora encountered. Incubation 

 periods from 10 to 14 days may be used in some cases. 



To rehydrate the medium, suspend 80 grams of Bacto-W. L. Nutrient Medium 

 in 1000 ml. cold distilled water and heat to boiling to dissolve the medium com- 

 pletely. Distribute in tubes or flasks and sterilize in the autoclave for 15 minutes 

 at 15 pounds pressure (121°C.). Final reaction of the medium is pH 5.5. The 

 Nutrient Medium with a final reaction of pH 6.5 is prepared by adding sodium 

 carbonate according to directions given on the label of each bottle. 



One pound of Bacto-W. L. Nutrient Medium will make 5.6 liters of medium. 



I Paper read at Am. Soc. of Brewing Chemists ^ \^rallerstein Lab. Comm., 13:357: i950- 

 Meeting, Detroit, May, 1950, 8 WaUgrstein Lab. Comm., 14:169:1951. 



BACTO 



W. L. DIFFERENTIAL MEDIUM (B425) 



DEHYDRATED 



Bacto-W. L. Differential Medium has the same composition as Bacto-W. L. 

 Nutrient Medium described above except for the addition of 0.004 g. of actidione 

 per liter. This medium permits unrestricted growth of bacteria and inhibits the 

 development of yeast and molds. 



To rehydrate the medium, suspend 80 grams of Bacto-W. L. Differential 

 Medium in 1000 ml. cold distilled water and heat to boiling to dissolve the 

 medium completely. Distribute in tubes or flasks and sterilize in the autoclave 

 for 15 minutes at 15 pounds pressure (121°C.). Final reaction of the medium is 

 pH 5.5. The Differential Medium with a final reaction of pH 6.5 is prepared by 

 adding sodium carbonate according to directions given on the label of each 

 bottle. Avoid overheating or repeated re-melting of the Differential Medium. 



One pound of Bacto-W. L. Differential Medium will make 5.6 liters of medium. 



