66 DIFGOMANUAL 



1919 prepared a satisfactory substitute for beer wort from malt extract. His 

 medium was used by Thom and Church^ in their studies of the Aspergilli. A 

 Malt Agar was also employed by Fullmer and Grimes* for their studies of the 

 growth of yeasts on synthetic media. 



The increasing importance of dairy sanitation has already brought milk under 

 strict microbiological control, and this control has now been extended to include 

 other dairy products, such as butter, cheese, concentrated and dried milk. Malt 

 media are readily adapted to the cultivation of fungi encountered in dairy sani- 

 tation. Butter and cheese manufacturers have recognized the yeast and mold 

 content of their products as an index of their sanitary classification. The damag- 

 ing effects of certain molds, which are of tremendous importance, particularly 

 in butter, may be greatly reduced through eradicating the sources of contamina- 

 tion by means of strict sanitary control and frequent checking by routine micro- 

 biological examination. Test samples may be taken at each step of the manufac- 

 turing process to indicate the sources of contamination. 



Hood and White^ have reported their extensive studies of media commonly 

 employed for counting molds and yeasts in butter, and evolved a tentative 

 procedure for making these determinations, recommending Bacto-Malt Agar 

 acidified with lactic acid U.S. P. to a reaction of pH 3.5 just prior to pouring 

 of plates. This more acid reaction inhibits the growth of bacteria which would 

 otherwise form colonies that would be confused with colonies of molds and 

 yeasts. 



The committee of the American Dairy Science Association^''^ recommended 

 the use of Bacto-Malt Agar in its 1930 and 1933 reports on the microbiological 

 analysis of butter. Parfitt^ has also recommended Bacto-Malt Agar in order to 

 secure comparative mold and yeast counts of butter. As mentioned above, 

 "Standard Methods"^ suggest the use of dehydrated Bacto-Malt Agar for mold 

 count of dry milk. For this purpose the reaction of the medium should be ad- 

 justed to pH 4.5. 



Bacto-Malt Agar is recommended for carrying stock cultures of yeast and 

 molds used in microbiological assays. The American Association of Cereal 

 Chemists^ specifies the use of Bacto-Malt Agar in carrying cultures of Sac- 

 charomyces carlsbergensis in stock and also for the preparation of the inoculum 

 in the assay of pantothenic acid and Vitamin Bg complex. Miller and Golding^^ 

 used Bacto-Malt Agar in their study of the oxygen requirements of molds. 



Heating processes during the rehydration and sterilization should be completed 

 in as short a period as possible. Excessive exposure to heat causes partial hydroly- 

 sis of the agar with resultant inability to solidify properly when cooled. Normally, 

 a medium prepared from Bacto-Malt Agar is slightly soft and is ideal for plating 

 purposes. However, if the medium is desired for streaking, use 54 grams of Bacto- 

 Malt Agar per 1000 ml. distilled water, or include 5 grams Bacto-Agar with the 

 45 grams of Bacto-Malt Agar per liter. 



To rehydrate the medium, suspend 45 grams of Bacto-Malt Agar in 1000 ml. 

 of 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.). The final reaction of the medium will be pH 5.5. 



Frequently, in establishing the yeast and mold count, it is desired to avoid 

 confusion from bacterial growth by acidifying the medium to pH 4.5 or 3.5. On 

 the label of each package of Bacto-Malt Agar there is specified the amount of 

 lactic acid U.S. P. (85 per cent) which should be added to 100 ml. of sterile 

 melted Bacto-Malt Agar to adjust the reaction to pH 4.5 or 3.5. After the acid 

 has been well mixed with the Malt Agar, plates are poured as usual, allowed to 

 harden, and incubated for 5 days at 25 °C. before counts of yeast and mold 

 colonies are made. The medium should never be heated after the acid is added, 



