MOLDS, YEASTS, TORULAE, AND BACTERIA 191 



10. After_28 hours count the number of colonies 

 of bacteria and yeasts present on both plates. Cal- 

 culate the numbers in a gram of dry yeasts. 



Bacteria do not multiply well on wort agar, the acid 

 reaction being imfavorable. The coimt on meat agar 

 will, therefore, be higher than on worj; agar. 



11. Examine the yeast colonies under the micro- 

 scope, make stains, examine in water, and transfer the 

 different colonies to wort tubes. 



12. Study the cultures prepared as before. 



SECTION s 

 EXAMINATION OF YEAST OF SALT-RISING BREAD 



1. Make smears and stains from the yeast. 



2. Inoculate litmus milk and incubate at 37° C. for 

 two or three days. 



3. Make gentian violet and Gram stains, and note 

 the dark red color of the litmus. 



4. Inoculate a flask containing 250 c.c. sterile milk 

 and incubate for seven days at 37° C. 



5. Determine the amount of acid formed from day 

 to day with 1/20 N • NaOH and phenolphthalein as 

 indicator. Record the results. 



6. Plate in meat extract agar and wort agar. 



7. Study the colonies in both plates. 



Meat extract agar is not suitable for the organism, 

 active in this yeast and probably no colonies will 

 appear. Beerwort agar is more suitable. The active 

 organism is a large bacillus, which forms an amount 

 of lactic acid often as high as 3 per cent. When used 

 for baking this acid combines with the soda added to 



