ORGANIC ACIDS SERVE AS A FOOD 169 



EXERCISE 4. TO SHOW THAT ORGANIC ACIDS MAY 

 SERVE AS A FOOD FOR SOME ORGANISMS 



Apparatus. Two sterile 200 c.c. Erlenmeyer flasks; 

 sterile 5 c.c. pipettes; 200 c.c. whey, soured by Bact. lactis 

 acidi. 



Cultures. Oospora lactis. 



Mycoderma (pickle scum yeast). 



Method. 1. Titrate the acid liquid and record the titre. 



2. Place 100 c.c. in each flask. 



3. Heat for one hour, cool and inoculate each flask with 

 one organism. 



4. Titrate every two days from the time growth shows 

 until the reaction becomes constant. Always use sterile 

 pipettes for obtaining a sample for titration. 



5. Plot curves on the same paper, using the same zero 

 point. 



6. Did you place the organism in its natural habitat? 

 Will either of these organisms use another acid except 



that common to its habitat? 



7. What is the chemical nature of this organic acid? 



8. What has happened to the organic acid in question? 

 Write the chemical equation showing this action. 



9. What type of enzyme is concerned in the change which 

 takes place? 



10. State your results in detail and point out any con- 

 clusions to be drawn. Point out any practical applications 

 that may be made. 



REFERENCES 



LAFAR: Technical Mycology, English Ed., Vol. II, Part 2, pp. 417, 418, 



452-455. 

 LASER: Biological Test for Butter. Zeitschrift fur Hygiene (1891), 



Vol. X, p. 513. 



MARSHALL: Microbiology, p. 111. 

 NORTHRUP: Tech. Bui. No. 15, Michigan Experiment Station. The 



Influence of Certain Acid-destroying Yeasts upon Lactic Bacteria, 



pp. 5-7. 



