112 GENERAL MICROBIOLOGY 



EXERCISE 38. TO DETERMINE THE ACIDITY CHANGES 

 PRODUCED BY MOLDS IN CIDER (OR OTHER 

 LIQUID MEDIA HAVING A LOW ACIDITY AND 

 LOW SUGAR CONTENT) 



Apparatus. Four 100 c.c. Erlenmeyer flasks; 200 c.c. 

 of cider; six 1-c.c. pipettes for titration (sterile); normal 

 NaOH; N/20 NaOH. 



Cultures. Pure cultures of four molds. 



Method. 1. Determine the titre (reaction) of the cider 

 and neutralize with normal sodium hydrate. 



2. Place 50 c.c. in each of the Erlenmeyer flasks and 

 sterilize by the Tyndall method. 



3. Inoculate each, using a different mold for each flask. 



4. As soon as the mold mycelium shows in the flask 

 (examine by looking through the flask toward the light), 

 titrate. 



Note. One cubic centimeter of neutral cider is diluted to 50 c.c. 

 with distilled water for titrating as the larger quantity, 5 c.c., when 

 diluted, is of such a dark color that it is practically impossible to obtain 

 a uniform or satisfactory end point. The burette reading must be 

 multiplied by 5. 



Each pipette must be used only once. After using a pipette once, 

 clean and resterilize it for future use. 



5. Titrate every three days thereafter, making eight 

 titrations in all. 



6. Tabulate -your results. Plot a curve showing the rise 

 in acidity, making all curves on one sheet, starting from the 

 same zero point, and using different inks or different kinds 

 of lines to represent the different acidity curves. Use 

 acidity values as ordinates, days as abscissae. 



7. State fully any conclusions which may be based 

 upon your data and point out the practical application 

 which may be made. 



REFERENCE 

 LAFAR: Technical Mycology, Vol. II, Part II, pp. 353-361. 



