EXERCISE 9

 

Microbial Fermentations: Wine and Yogurt Production

A. Wine

 

Supplies required: 1 tube of fruit juice (grape or apple), no preservatives added, broth cultures of Saccharomyces cerevisiae variety ellipsoideus.

 

Protocol:

 

PERIOD 1

 

1.         Label your tube of fruit juice in the usual fashion.

2.         Aseptically add 0.1 - 0.2 ml of the S. cerevisiae var. ellipsoideus culture to the tube. This strain of S. cerevisiae is a wine strain selected for its tolerance to ethanol and ability to produce other flavor enhancing agents.

 

3.        Incubate all tubes for 48 hours AT ROOM TEMPERATURE. Leave the tube caps     cracked open just a bit to prevent an explosion due to CO2 production

(glucose --> 2 ethanol + 2 CO2).

 

PERIOD 2

 

Note the aroma and flavor of your wine. Remove a portion of wine with a pipette and taste it--is your wine a big success?

 

 

B. Yogurt

 

Supplies required: 120 ml 2% lowfat milk supplemented with powdered dried skim

milk, plain commercial yogurt, aluminum foil, paper cup, disposable plastic bulb pipette,

plastic spoons

 

Protocol:

 

Period 1

 

1.         Label a paper cup with your name and add about 120 ml (4 fluid ounces) of milk. There will be a marked cup available to allow you to approximate this volume.

 

2.        Measure the pH of a drop of the milk with pH paper as demonstrated by the Laboratory Instructor. Do not dip the pH paper in the milk. Reading the pH paper will be easier if you read the opposite side of the strip from where you placed the drop of milk. Record the pH.

 

3.        Inoculate 1 ml of starter culture (commercial plain yogurt) into the cup with a plastic disposable bulb pipette; the (R) marks about 1 ml on the pipette. Stir the inoculum into the milk vigorously for 20 seconds, which will help produce a smooth product.

 

4.         Cover your cup with foil. Write your initials on the foil. Incubate your cups at 40oC (37oC - 42oC is the temperature range for yogurt). Note that yogurt is produced at a much higher temperature than wine.

 

5.        Place a small drop of water on a microscope slide. Using your sterilized loop, transfer a small amount of the starter yogurt to the drop. Make a smear by spreading, drying the slide, and then stain with methylene blue. Inspect your stained slide under the microscope and draw the bacteria you see. Also measure and record the pH of the commercial yogurt.

 

6.         The next day your yogurt should be done and will be refrigerated.

 

Period 2:

 

1.         Check your yogurt. How does it compare with the starter yogurt in appearance and consistency? Measure and record its pH.

Eat your yogurt, plain or with the provided fruit preserves. (Here we make an exception to the no food in the lab rule).