EXERCISE 8

 

Food Microbiology - Bacterial Counts of Foods

 

Supplies Required: 3 Eugonagar deeps, 3 sterile Petri plates, 9.9 ml H2O blanks, food samples such as ground beef, raisins, frozen vegetables previously diluted to 10-1 (grams of food/ml of water) with sterile water .

 

Protocol:

 

PERIOD 1

 

1.   Label 3 Petri plates with the usual information plus 10-2,10-3, or 10-4.

2.    Select a single food product. These have already been diluted (aseptically) by 10-1

     and blended for 5 minutes. Shake the 10-1 dilution and make from it a 10-3 dilution by

transferring 0.1 ml into a 9.9 ml H2O blank. Shake the 10-3 dilution thoroughly.

3.   Plate out as follows: Transfer 0.1 ml of the original 10-1 dilution of the food product into the plate labeled 10-2. With a fresh pipette, aseptically transfer 1 ml of the 10-3 dilution into the correct plate, and with the same pipette aseptically transfer 0.1 ml of the 10-3 to the plate labeled 10-4.

  

4.   Briefly cool your 3 deeps of eugonagar to about 50oC, and pour the contents of

            one into each of the dishes. Rotate the plate well to get good mixing but do not

      splash agar up on the underside of the dish lid. After the agar has solidified, tape

       your plates together, invert and incubate at 32oC.

After 48 hours the plates will be refrigerated.

 

PERIOD 2

 

Examine your plates. Choose the plate(s) containing 30-300 colonies and count the total number of colonies present. Calculate the concentration of living bacteria per gram of food as in Exercise 7.

  Which food product(s) contained the highest total counts? Which contained the lowest?