EXERCISE 7

 

Dairy Microbiology: The Standard Plate Count of Milk

Supplies required: 4 Plate Count Agar deeps (tryptone, glucose, yeast extract, agar) 4 sterile plates, pipettes, 9.9 ml water blanks.

 

Protocol:

 

PERIOD 1

 

The Laboratory Instructor will assign each person either a good quality or a poor quality milk sample to analyze.

 

Good Quality Milk

 

1.        Label your plates in the usual fashion plus "undiluted, 1 ml", "undiluted, 0.1 ml",

"10-1, 1 ml", "10-1, 0.1 ml".

2.        Dilute your milk sample by 10-1. Aseptically transfer 1 ml and 0.1 of the undiluted good quality milk directly into their respective Petri plates. Aseptically transfer 1 ml and

0.1 ml of the 10-1 dilution into their respective plates.

 

3.         Cool four molten plate count agar deeps to 50oC and pour the contents of one tube into each plate. Rotate the plates carefully to ensure proper mixing with the milk. Tape your plates together and incubate at 32oC.

 

Poor Quality Milk

 

1.         Students using the poor quality milk perform the same operations as for the good quality milk with the following changes. Dilute your poor quality milk sample by 10-1, 10-2,

10-3, and 10-4, and 10-5. Shake each dilution thoroughly. Plate 0.1 ml of the 10-1 sample, 0.1 ml of the 10-2 sample, 0.1 ml of the 10-3 sample, 0.5 ml of the 10-4 sample, and 0.5 ml of 10-5.

 

2.        Tape your plates and incubate inverted at 32oC.

 

After 48 hours your plates will be refrigerated.

 

PERIOD 2

 

1.         Choose those plate(s) containing 30-300 colonies and count the total colonies present. Remember, some colonies will be in the agar, some on the bottom, and some on the top. Using the formula:

 

  original cell concentration = colonies counted x 1/dilution                                                                       volume plated

 

       calculate the total number of viable cells present in your milk sample.

 

 

Example: on the plate labeled 10-3, 0.1 ml there are 45 colonies. 1/dilution is 1/10-3.

 

            original cell concentration = 45 x 103

                                                            0.1

 

            original cell concentration = 4.5 x 105

 

 

2.        Compare the Standard Plate Count of the good and poor quality milk.