Question

California law stats that milk prior to pasteurization have no more than 750 coliforms per mL and no more than 50,000 total bacterial CFU's per mL. Assuming that 1 mL of milk is diluted into 99 mL of water and 0.5 mL of the diluted milk is used to inoculate a plate, what is the maximum number of colonies- both total and coliform- that may grow for the milk to be judged acceptable?

Answer 1

Answer:

The number of colony-forming units (CFU) = [no of cells]/ [volume of dilution x volume of diluted sample added to the plate]

Now CFU per ml of dilution is 50,000 colonies divided by 0.5 ml plated, or 50,000 X 0.5 = 1 x 10^5 colony-forming units/ml of dilution.

Dilution factor --> 1 mL of milk is diluted into 99 mL of water ---> [1]/[1+99] ---> 1/100 or 10^-2

0.5 mL of the diluted milk is used to inoculate a plate

Total bacteria CFU/ml: Dividing this number by the 10^-2 dilution = 1 x10^5 / 10^-2 = 10⁵ x 10² = 10⁷ colony-forming units/ml of original sample.

The maximum number of colonies- both total and coliform:

The number of colony-forming units (CFU) = [no of cells]/ [volume of dilution x volume of diluted sample added to the plate]

Now CFU per ml of dilution is 750 (coliforms) colonies divided by 0.5 ml plated, = 1500 colony-forming units/ml of dilution.

Dividing this number by the 10^-2 dilution = 1500 / 10^-2 = 1500 x 10² = 1.5 x 10⁵ coliform units/ml of original sample.