Last Updated 06 Sep 2020

Quantitave Plating

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Pasteurization, a process named after scientist Louis Pasteur, involves the application of heat to destroy the majority of human pathogens in foods.

In the dairy industry, pasteurization involves the “heating of every particle of milk or milk product to a specific temperature for a specified period of time without allowing recontamination of that milk or milk product during the heat treatment process. ” For public health purposes, pasteurization is the process of making milk and milk products safe for human consumption by destroying all bacteria that may be harmful to health. A serial dilution is a laboratory technique in which a substance is decreased in concentration in a series of proportional amounts. Dilutions are usually made in multiples of 10. The viable plate count procedure allowed for live cells in the milk samples to be analyzed. The raw milk (unpasteurized) sample demonstrated too many colony forming units to count in the first three dilutions of 10-3, 10-4, and 10-5. The 10-6 dilution demonstrated 284,000,000 cells/mL and the 10-7 dilution demonstrated 440,000,000 cells/mL. This demonstrates that raw milk contains a lot of bacteria even after multiple serial dilutions. In the 10-3 pasteurized sample, the plate exhibited 71,000 cells/mL. The results of the additional dilution samples contained too few colony forming units to count.

However, in the 10-7 dilution, although the plate demonstrated 12 colonies, there should have been no colony forming units on this plate. The reasons for this could have been that this sample was contaminated from “double-dipping” the sample before dispensing it onto the plate or when using the pipette, it mistakenly was inserted in a higher concentration sample and then immediately to a lower concentration sample before it was dispensed onto the plate. The results suggest that pasteurization kills most pathogens in milk by brief exposure to relatively high temperature.

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This was demonstrated most beneficially with the 10-6 and 10-7 dilutions wherein the raw milk exhibited 284,000,000 cells/mL and 440,000,000 cells/mL respectively and the pasteurized milk had too few cell colony forming units to even count. Unpasteurized milk is not healthy for people to drink. For example, in a study performed in December of 2001, the bacteria Campylobacter jejuni, a microaerophilic bacteria commonly found in animal feces, was found in unpasteurized milk at an organic dairy farm in Wisconsin and caused 75 people, ages 2 to 63, to become ill. Therefore, pasteurized milk is essential for ensuring good quality, nearly bacteria-free drinkable milk.


  1. BioSci 101, Section 810 – PowerPoint Lab 9A
  2. http://www. foodsci. uoguelph. ca/dairyedu/pasteurization. html
  3. BioSci 101, General Survey of Microbiology Laboratory Supplement Fall 2012
  4. http://foodsafety. ksu. edu/articles/1138/Raw_Milk_Outbreak_Table. pdf

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