BioSci 101 – Lab Section 810
The purpose of this lab is to see the effects of pasteurization while emphasizing the process for serial dilutions.
See references (1)
As the dilution factor increased for both the raw milk (unpasteurized) and pasteurized milk samples, the number of colonies decreased. The number of cells/mL in the pasteurized milk sample is considerably less than the number of cells/mL in the raw milk sample.
RAW (UNPASTEURIZED) SAMPLE Dilution Factor | Number of Colonies | Number of cells/mL | 10-3 | TMTC | TMTC | 10-4 | TMTC | TMTC | 10-5 | TMTC | TMTC | 10-6 | 284 | 284,000,000 cells/mL | 10-7 | 44 | 440,000,000 cells/mL | | AVERAGE |…show more content…
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. 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.