Analysis of Pasteurized Milk

The routine pasteurization of milk is a heat treatment process intended to destroy the most thermoduric pathogens likely to be present in cow’s milk. Milk cannot be sold if it contains over 20,000 CFU/ml, but it usually contains less than 500 CFU/ml when it is newly pasteurized (“Basic Dairy,” 2010). This procedure involves raising the temperature of the milk to 63 °C for 30 minutes or 72 °C for at least 15 seconds (flash pasteurization). Please note that neither of these treatments would be sufficient to destroy endospores. Thus, pasteurization should not be regarded as a method of sterilization. Milk can be essentially sterilized by Ultra High Temperature (UHT) pasteurization in which it is heated to a higher temperature than is used for normal pasteurization, but just for a few seconds (138° C for at least two seconds). This milk can be stored for 60-90 days without spoiling (“Basic Dairy,” 2010).

Some organisms will survive the pasteurization process and be present in commercially sold milk. While these organisms are not pathogens, they will continue to reproduce in the milk over a period of time and gradually reduce the overall esthetic appeal of the milk. It is for this reason that pasteurized dairy products are generally given an expiration date after which they may not be sold.

In this experiment, fresh pasteurized milk and outdated pasteurized milk (still in the unopened container) will be compared for sanitary quality.

Important: Formulate a hypothesis regarding the results you expect to find next week. Write your hypothesis in your lab notebook and remember it for the lab report.

Procedure: – Day 1

1. You will perform serial dilutions on only one of the milk samples. Label the tubes and plates “fresh” or “outdated” accordingly.

a. Label four 9 ml dilution blanks with your name and “outdated” or “fresh”. On one, write 1:10, on a second write 1:100, on a third write 1:1000, and on the last write 1:10,000. Also label five empty petri plates with your name and “outdated” or “fresh”. Label one of the plates “undiluted”, one “1:10”, one “1:100”, one “1:1000”, and one “1:10,000”. Remember to label the plates on the agar side, and to write in small letters around the outer part of the plate. Once labeled, turn all plates right side up

b. Aseptically transfer 1.0 ml from the original, outdated milk sample to the plate labeled “undiluted”. As long as you did not contaminate the pipette, use the same pipette to transfer 1.0 ml of the milk sample to the tube labeled 1:10. Mix. Discard the pipette in the pipette container.

c. Aseptically transfer 1.0 ml of the 1:10 tube to the petri dish so labeled, taking care not to touch the tip of the pipette to anything. Using the same pipette, aseptically transfer 1.0 ml from the 1:10 tube to the tube labeled 1:100. Mix. Discard the pipette in the pipette container.

d. Aseptically transfer 1.0 ml of the 1:100 tube to the petri dish so labeled, taking care not to touch the tip of the pipette to anything. Using the same pipette, aseptically transfer 1.0 ml from the 1:100 tube to the tube labeled 1:1000. Mix. Discard the pipette in the pipette container.

e. Aseptically transfer 1.0 ml of the 1:1000 tube to the petri dish so labeled. Using the same pipette, aseptically transfer 1.0 ml from the 1:1000 tube to the tube labeled 1:10,000. Mix. Discard the pipette in the pipette container.

f. Aseptically transfer 1.0 ml of the 1:10,000 tube to the petri dish so labeled. Discard the pipette in the pipette container.

g. Take care! The agar will be hot. Mix 1 tube of melted agar into each petri dish. Replace the cover. Swirl gently to mix, ensuring that the entire bottom of the dish is covered, and allow to harden. Once the agar has hardened, invert.

g. Repeat steps ‘a’ through ‘g’ using the fresh milk sample, changing the labeling accordingly.

h. All tubes can go in the racks on the waste cart. Incubate your plates (you should have 8) in your cabinet.

Reference

Basic Dairy Bacteriology. (2010, June 1). Cornell University. https://foodsafety.foodscience.cornell.edu/sites/foodsafety.foodscience.cornell.edu/files/shared/documents/CU-DFScience-Notes-Bacteria-General-Dairy-Micro-06-10.pdf