Practice Assignment 03
Checking up on milk quality
Milk needs to be kept cold at all stages in its journey from farm to kitchen. Pasteurisation kills the pathogenic micro-organisms but some micro-organisms survive pasteurisation. They will multiply quickly if the milk is not kept cold. Micro-organisms cause 'off' flavours to develop, and in large quantities cause changes in the milk which can be easily seen and smelt! This assignment is to check exactly how 'fresh' some samples of milk are by counting the number of micro-organisms present.
What you have to do
- Work in a small group. Other groups will carry out the same tests on the same set of samples of milk.
- Get a copy of the Standard Procedure SP 0003:2005.
- Use the procedure to test the samples of milk provided. Produce a report on the quality of each sample of milk, based on your group’s results.
- Compare your reports with those from other groups. Prepare an overall report on the quality of the milk samples that you tested. Include comments on the amount of agreement, and variation, in the results.
Some hints
- It is important to use aseptic techniques when taking samples, preparing decimal dilutions and setting up the cultures. Make sure that you know exactly what you are going to do before you start.
- Each milk sample will need a separate Petri dish of nutrient agar. Label your dishes carefully.
- Each group is testing the same samples. If you work carefully the results should be similar.
Some questions
- Why is it necessary to test each sample at several different dilutions?
- Why is it important to do each test more than once?
- What is pasteurisation?
- What is ‘long life’ UHT milk? Explain why it has a much longer life than pasteurised milk.