Practice Assignment 03
Checking up on milk quality
The standard procedure can be used directly for this assignment. The aseptic techniques required should be demonstrated beforehand and the students given an opportunity to practice with non-sterile equipment until they are confident and understand the procedures.
Samples of milk need to be prepared in advance. (See below.) Three samples of milk should be available for testing.
The need for careful measurement of volumes during serial dilutions should be emphasised. Small errors become magnified at each subsequent dilution.
Health and safety note
Agar plates should not be opened after incubation. If necessary, extra tape should be used to seal the plates before examination.
Apparatus and reagents
- Standard Procedure:
- SP 0003:2005 A method for counting micro-organisms in dairy products
- •sterile apparatus:
- sample bottle
- rack of 5 test tubes plugged with cotton wool
- 10 cm3 graduated pipette and filler, or syringe
- 1 cm3 graduated pipette, or syringe
- fine dropper (to deliver about 50 drops per cm3)
- Petri dish of nutrient agar
- marker pen to write on glass
- 5 cm3 or 10 cm>
- adhesive tape
- incubator set at 30 ± 1oC
- sterile saline-peptone solution (0.1% peptone, 0.85% NaCl)
[Available commercially as ‘Oxoid’ MRD (Maximum Recovery Diluent) CM0733]
Test Specimens
- freshly bought pasteurised milk that has been kept refrigerated;
- fresh (in date) pasteurised milk that has been stored in a warm room for 2 days;
- pasteurised milk that has been stored refrigerated until 2 days after its use by date.