Some of you are familiar with microbial testing using plates or films. For those of you that aren’t, hopefully this will serve as an introduction. I’m no technician or scientist, indeed, my first experience with microbial testing was at the Cheesemaking Technology Course at the University of Guelph in 2003. My partner Elizabeth and I started doing some of our own testing this past spring, and can now quickly and economically test milk and cheese for total coliforms,
E. coli and
Staph aureus. The availability of high quality cultures, ingredients, and technical information has helped fuel the growth of small-scale cheesemaking in North America. The ability for the small milk producer or cheesemaker to accurately, easily, and affordably carry out microbial testing in-house is another important tool for ensuring the highest quality and safety standards at all stages of the production chain.
Equipment:
Here is a list of everything you need to get started with your own Petrifilm microbial testing protocol:
- A pH meter or pH test strips: Depending on what you are testing, you may need to adjust the pH of your sample. Strips are about $30. Hand-held pH meters and probes vary in price, but a good setup can be purchased for under $600.
- A bottle of Sodium Hydroxide 1N for adjusting pH when necessary
$21/litre
- 99 mL Dilution Bottles BFB (Butterfield Phosphate Buffer
$89/case of 75
- Stomacher Whirl Bags
$143/500
- Disposable Sterile Transfer Pipettes
$68/400
- Incubator (0.1 cubic ft
$447
- A magnifying glass or illuminated magnifier (optional)
$110
- Petrifilm™ Staph Express Count Plates and Petrifilm™ E. coli/Coliform Count Plates
$200/100
- A gram scale
$100 (you don’t need an expensive scientific balance)
Assuming you use strips for testing pH, the start-up cost is $1208. This works out to about $3.65 per test in materials.
Method
All Petrifilm Count Plate tests share the same basic steps. To test a cheese sample for total coliforms and E. coli, mix an 11 gram sample of the cheese with the diluent solution (99 mL) in a sterile plastic bag. This yields a dilution factor of 1:10. Blend the cheese into the solution (using a stomacher, a sterilized blender or a roller homogenizer) and add drops of Sodium Hydroxide until the pH of your solution is within the recommended range of 6.5-7.5. Let the mixture settle. Using a sterile pipette, transfer 1 mL of this solution on the Petrifilm E.coli/Coliform Count Plate.
Put the Petrifilm plate in the incubator for the required time and temperature and then read the results from the plate. Coliforms can be counted after 24 hours and E.coli after 48 hours. We’ve been using 3M products, but there are other manufacturers of testing media. 3M provides a binder with all the information you need to begin testing and interpreting your plates and provides great technical support. As with any new procedure, the process seemed a little awkward at first, but we soon found a routine that can be repeated quickly and easily.
The equipment we use to test with Petrifilm plates.
This raw milk sample film shows 6 colonies forming units associated with gas. After adjusting for the dilution factor, the total coliform count is 60 cfu/mL. (The Dairy Farmers of Ontario standard is less than 100 cfu/mL.) There are no E. coli colonies showing in this sample. These would appear as blue spots associated with gas and would be included in the total coliform count. The E. coli count is therefore reported as less than 10 cfu/mL (testers never say zero!). The coliforms in this milk sample probably came from residue in the milking equipment.
There are no coliform colonies or E. coli colonies visible in this raw milk sample so both counts are reported as <10 cfu/ml.
Some Useful Sites:
The Canadian Food Inspection Agency microbial standards for milk and cheese.
For comparison, the UK standards.
There is an informative site from the US FDA called “Bad Bugs”
Testing equipment from different manufacturers can be sourced at
www.rapidmethod.com
