The first thing to understand is BRCGS Global Standard Food Safety reference kill step, six log reduction or full cook are different terms but deliver the same outcome. Before we can understand what a kill step, six log reduction and full cook are, we need to understand what a colony forming unit is first.
We can now look a the petri dish to determine if there are any ‘colonies’ on the petri dish. This is where any single bacteria has grown and made a colony which is visible in the agar.
A colony starts off as one bacteria that was transferred onto the petri dish from the swab. As the petri dish is incubated, the bacteria goes through a process called binary fission.
Binary fission
This is where one bacteria splits to form two bacteria, two bacteria splits to form four bacteria, four bacteria splits to form eight bacteria and over time this doubling of bacteria produces a visible colony on the petri dish. That one initial bacteria through binary fission becomes a colony forming unit (CFU).
Now we know how colony forming units are created, we need to count them. This is done by counting each visible colony on the petri dish and can either be done visually or by microscope.
Often, the number of micro-organisms (CFUs) is reported as a log rather than as a number. This is because the numbers can be so big it is difficult to count all of the noughts.
Let’s begin with explaining the steps we need to take to generate a colony forming unit:
| Step one:
Select the swab to be used. |
| Step two:
Take the swab and swab the area that requires verification, for example: equipment that has just been cleaned. After swabbing, send the swab to the laboratory. |
| Step three:
The laboratory prepares the petri dish with the agar and gently rubs the end of the cotton swab across the surface, transferring the bacteria on the swab to the agar. The type of agar used allows different bacteria to be identified. In this example, the agar used is Plate Count Agar (PCA), which is a microbiological growth medium commonly used to assess all bacteria that are present on the petri dish and does not allow for identification of specific bacteria. |
| Step four:
The petri dish is placed in the incubator, this is then set to the optimal growth temperatures for the organisms of concern and a time frame suitable for their growth. |
| Step five:
Remove petri dish from the incubator. |
So what is a colony?
As we move from log one to log two, we increase the number of colony forming units by a factor of ten. We can see from the table below, one is equivalent of 0 – 10 colony forming units and six log is 100,001 – 1,000,000.
Log:
1 = 0 – 10
2 = 11 – 100
3 = 101 – 1000
4 = 1001 – 10,000
5 = 10,001 – 100,000
6 – 100,001 – 1,000,000
Understanding log reduction
A 1 log reduction will kill 90% of the colony forming units with the colony forming unit count being reduced by a factor of 10.
If we have 1,000,000 (1 million) colony forming units and we want to reduce the units to 1 colony forming unit, we need a 6 log reduction.
The example shows how a 6 log reduction takes 1,000,000 colony forming units to 1 colony forming unit.
Log reduction:
1 log reduction reduces 1,000,000 to 100,000
2 log reduction reduces 100,000 to 10,000
3 log reduction reduces 10,000 to 1000
4 log reduction reduces 1000 to 100
5 log reduction reduces 100 to 10
6 log reduction reduces 10 to 1
So practically how do we achieve a 6 log reduction? Using mathematics, we can calculate core temperature of a product and time required to deliver a 6 log reduction.
Fortunately for us, this has already been done and has been calculated at: 70°c for 2 minutes or equivalent (think hotter temperatures requires less time, lower temperatures require more time). In simple terms, if we are cooking chicken and want it to be safe for human consumption then we need to cook it with core temperature of 70°c for 2 minutes which will deliver a 6 log reduction. Cooking a product at 70°c for 2 minutes is also know as a kill step and full cook.
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