Understanding what is meant by a kill step, six log reduction or a full cook

  1. Understanding what a Kill step, full cook and 6 Log reduction are.

    In this blog I am going to explain what a Kill step, full cook and 6 Log reduction is.

  2. Introduction

    The first thing to understand is BRCGS Global Standard Food Safety reference kill step, 6 log reduction or full Cook are different terms but deliver the same outcome. Before we can understand what a kill step, 6 log reduction and full cook are we need to understand what a colony forming unit is.

    I’ll begin by explaining the steps we need to take to generate a Colony Forming unit.

    Step 1
    Select the swab to be used.

    Step 2
    Take the swab and swab area that requires verification, e.g equipment that has just been cleaned, and after swabbing send the swab to the laboratory.

    Step 3
    The Laboratory prepares the Petri dish with the agar and gently rubs the end of the cotton swab across the surface of the agar 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 the identification of specific bacteria.

    Step 4
    The Petri dish is placed in the incubator this is set to the optimal growth temperatures for the organisms of concern and a time frame suitable for their growth.

    Step 5
    Remove petri dish from the incubator
  3. So what is a colony forming unit?

    We can now look at 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 see step 3 above. As Petri dish is incubated the bacteria goes through a process called binary fission.

  4. Binary fission

    Is where one bacteria splits to form 2 bacteria and over time this doubling of bacteria produces a visible colony on the petri dish. We do not know how many bacteria there are but we do know that the CFU was started by one live bacteria.

    Binary Fusion

    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 be done visually or with a microscope

    Often the number of micro-organisms (Colony Forming Units) is reported as a log rather than as a number. As we move from log 1 to log 2 we increase the number of Colony Forming Units by a factor of 10. We can see from the table below 1 log is equivalent of 0 – 10 Colony forming units and 6 log is 100,001 – 1,000,000


    1 = 0 – 10

    2 = 11 – 100

    3 = 1001 – 1000

    4 = 1001 – 10,000

    5 = 10,001 – 1000,000

    6 = 100,001 – 1,000,000

  5. Understanding log reduction

    A 1 log reduction will kill 90% (percent) 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

    1 log reduction reduces 1000000 to 100000

    2 log reduction reduces 100000 to 10000

    3 log reduction reduces 10000 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 statistics we can calculate core temperature of a product and time required to deliver a six log reduction.

    Fortunately for us this has already been done and has been calculated at:
    70 °c for 2 minutes. or equivalent (think hotter temperature requires less time, lower temperature requires 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 known as a Kill step and full cook.

    To see the range of BRCGS, HACCP and Food Safety courses offered by totrain click here.

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Understanding what a Kill step, 6 Log reduction or full cook is.

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John Husband – Consultancy & e-Learning Director


John Husband is a BRCGS Principle Training Partner (Traditional and Virtual), Third Party Auditor and RSPH approved trainer. Since 1998, he’s helped hundreds of food and manufacturing businesses work smarter, stay compliant and develop their management systems. When he’s not training or consulting, he uses the totrain blog to share everything he knows.
E: john@totrain.co.uk
M: 0780 3136598
W: totrain.co.uk