FOOD90023 · Food Microbiology
Microbial Inactivation
At a lethal temperature a constant fraction (not a constant number) of cells dies each minute, so on a log scale the survivor curve is a straight line — you can halve and halve again forever but never truly reach zero. That first-order behaviour is why canning aims for commercial sterility, a statistical safety margin, not absolute sterility. The chapter is built around three numbers: the D-value (time at a fixed temperature to kill 90%, a one-log reduction), the z-value (the temperature change that alters D ten-fold, linking D across temperatures), and the F-value (the lethality of a whole process), with the 12-D botulinum cook as the canonical worked calculation. It then contrasts pasteurisation vs sterilisation and thermal vs non-thermal methods, and finishes with the HACCP framework — control the process, not just the end product — where predictive models set the critical limits.
What this chapter covers
- 01First-order (log-linear) thermal death — why 'sterile' is a probability
- 02D-value (decimal reduction time) — 90% kill per D
- 03Worked calculation — read D off two counts, size a process time
- 04z-value — linking D across temperatures
- 05F-value and the 12-D botulinum cook
- 06Pasteurisation vs sterilisation; thermal vs non-thermal/chemical methods
- 07HACCP — control the process, not the end product
Worked example: read a D-value and size a process
- +1(a) Count the log reductions. log₁₀ 106 − log₁₀ 102 = 6 − 2 = 4 logs.
- +1(a) Solve for D. D = t / (log reductions) = 12 min / 4 = 3 min at 65 °C (written D₆₅ = 3 min).
- +1(b) Use D forward. For a 5-log reduction: t = D × 5 = 3 × 5 = 15 min at 65 °C.
- +1(c) Constant fraction, not number. Each D removes the same percentage (90%), not the same count: 106→105 takes one D, and 102→101 takes the same D.
- +1(c) Consequence. So a higher starting load means more logs to remove and therefore more total heating time — another reason to limit initial contamination before processing.
Key terms
- D-value (decimal reduction time)
- The time, at a fixed temperature, to kill 90% of a population — a one-log (one decimal) reduction. Because thermal death is log-linear, each D destroys 90% of whatever is left. A low D means easy to kill; a spore's large D sets the cooking process.
- z-value
- The temperature change (in °C) that changes the D-value ten-fold. It captures how sensitive an organism's heat resistance is to temperature and lets you convert a D at one temperature into a D at another, so you can compare processes run at different temperatures.
- F-value
- The total lethality delivered by a whole thermal process, expressed as an equivalent number of minutes at a reference temperature. It combines the come-up, hold and cool-down into one figure, so you can verify that a process achieves the required kill (e.g. the 12-D botulinum target).
- 12-D botulinum cook
- The canning standard for low-acid foods: a thermal process delivering a 12-log reduction of Clostridium botulinum spores. It is the reference 'botulinum cook' that defines commercial sterility for low-acid products and is the canonical D/F-value calculation.
- Pasteurisation vs sterilisation
- Pasteurisation is a mild heat treatment that kills vegetative pathogens and most spoilage organisms but not spores, extending shelf life with refrigeration. Sterilisation (commercial sterility) is a more severe process that also inactivates spores to give a shelf-stable product — the difference is whether spores are targeted.
Microbial Inactivation FAQ
Why is 'sterile' described as a probability rather than a guarantee?
Because thermal death is first-order (log-linear): each unit of time removes a constant fraction (90% per D), so the survivor curve is a straight line on a log scale that approaches zero but never reaches it. You can always halve the survivors again, but never prove that the last cell is gone. So processors aim for commercial sterility — a defined statistical safety margin such as the 12-D botulinum reduction — rather than absolute sterility, which is unattainable in principle.
What is the difference between D, z and F?
They answer three different questions. D is how long to kill 90% at one temperature (the organism's heat resistance there). z is how D changes across temperatures (the temperature shift that changes D ten-fold). F is the lethality of the whole process expressed as equivalent minutes at a reference temperature. In short: D = at one temperature, z = between temperatures, F = the whole process.
Why does a higher starting count need more heating, not the same?
Because each D removes the same percentage, not the same number, of cells. Going from 106 to 105 takes exactly one D, and so does going from 102 to 101. So if you start with more cells you have more logs to remove, and each log costs another D of heating time. That is a key reason to limit initial contamination before processing — it directly shortens the cook needed for a safe product.
How does this chapter connect to HACCP?
Thermal processing is usually a critical control point (CCP): the cook is the step that controls the spore or pathogen hazard, and the D/z/F calculations set its critical limits (e.g. a minimum F-value or time-temperature combination). HACCP is the framework that says control the process rather than rely on testing the end product, and predictive microbiology supplies the numbers that justify each critical limit. So D/z/F, predictive models and HACCP form one chain from kinetics to a documented safe process.
Exam move
Get the first-order death idea locked in — constant fraction, log-linear survivor line, 'sterile' is a probability — because it explains everything else. Then make the D-value calculation automatic: count the log reductions, D = t / (log reductions), and use D forward to size a process (t = D × number of logs). Be precise about what each number means: D at one temperature, z between temperatures, F for the whole process, with the 12-D botulinum cook as the worked example. Learn the pasteurisation vs sterilisation distinction (spores or not) and a couple of non-thermal methods. Finish with HACCP — control the process not the product, thermal step as a CCP, D/z/F set the critical limits — which ties inactivation back to predictive microbiology.
