Listeria inactivation at mild heat with oregano oil: D- and z-values

Discovery · Updated 2026-11-10

A 2025 study found that a carvacrol-rich fraction of Mexican oregano oil made mild heat kill Listeria monocytogenes far faster in a meat-like broth. Here is how to fit D and z values yourself.

Listeria monocytogenes is the bacterium behind listeriosis, a rare but serious illness often linked to ready-to-eat meats. Heat kills it, but strong heat also spoils texture and flavour. A 2025 paper in Applied Sciences tested whether a small dose of oregano essential oil lets milder heat do the job. In a broth made to resemble meat, one fraction of the oil, rich in carvacrol and thymol, gave up to about 5.5 log reductions (a 300,000-fold drop) within 3 minutes at 57.5 °C, against under 2 logs with heat alone. The F-value calculator fits the same kind of D- and z-values to your own survivor counts, and the curve fitter handles curves that bend.

What they found

The team heated one outbreak strain of L. monocytogenes at 52, 54, 57.5 and 63 °C, with no oil, with the whole oil, or with a fraction of it called FIV, each at 0.06 % by volume. The oils made the bacteria die much faster at every temperature. The time for the first tenfold drop at 52 °C, for example, fell from about 8.5 minutes without oil to about 0.6 minutes with FIV.

The z-value, the rise in temperature that makes killing ten times faster, barely moved: the authors report 5.75 °C without oil, 5.20 °C with the whole oil and 5.00 °C with FIV. So the oil did not change how the bacteria respond to heat; it simply added killing power on top. The survival curves bent and flattened with time, a pattern called tailing, which suggests some cells resist better than others.

Why it matters

Ready-to-eat products such as cooked ham are often given a short heat treatment after packing to kill any Listeria picked up during slicing. If a natural antimicrobial lets that step run cooler or shorter, the food may keep a better texture and use less energy. The tailing matters for safety too: a straight-line model that ignores it can promise more killing than really happens at long times.

How they did it

1. The oil from Poliomintha longiflora, a Mexican oregano, was split into fractions; FIV was about 60 % carvacrol and 21 % thymol.
2. Tubes of a meat-like broth with the oil were brought to temperature in a water bath, then the bacteria were added.
3. Samples were taken over up to 30 minutes, diluted, spread on Oxford agar (which selects for Listeria) and counted after 48 hours at 37 °C.
4. Two models were fitted to each survival curve: the classic straight line on a log scale (the Bigelow model, which gives a D-value), and a Weibull model with a shape parameter. The Weibull model fitted better, with a shape parameter below 1 in every case, meaning a curve that flattens.
5. The temperature effect was then fitted across the four temperatures to give the z-values.

Check it yourself with fibub

The F-value calculator fits the straight-line (log-linear) model. Here is a worked example with our own made-up numbers, not the paper’s data.

D-value at one temperature. Paste these invented counts per mL at 57.5 °C, one line per sample, into D-value from survivor counts: 0, 2e7; 1, 6.5e6; 2, 1.8e6; 3, 6.0e5; 4, 1.7e5; 5, 6.0e4. The tool fits a line to log₁₀(count) against time and reports D = 1.96 min, the time for each tenfold drop, with R² = 0.9994 and 2.55 log reductions seen over 5 minutes.

Now try a tailing curve: 0, 2e7; 1, 1.5e6; 2, 4.0e5; 3, 1.6e5; 4, 8.0e4; 5, 5.0e4. D comes out almost the same, 2.03 min, but R² drops to 0.9086 and the plot shows the points bowing away from the line: fast deaths early, then a stubborn tail. That is the sign to fit a Weibull model instead. In the curve fitter you can type your own formula, such as log₁₀ N = a − (t/δ)^p, and get δ and p with their uncertainties.

z-value across temperatures. In z-value from D-values, paste these invented pairs: 52, 9.0; 54, 4.5; 57.5, 1.0; 63, 0.11. The tool gives z = 5.69 °C with R² = 0.9994, close in size to the paper’s values. Set the reference temperature at the top to 57.5 °C and the tool reads back D at 57.5 °C from the line: 1.02 min.

F for a target kill. In F needed for a log reduction, enter D = 1.02 min, a starting count of 1e5 and a count wanted of 1. The answer is F = 5.10 min at 57.5 °C, for 5 log reductions.

F from a heating curve. Keep the reference at 57.5 °C and z at 5.69 °C, and paste a product’s core temperatures every 2 minutes: 20, 40, 52, 56, 58, 58, 58, 50, 35 °C. The general method adds up the lethal rate over time and gives F = 8.75 min, or 8.58 log reductions with D = 1.02 min. Most of it comes from the minutes near 58 °C.

To write the step into a food safety plan, record the critical limit and monitoring in the HACCP plan builder.

The fine print

• One strain was tested, in broth rather than real meat. Fat and protein in food can shield bacteria and soak up oil compounds.
• Only one oil dose was used, chosen low enough that survivors could still be counted, and each temperature had two independent runs.
• The paper did not measure taste or texture, so whether 0.06 % oregano oil is acceptable in a product is still open.
• Our worked examples are invented. The calculator is for study and checking sums; a commercial heat process must be set and validated by a qualified process authority under the rules where the food is sold.