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Pasteurisation F-Value Calculator

Pasteurisation F-Value Calculator

Time-temperature equivalence using Bigelow's z-value model

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Convert between time and temperature to find equivalent pasteurisation processes, and read the P* pasteurisation value, using Bigelow's z-value model. Useful for comparing processes, judging temperature deviations, and checking whether an equivalent time–temperature combination meets the standard.
Need help with pasteurisation f-value design, validation, or troubleshooting? Discuss your project →

The Formula

The fundamental relationship for thermal equivalence is Bigelow's first-order log-linear inactivation model. For a given microorganism, the time required to achieve equivalent lethality at two different temperatures is:

t2 = t1 × 10(T1 − T2)/z t = exposure time at temperature T (s) · z = temperature change for 10× rate change (°C)

The z-value describes the temperature change that changes the modelled inactivation time tenfold. It depends on the organism or enzyme, product and test conditions. Select a value supported for the purpose of the calculation.

The calculator also displays a reference index using 15 seconds at 72°C and z = 8°C. This is an engineering comparison under stated assumptions; a value of one or more does not establish pasteurisation compliance or validate a different processing schedule.

The milk heat-treatment combinations in the applicable legislation must be considered separately, together with product scope, the required phosphatase outcome, equipment controls and validation of the fastest product path. See Regulation (EC) No 853/2004, Annex III, Section IX, as applicable in Great Britain.

Worked example: effect of the selected z-value

Starting from 15 seconds at 72°C, a comparison at 71°C using an illustrative z of 8°C gives:

15 × 10(72−71)/8 = 20.00 seconds (rounded).

This describes the selected mathematical model. It does not authorise operation at 71°C or establish a validated alternative pasteurisation schedule.

Need help applying this calculation to your specific plant or process?

Calculations on paper are one thing; real plant operation, validation, and commercial decisions need expert review. Schedule a call with Watson Dairy Consulting →

Interactive Calculator

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Frequently Asked Questions

What is the z-value for milk pasteurisation?

There isn't a single one. The law (retained Regulation (EC) No 853/2004) does not specify a z-value at all — it defines pasteurisation by time and temperature (72 °C/15 s, 63 °C/30 min, or equivalent) plus a negative phosphatase test. In engineering practice three z-values are used: 8 °C for the P* equivalence index (the recognised basis for judging alternative processes), ~4.3 °C for Coxiella burnetii death (which explains why 72/15 and 63/30 match, after Enright et al., 1957), and ~5 °C for alkaline phosphatase.

Why are 72°C/15 seconds and 63°C/30 minutes both listed?

They are specified heat-treatment combinations in the relevant milk-hygiene rules. Their legal status does not depend on choosing a z-value that makes a simple model fit both points. Alternative schedules require evidence appropriate to the product and applicable requirements.

What does UK law actually require for pasteurisation?

Retained Regulation (EC) No 853/2004 (Annex III, Section IX) requires milk to be pasteurised by at least 72 °C for 15 s, at least 63 °C for 30 min, or any equivalent combination, such that it gives a negative alkaline phosphatase test immediately afterwards — for cows' milk, an activity of ≤ 350 mU/L by ISO 11816-1. The applicable requirements and validation must be checked for cream, concentrated, sweetened and other products; do not apply a universal cream minimum across jurisdictions. Always confirm the current wording of the regulation that applies to you.

Can I use this for sterilisation (UHT) calculations?

The same model applies but with different z-values. For sterilisation targeting Clostridium botulinum spores, z is typically 10°C. F0 values for UHT are calculated at the reference 121.1°C. The principles are identical.

Need expert support on pasteurisation f-value? Watson Dairy Consulting provides independent support including process design, plant audit, and troubleshooting. Contact Watson Dairy Consulting.

References & Further Reading

  1. Bigelow, W. D. (1921). "The logarithmic nature of thermal death time curves." Journal of Infectious Diseases, 29(5), 528-536.
  2. Enright, J. B., Sadler, W. W., & Thomas, R. C. (1957). "Pasteurisation of milk containing the organism of Q fever." American Journal of Public Health, 47(6), 695-700.
  3. Codex Alimentarius: CAC/RCP 57-2004 Code of Hygienic Practice for Milk and Milk Products.
  4. Lewis, M., & Heppell, N. (2000). Continuous Thermal Processing of Foods. Aspen.
  5. Regulation (EC) No 853/2004 (retained in UK law), Annex III, Section IX — pasteurisation time–temperature and alkaline phosphatase requirements.
  6. ISO 11816-1 / IDF 155-1 — determination of alkaline phosphatase activity in milk (reference method).

Further reading: John Watson publishes articles on dairy industry topics on LinkedIn. Browse all articles by John Watson on LinkedIn →

Disclaimer: This calculator and the calculations on this page are provided for educational and indicative purposes only. Real plant performance depends on many factors not captured by simple models. Always verify against your specific plant data, ingredient specifications, regulatory requirements and safety procedures. Watson Dairy Consulting accepts no liability for production, regulatory, safety or commercial decisions made on the basis of this calculator alone. For project-specific support, please contact Watson Dairy Consulting.

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