Let me start by saying I know why, but I am inquiring more about the origin.

My question is more related to literature that I cannot seem to find. I've found some helpful papers and information on the origin of the using 121°C for sterilizing equipment, and killing bacteria but I wanted to ask the question anyways and see if anyone knew something more about it.

So to restate my question, I want to see data as to why 121°C and not 120°C?

Originally I though it is because the experiments might've been conducted in F and the inter-conversion from F to C yielded 121°C which is an odd number.

From what I could gather it started in the food industry with a company that made canned products and their canned products were exploding. This company approached MIT and after some research they found that at 121°C (250F) for 60 min in retort would kill the spores surviving the previous processing. (further exp. reduced the time to 10 min, for canned clams)

This is detailed in the paper which was published in 1897


The autoclave was invented in 1879 (18 years prior to the publication). At the end of the publication it says:

Detailed descriptions of the organisms and of many more experiments will be given in our full paper on this subject to appear in a forthcoming number of the Technology Quarterly

I have not been able to find this specific issue of Technology Quarterly

The authors (S. C. Prescott and W. Lyman Underwood) also say:

As we have shown in our previous paper, in order to insure sterilization in practice, it is necessary to obtain and maintain a temperature in excess of 121°C (212F) throughout the contents of the can. Intermittent sterilization may be employed, but is less efficient and is not practicable upon a large or commercial scale. We have found by experiment that sixty minutes at 121°C (250F) is sufficient time for sterilizing corn, and it seems probable that this can be shortened somewhat or the temperature reduced.

They are using °C, but since it's 1890's retorts (autoclaves) used in food industry might be F and the authors probably converted retort eqipment readings to °C for scientific community.

Also, this was what pioneered thermal death study and if you look at the temperature that they used for the study they are kind of sporadic. Probably because they wanted to see what temperature + time would reach the center of the can. The length of heating, or processing, and the pressure which is given vary somewhat in different factories and that might’ve been what led them to the 121°C and not 120°C.

Further research was done in this field: In 1956 and 1958 F. H. Deindoerfer & A. E. Humhrey found that 250F (121C) is the optimal temperature to avoid damage nutrient media. They also discussed analytical methods for heat sterilization times.



What I've been specifically looking for is the first thermal death study with D&Z values that led to the widespread usage of 121°C. Andy why the time and psi required for sterilizing biological samples. I know and found papers on what is used to validate Autoclaves.

Geobacillus sterothermophilus (known as Bacillus sterothermophilus also) is used for validating autoclaves and that is due to it being very heat resistant, making a good biological indicator of microbial life post sterilization. The one paper I found which had some graphical data about its thermal death rate uses °C instead of F. I cannot seem find any information on the bacterial Log reduction at 120°C comparing it to 121°C. This answer might as well as be a mathematical equation leading to the that temperature, but that is only venue I haven't explored

Paper on Geobacillus sterothermophilus: https://www.ncbi.nlm.nih.gov/pmc/articles/PMC1057891/

If anyone can find that Technology Quarterly paper I mentioned at the start that'd be great.

  • 4
    $\begingroup$ I think your well-researched question is probably as close as you'll get to an answer here. Still gets an upvote from me but I just doubt we have an expert in 19th century autoclaves. From a practical perspective, I doubt anyone has ever compared 120 to 121 - why would you? For one, the tolerance in the thermometer could very well include that degree of difference, especially decades ago, and the degree of sterilization is unlikely to be meaningfully different over any small difference in time or temperature. $\endgroup$
    – Bryan Krause
    Commented Sep 14, 2020 at 22:02
  • 2
    $\begingroup$ At some point you might consider posting a different (i.e. not identical) but related question in History of Science and Mathematics SE. Have a look around and see if something can be written that fits the site. $\endgroup$
    – uhoh
    Commented Sep 15, 2020 at 0:14
  • $\begingroup$ Thank you, I'll look into that and rephrasing my question. $\endgroup$
    – m4rio
    Commented Sep 15, 2020 at 0:38
  • 1
    $\begingroup$ +1. Interesting question. Why do you write the link to the same F. H. Deindoerfer & A. E. Humhrey papers twice? $\endgroup$
    – Hans
    Commented Sep 15, 2020 at 4:53
  • 2
    $\begingroup$ I have a strong suspicion that 250F was picked because humans like to round numbers to the nearest 5s, 10s, and consequently 50s. The conversion to centigrade leaves it at 121. If it ain't broke, don't fix it. Scientists can be lazy like that. $\endgroup$
    – S Pr
    Commented Sep 15, 2020 at 12:24

1 Answer 1


Quick answer:

For historical and engineering reasons, standards for sterilization with autoclaves were developed with steam pressure and time being the operational parameters. Temperature wasn't even measured by autoclaves until decades after they were in widespread use. For human factor reasons, the most appealing combination of sufficient pressure and time was 15 pounds per square inch pressure (one rounded off Earth atmosphere) for 15 minutes. The temperature that corresponds to 15 psi steam pressure is 121°C.

Long answer:

It's all about the history of steam technology and the numbers people find easy to use and remember.

Steam power was the basis of industrial process in the 19th century, and to use it safely required monitoring and controlling boiler pressure. Boiler explosions were not uncommon in the early days of steam power. The Bourdon pressure gauge was invented in 1849 and was a robust technology that yielded reliable and accurate measurement of steam pressure.


When Chamberland invented the autoclave in 1879 from Papain's earlier pressure cooker, it relied exclusively on pressure measurement for control. Not until 1933 were autoclaves produced that monitored temperature at the drain line to confirm that air was exhausted and the steam was truly saturated. https://www.steris.com/healthcare/knowledge-center/sterile-processing/everything-about-autoclaves

Study of steam engines was the basis for the development of thermodynamics and the kinetic molecular theory of gasses. Although steam is far from an ideal gas, there is a predictable relationship between temperature and pressure in saturated steam as well as well established properties of heat transfer. These can be found on standard engineering steam tables.

Examples: https://www.engineersedge.com/thermodynamics/steam_tables.htm

Saturated steam is very efficient in transferring heat. This is why scald injuries are so severe compared to simple burns at the same temperatures. It is also how saturated steam kills even heat resistant microorganisms in an autoclave. https://learnaboutgmp.com/good-manufacturing-practices-cgmp/saturated-steam-efficient-sterilization/

Many combinations of pressure and time will do the job, but some are more appealing to people than others. One earth atmosphere of pressure at sea level is defined as 14.69 pounds per square inch (psi), though in reality it varies a little bit due to temperature and weather. Because of this variation and because humans using Arabic numerals, to do arithmetic and algebra like integer units in 5's, engineers often round off 1 atmosphere of pressure to 15 psi.

This amount of saturated steam pressure was well within the capacities of typical steam boilers of the late 19th century. And a nice round 15 psi of saturated steam will kill even highly resistant bacterial endospores in just 15 minutes. This pressure/time combination yielding an obvious mnemonic for adequate sterilization: 15 psi for 15 minutes.

The temperature of that 15 psi of saturated steam? 121°C.

And that is why we use the very unusual 121°C for typical autoclaving. It's because we are really using a rounded one earth atmosphere of steam pressure for the same number of minutes as psi of pressure. And we did so without concern for direct temperature measurement for the first 50 years of autoclave use. Even today, the temperature of the autoclave is of secondary importance.

  • $\begingroup$ Nice answer. +1. Might I recommend taking a play out of one of our best answer writer's playbook: the summary/long answer style. See here, here, and here for examples. I write long posts like you, and I know my posts (e.g., here) began attracting more attention once I started emulating @AliceD. Not necessary, but it works well. cheers. $\endgroup$ Commented Feb 14, 2021 at 3:18
  • $\begingroup$ Thanks for the advice! I've used the short answer/long answer approach in other contexts, you're right it would be useful here. I've also been trying to tighten up my answers more generally partly for readability, but also time writing, and less wear on my arthritic hands. $\endgroup$
    – DrRadium
    Commented Feb 22, 2021 at 13:36
  • $\begingroup$ whenever you find a weird standard value in science the answer is usually we didn't use to use those units, but after standardization there was no reason the change the value when we changed units. $\endgroup$
    – John
    Commented Oct 23, 2023 at 0:18

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