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.