Acidity and food safety

Why the rules exist

Updated October 2026

On this page5

The organism Why 4.6 Why a sealed jar is the risk What this means at the bench References

Every requirement on an acidified food traces back to one organism and one number. Knowing why makes the rules easier to follow correctly rather than approximately.

The organism

Clostridium botulinum is a soil bacterium, which means it is on your peppers, your garlic and your carrots as a matter of course. Washing reduces it and does not remove it.

It survives ordinary cooking as a spore. The spore is heat resistant in a way the growing cell is not, so a simmer that kills everything else leaves the spores intact. This is the part that surprises people: cooking is not the control.

Why 4.6

The spore germinates and produces toxin only within a range of conditions. Below about pH 4.6 it does not. That is the whole basis of the number, and it is a biological threshold rather than an administrative one.

The toxin is among the most potent known and is not detectable by taste, smell or appearance. A contaminated jar looks and smells like a good jar. There is no sensory check and no amount of experience substitutes for the measurement.

Why a sealed jar is the risk

The organism is anaerobic: it grows where there is no oxygen. A sealed bottle of sauce at room temperature is close to an ideal environment if the acidity is wrong. Refrigeration slows it; a shelf-stable product has no such help.

So the risk is not in the kitchen, it is on the shelf, weeks later, in someone else's house. That is why the control is a measurement taken before the bottle closes rather than a judgment made when it opens.

What this means at the bench

Target well below 4.6 rather than at it, because ingredients vary and measurement has error. Measure the equilibrated product rather than the mix. Measure in more than one place, because a pocket above the line in a thick sauce is the exact failure mode. And record it, because a batch you cannot evidence is a batch you cannot defend or recall precisely.

None of this is caution for its own sake. It is the shortest path between a recipe and a product that is safe on a shelf for a year.

References

21 CFR Part 114, Acidified Foods. Low-acid foods to which acid is added, water activity above 0.85, finished equilibrium pH 4.6 or lower reached within the time designated in the scheduled process. 114.3 excludes carbonated beverages, jams, jellies and preserves, foods stored, distributed and retailed under refrigeration, and acid foods including condiment sauces containing small amounts of low-acid food whose finished equilibrium pH does not significantly differ from the predominant acid. ecfr.gov

US Food and Drug Administration. Draft Guidance for Industry, Chapter 16: Acidified Foods. fda.gov

Written from published extension guidance and federal publications, not from a laboratory. Your state's requirements and your own process govern anything general said here.