The regime

Two numbers, both required

Updated October 2026

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The definition Finished equilibrium pH Water activity above 0.85 The tomato carve-out And alcoholic beverages are excluded by the definition So the measurement is the gateway Which is why 4.6 is the number everyone in food knows References

Both have to be over before a food is low-acid, which is the one piece of good news in this section.

The definition

Low-acid foods means any foods, other than alcoholic beverages, with a finished equilibrium pH greater than 4.6 and a water activity greater than 0.85. Two tests joined by an and, not an or.

Finished equilibrium pH

Not the pH of the brine, the stock or one ingredient, and not the pH during production. The figure after the product has equilibrated, which for a chunky product with solids in liquid can take time to reach.

Water activity above 0.85

Which is the second route out, and it is why a dried or a very high sugar or salt product can sit outside the definition even at a high pH. For a soup or a broth it is not available; they are almost all water.

The tomato carve-out

Tomatoes and tomato products having a finished equilibrium pH less than 4.7 are not classed as low-acid foods. Note the figure is 4.7 there rather than 4.6, which is a specific allowance for a specific commodity.

And alcoholic beverages are excluded by the definition

Which is a jurisdictional carve-out rather than a safety one, and it matters only if you are making something that is both.

So the measurement is the gateway

A pH meter, properly calibrated, on the finished equilibrated product, and a water activity measurement. Those two numbers decide which regime you are in, and guessing either is how people end up in the wrong one.

Which is why 4.6 is the number everyone in food knows

It is the line Clostridium botulinum will not grow below, and the whole architecture of both part 113 and part 114 is built around it. The written site on Pickles is about getting under it deliberately.

References

21 CFR 113.3, the definitions for thermally processed low-acid foods packaged in hermetically sealed containers. Paragraph (j) defines a hermetically sealed container as one designed and intended to be secure against the entry of microorganisms so as to maintain commercial sterility; (n) defines low-acid foods as those with a finished equilibrium pH greater than 4.6 and a water activity greater than 0.85, excluding tomatoes and tomato products below pH 4.7; (r) requires a scheduled process at least equivalent to one established by a competent processing authority. law.cornell.edu

21 CFR part 114, Acidified foods, the regime for foods to which acid or acid foods are added to produce a finished equilibrium pH of 4.6 or below, which is the alternative route for a product that would otherwise be low-acid. law.cornell.edu

Written from federal regulation, not from a production kitchen. Nothing here is a scheduled process, and nothing here is a substitute for a competent processing authority. If you are considering a shelf-stable low-acid product, the first call is to a process authority rather than to a packaging supplier.