Sugar work

Water activity, and why candy keeps

Updated October 2026

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The mechanism Which puts most of this outside the acidified foods regime Added water moves the line It works in the other direction too Which is why the package is a moisture barrier Measure it if you add anything wet References

Candy keeps for the same reason a syrup does: there is sugar in the way of the water, and nothing can get at it.

The mechanism

Dissolved sugar holds water molecules and makes them unavailable to microorganisms, which is measured as water activity rather than as moisture content. A hard candy is extremely dry and a fudge is less so, and both are usually well below the level anything needs to grow.

Which puts most of this outside the acidified foods regime

The federal definition of an acidified food applies only to foods with a water activity greater than 0.85. A properly made confection is below that, so the whole process-filing regime that governs a sauce or a pickle does not reach it. That is why cottage food law can cover candy and cannot cover salsa.

Added water moves the line

Fresh fruit, cream, or a soft filling raises the water activity, and enough of it takes a product out of the shelf-stable category. A chocolate with a fresh cream center is a refrigerated product with a short life, not a confection with a long one, and that is a formulation decision rather than a detail.

It works in the other direction too

These products pull water from humid air, which softens a hard candy, makes brittle sticky and can eventually give something enough free water to support growth. The hygroscopic behavior that makes candy keep is the same behavior that ruins it in a damp room.

Which is why the package is a moisture barrier

A jar with a good seal, a wrapped piece, or a tin that actually closes. The container is doing one job above all others here, and it is keeping the air out.

Measure it if you add anything wet

For a plain sugar confection the answer is obvious. For anything with fruit, dairy or a soft center it is a question with a number, and having that number once per formulation settles whether you are making a shelf-stable product at all.

References

21 CFR 114, Acidified Foods, whose 114.3 definition applies only to foods with a water activity greater than 0.85 — which is why most confectionery sits outside it. law.cornell.edu

Michigan State University Extension. Cottage food law: which preserved products are covered, under which confectionery is one of the products most often permitted. canr.msu.edu

Written from federal regulation and the published physical chemistry of sugar and cocoa butter, not from a laboratory. The chocolate standards of identity are specific and the sugar confectionery names are not, which is the main thing to carry away. Anything here is a starting point for a conversation with your state department of agriculture, not a determination that you comply.