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Jam and preserves
All Containers›Shop by use case›Jam and preserves›What goes wrong›Mold on top, or it is fermentingShop all jams and preserves →
What goes wrong

Mold on top, or it is fermenting

By Dwight Davis, Founder

Updated

On this page5
Mold and fermentation are listed for both jellies and preserves, with the same three causes: yeasts and molds growing on the product, imperfect sealing, and improper storage. This is the entry on the list that is not about texture.

The three causes

Three things produce this and they are not equally likely, which matters because the usual response is to blame the seal when the seal is rarely the problem. Two of the three happen before the jar is ever closed.

  • Yeasts and mold growing on the product, which is what too much free water allows.
  • Imperfect sealing, which lets them in.
  • Improper storage, which lets them work.

Why the sugar concentration is the first defense

The 65 percent figure is not only about texture. A solution that concentrated is hostile to most of what would otherwise grow in it, which is why the percentage is specified rather than recommended and why cutting it to make a lighter product changes the keeping qualities as well as the taste.

The 65 percent figure in the standard is doing this job as well as holding the gel. A product below concentration has water available, and then the seal and the storage are all that is left between it and spoilage. That is on 65 percent soluble solids.

The bubbles warning

Rising bubbles in a sealed jar are the signal that separates a cosmetic problem from one that means the jar does not get sold. They are worth looking for deliberately rather than noticing by accident. Hold a jar up to a window and look at the body of the set rather than the surface, because the surface is where the cosmetic case lives and the body is where the other one shows.

Extension guidance puts a safety instruction inside the bubbles entry rather than this one: bubbles may denote spoilage, and if the bubbles are moving, do not use it. That is the only "do not use" on the whole problem list.

What not to do

The instinct when a jar has mold on top is to deal with the mold, and that is the one response the published guidance is unambiguous about.

Do not scrape mold off and keep the rest. The visible growth is not the extent of it, and a jar that has gone is a jar that has gone.

References

National Center for Home Food Preservation, University of Georgia. Causes and possible solutions for problems with jellied fruit products. nchfp.uga.edu
21 CFR 150.160, Fruit preserves and jams. Code of Federal Regulations, Title 21. law.cornell.edu
Written from federal regulation and published extension guidance, not from a laboratory. A standard of identity is a federal floor and your state adds to it. Anything here is a starting point for a conversation with your state department of agriculture, not a determination that you comply.