Mold on the surface

Treat it as a signal, not a nuisance

Unlike separation or a grainy texture, this one says something was wrong. Either water was introduced, or the formulation carries more moisture than you believed, or the jar was contaminated and then given moisture to work with. All three are worth running down properly.

Check the formulation first

Honey, fruit, syrup or any wet addition raises the water activity, and enough of it takes the product out of the low-moisture category without anything looking different. A maker who added a wet ingredient to a recipe and did not re-examine the preservation question has found the answer here.

Then the fill and the jar

Condensation under a lid gives mold a wet surface to start on even when the product beneath is dry. Filling warm into a cool jar is the usual way that happens, and it concentrates at the top of the jar, which is where the mold appears.

Then the customer

A wet utensil introduces both water and the organism in one movement. If it is a single opened jar, this is the likely story, and a line on the label about using a clean dry knife is the cheap prevention.

Do not skim and sell

And do not treat one report as one jar. If the cause is in the formulation or the fill, every jar in that batch shares it and most have simply not shown it yet. The batch code is what lets you answer that rather than guess.

It is worth measuring the water activity

If you have added anything wet, the finished product’s water activity is a question with an answer rather than a matter of judgment. Having that number once, for each formulation, settles whether you are making a low-moisture food at all, and it is the difference between knowing and believing.

References