Darkening of top layer of jam

What causes the jam to darken at the top of the container ?

Hmm jam surface darkening …?
might be .
too much bubbles during processing and excess headspace during bottling…so as there is excess air it will contribute to “oxidative” browning .
Another reason not enough syrup to cover up the fruit bits so again it will lead to browning due oxygen exposure…

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What is container capacity??

The first thought will be oxidation. Just another thought, it could be as a result of condensation which leads to dilution at the surface, higher water activity hence some microbial activity.