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Why does jam go mouldy?

The complete question was:
A product with little moisture and a high sugar content goes mouldy after approximately one month. Wet spots seem to appear on the product. What causes this?
Fungi can grow at very low water activity (degree of available water), but not in jam. The water in jams is to much bound which makes it to dry for the fungi. However, a product of high sugar content does attract moisture from its surrounding (hygroscopic). This can create a thin film of water on, for instance, jam. This in turn creates a local lower sugar level allowing fungi to grow.