Mould

Mould (or mold) is a kind of fungi, which use spores to reproduce. Fungi spores are ubiquitous; present in the air, dust and water around us continuously. When they make contact with old, stale or rotting food or drink of a suitable type, these spores develop into more fungi which grow and reproduce rapidly.

Moulds play an essential role in the natural environment; breaking down and recycling organic matter that has reached the end of it's life. Without it, the world would be a much messier place.

Moulds can also be hazardous to human health, with the spores (in high quantities), potentially causing allergic reactions & respiratory issues and some moulds producing mycotoxins that can pose a serious health risk to humans if consumed.

Mould typically grows inside of food or liquid using a root-like structure similar to that of plants, called mycelium. The visible mould you see on the surface usually doesn't appear for a day or two, until the mould is ready to reproduce and spread spores into the environment. Thus, a small amount of visible mould usually indicates there is far more beneath the surface.

Mould grows quickly in warm and moist environments and more slowly in cold, dry, acidic, salty or sugary environments. This is why you're more likely to encounter mould on unrefrigerated bread, fruit or vegetables and far less likely to on jam or picked gherkins.

Warning

I did intend to add a section here on which foods can safely be eaten after the mould has been removed and which cannot but I came across a great deal of conflicting advice from various sources and it appears to be simpler and safer (especially for the young, elderly, ill, pregnant and immunocompromised) to be cautious and discard any food or drink that you find has mould.