Return to index Yeasts and moulds Both of these are types of fungus. Each individual is too small to see without using a microscope, but in a mass they are easy to see. Yeasts feed on sugar. If there is no air in their environment, they turn the sugar into alcohol and carbon dioxide gas after removing some energy. There are many different kinds - many living on the skins of fruits.
Moulds occur naturally in the soil and elsewhere, feeding mostly on dead organic material.
They can grow in quite cool conditions - even in a refrigerator. Their minute spores float around in the air. Sometimes you can see a cloud of them rising from a piece of mouldy bread. The vast majority of moulds are involved in the breakdown of dead plants or animals, but some species are parasites and attack live plants. A few species even live on animals, causing irritating skin conditions.