Microbes and food 1. Menu - you are what you eat
Packets of mushrooms
Picture 1.4a .Some different mushrooms on sale in the supermarket.

1.4 Mushrooms
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What are they?
Mushrooms are filamentous fungi that produce large, often edible fruiting bodies. They live on organic material, thriving on compost, fallen leaves and damp wood and any other dead plant or animal matter. Their role in causing decay is important in maintaining ecological cycles.

The fungus is made up of a mycelium – a network of fine threads, called hyphae – which branch and grow over and through their food source. The hyphae secrete enzymes that digest organic matter into soluble products. These diffuse back into the fungal threads.

The fruiting body is produced when conditions are favourable. It begins as a small button-shaped structure underground growing into the typical mushroom shape with a stalk and cap. It is made up of hyphae that have fused together. Its function is to produce and disperse spores. The spores grow in special structures called sporangia on gills on the underside of the cap and they burst periodically. The spores are released and dispersed by air currents, germinating if they land on a suitable food source.

Growing mushrooms photo
Picture 1.4b .Growing mushrooms. You can see the gills on the underside of the caps.
How are they produced?
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Cultivating mushrooms is a traditional biotechnology which has been used since the 1600s. Shi-take mushrooms have been cultivated for even longer in the Far East. The cultivated mushroom seen most in the shops is called Agaricus bisporus, although more exotic types of mushrooms like ceps, chanterelles and oystercaps are becoming popular. Growing mushrooms for food is now big business; they are the highest value crop grown in the UK. On mushroom farms, beds of a rich compost mixture are inoculated with a pure fungal culture known as ‘spawn’. It develops into a mycelium throughout the substrate before mushroom formation is artificially induced. The crop is gathered, packed and distributed.

Go to www.mushroomcouncil.com for a detailed description of commercial mushroom production.

How do they spoil?
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Mushrooms are not grown in sterile conditions, although the soil mix is given a form of pasteurisation which prevents growth of many of the microbes which might spoil the final product. A range of fungi, bacteria and viruses are pathogenic to mushrooms. Mushrooms can become contaminated from many sources during production and processing, including the humans harvesting the crop. Bacteria, yeasts and moulds cause most problems. Mushrooms have soft flesh which is easily bruised, helping bacteria such as Pseudomonas species to spread. These cause brown blotches on the caps. Other microbes induce shape distortions. Mushrooms spoil quickly if kept at room temperature for long.

Mushrooms keep best if they are stored free from moisture in cool conditions. They are often sold packaged under plastic film where the reduced oxygen level increases shelf life.

The Death Angel mushroom
Picture 1.4c The Death Angel mushroom. Despite its ordinary appearance, this mushroom can be deadly.

Can they be harmful?
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WARNING! Not all mushrooms are edible. Some wild mushrooms are very poisonous and can even cause death. Gathering and eating wild mushrooms is not sensible unless you are an expert in identifying them.

Cultivated mushrooms are usually cooked, which kills any harmful bacteria that may have contaminated them from soil, insect pests, rodents or handling during processing. The greatest risks are from faecal bacteria, Staphylococcus aureus and bacterial spores from the soil. However commercial mushroom production is carried out with great attention to hygiene and there have been few reported cases of food poisoning from this product.