Microbes and food 1. Menu - you are what you eat
Salad photo
Picture 1.9a .Salad vegetables.

1.9 Vegetables
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What are they?
A vegetable is any edible part of a plant. Vegetables are low in fat and protein; the main components are usually water, fibre, starch, vitamins and minerals. This mixture provides a good food source for the growth of any microbe which may have contaminated them from soil, air or water. Vegetables are eaten raw and cooked and are ingredients in a wide range of foods. They also may be chopped, canned, dried, frozen, salted, packaged, fermented (see pickles) or packaged in various ways.
How do they spoil?
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Field of blighted potatoes photo
Picture 1.9b Potato blight is a mould that attacks the leaves of potato plants and can spoil a crop in the field. It caused the Irish famines in 1846 and 1848.
Growing vegetables can be attacked by plant pathogens which damage the tissues. An example is potato blight, a fungal disease which can ruin whole crops. Plants fight back by:
  • having thick skins to repel microbial invaders
  • producing anti-microbial compounds

All vegetables can be spoiled during harvesting, processing, transport and storage. Spoilage is more likely if the crop is damaged as this enables microbes to enter the tissues. A broken skin releases nutrients which support microbial growth. The vegetable may become spongy and soft, as microbial damage to the plant cell walls releases the watery contents. Vegetables have a higher pH than fruit, though they are still slightly acidic. This encourages bacterial as well as fungal growth – around one third of total microbial spoilage of vegetables is due to bacteria.

Spoilage can be prevented by careful handling and transport. Fresh vegetables can have a good storage life if kept in the correct conditions. Refrigeration slows down microbial growth; control of humidity is also important as condensation will encourage bacterial growth and too dry an atmosphere will lead to wilting of the produce. Packaging can regulate the amount of oxygen and water reaching the plant and the microbes growing on it.

Any treatment of vegetables will affect the type of microbes present and their rate of growth.

Can they be harmful?
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Vegetables can become contaminated with human pathogens if they have been exposed to raw sewage or animal manure in the field, to dirty irrigation water or from unhygienic handling. The bacteria and viruses from these sources might then be eaten with the vegetable. Some bacteria e.g. salmonellae can even grow in certain vegetables. The increasingly popular organically grown vegetables are more likely to be contaminated than those cultivated using artificial fertilisers. There is little risk of ill health if the vegetables are washed and cooked properly, but salads are sometimes associated with cases of food poisoning. Steps can be taken by the farmer and producer to avoid these problems; use of sewage sludge is banned in some countries, or it may not be applied within a set period before harvest. Simple measures like chlorinating the wash water for vegetables are also effective. Processing and packaging must also be hygienic.