Microbes and food 1. Menu - you are what you eat
Bowl of rice photo
Picture 1.8a Cooked rice.
1.8 Rice
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What is it?
Rice is a cereal grain and is the main food of half the world's population. It is grown in many countries, although it originated in Asia. Rice is usually eaten after simple cooking, although some is ground into flour and used as an ingredient in other cooked foods. Puffed rice breakfast cereals are popular in Europe and America. Rice wine, made by fermenting the grains with fungi, is used in Japanese and Chinese cooking.
Paddy field photo
Picture 1.8b A rice paddy field.

How does it spoil?
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Moulds are the most important microbes associated with cereals during growth, harvest, storage and shipping. They are divided into two groups:
  • field fungi which cause disease in the growing plants
  • storage fungi which can spoil the dried grains.

Field fungi need plenty of moisture for active growth, but storage fungi such as Penicillium and Aspergillus can grow with much less. Fungal spores get into the harvested crop from processing equipment and dust in the environment.

Good storage conditions are the key to keeping rice fresh as the spores only germinate if the moisture, temperature and oxygen levels are just right. Moulds cannot grow in rice grains kept at below 14% moisture. Warmth also encourages fungal growth.

Spoiled rice grains can be discoloured, smell "off" and lose their goodness. Once they have started to break down, a wide range of other microbes complete the process of decay.

Packets of rice
Picture 1.8c Rice is stored in dry sealed packs.
Can it be harmful?
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Some field fungi can produce mycotoxins in the growing rice, although this is not as serious a problem as it is with cereals like wheat. These poisons, which can cause cancer if eaten, survive processing.

Correctly stored cereals are so dry that bacteria will not grow. But pathogens such as Salmonella picked up in the field or during processing from humans or animals may survive and multiply if the rice is used in a moist food. Cooking will kill the bacteria, but the cooked rice may become contaminated again from the environment. Care should be taken to avoid this.

Bacillus cereus food poisoning is associated with cooked rice. Spores survive the cooking; they germinate and grow if the food is not chilled properly - i.e. left out in a warm kitchen. The bacteria produce toxins which cause vomiting and diarrhoea.

Before being eaten, rice should always be

  • cooked in small quantities
  • refrigerated below 6°C if not being used immediately
  • reheated to a high temperature.