Microbes and food 1. Menu - you are what you eat
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1.1 Beans and pulses
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What are they?
Baked beans are a popular snack. They are made from processed haricot beans. Beans, along with peas and lentils (often called pulses), belong to the legume family. This group of plants uses nitrogen-fixing bacteria to help them grow. The bacteria are essential to the production of high yields of these food crops.
Legume root
Picture 1.1a The root of a legume showing the nodules.
How are they produced?
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When the seeds of a legume germinate, the seedlings release chemicals into the soil which stimulate the growth of Rhizobium bacteria. The bacteria then invade the roots of the seedlings where they live and multiply inside the cells. The enlarged root cells form a lump or nodule. Nitrogen is then fixed from the atmosphere by a symbiotic process of the plant and the bacteria. The plant provides the anaerobic conditions and growth nutrients for the bacteria, and the nitrogen gas converted to ammonia by the bacteria is incorporated into plant protein.
Baked beam cans
Picture 1.1b Baked beans are a popular food.

How do they spoil?
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Most beans and pulses are dried or canned after harvest and spoilage is not usually a problem.
  • Drying prevents the growth of microbes due to the low moisture content. Mould growth can occur if the product becomes damp in storage. In the home pulses should be stored in sealed containers in a dry place to avoid aerial contamination by microbial spores.
  • Canning gives pulses a long shelf life. Once the can has been opened the contents should be eaten quickly. Left over, canned baked beans should be stored covered in the fridge to avoid contamination by spoilage or food poisoning microbes.

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