Microbes and food 1. Menu - you are what you eat
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Cooked prawns photo
Picture 1.6a .Cooked prawns with their shells on.

1.6 Prawns
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What are they?
Prawns are a type of shellfish known as crustaceans, a group which also includes crabs, lobsters and shrimps. They are high in protein and low in carbohydrate and provide a good food source for the growth of bacteria. Prawns can be harvested from a range of aquatic environments – deep seas, shallow coastal waters and rivers. They are also grown in fish farms. Prawns are caught in nets and refrigerated on board ship until they can be delivered to the processing plant where they are washed, sorted, and usually peeled before being cooked. Some raw prawns are sold frozen.
How do they spoil?
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Prawn processing photo
Picture 1.6b Processing shelled prawns. Workers inspect the prawns and remove any bad ones. Notice that they wear gloves and protective clothes to prevent contaminating the prawns.
The prawns die immediately after capture and they start to deteriorate quickly. They can be contaminated with bacteria from mud and the surrounding water, from ice and the boat. Because it takes several days from being caught to being processed, the microbes multiply and by the time they get to the plant the prawns often contain large numbers of bacteria. Most of these are washed off, but more bacterial contamination can occur during peeling and other manufacturing steps. The bacteria are psychrophilic types i.e. cold-tolerant due to the chilled conditions. Strict attention to quality control and hygiene is very important as spoiled prawns taste unpleasant and have ‘fishy’ off odours. These are due to the proteins being broken down into amines and other volatile compounds.

Freshly cooked prawns must be kept chilled and eaten quickly. Frozen prawns must be used as soon as defrosted. The best way to keep crabs and lobsters fresh is to keep them alive until just before they are wanted for the table. Then they should be cooked and consumed straightaway.

Can they be harmful?
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Prawns and shellfish which have been harvested close to the shore may well come from polluted waters. There is a high risk of contamination with human sewage. This can contain enteroviruses as well as pathogenic bacteria which can cause gastric illness if the product is not processed and stored correctly. The viruses and bacteria should be killed by cooking, but poor hygiene during handling after this stage can lead to further contamination of the prawns by factory workers. In tropical waters the bacterium that causes cholera, Vibrio cholerae, may even be present.
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