There is very little chance of eating a contaminated oyster as long as you choose ones with their shells tightly shut. Oysters grown commercially will usually undergo a cleansing period in purified water for 36 hours. Fresh oysters that are tightly shut can be kept in the refrigerator for up to a few days. Wash off any mud before storing them, but do not soak them. Oysters that have been opened are best eaten straightaway but can be stored in the refrigerator for up to 2 to 3 hours.
Always buy opened oysters from a reputable fish market or retailer. Oysters are a very concentrated form of protein and some people find them difficult to digest for this reason. As oysters are often eaten on festive occasions, they may be consumed with more alcohol than usual, which can also contribute to adverse side effects. Oysters do seem to cause particularly severe problems if mixed with spirits, so it is better to drink chilled white wine or champagne, or even cold dark stout, with them. These are better suited to their taste.
Oysters are most often served raw, topped with black pepper and a squeeze of lemon juice. They may also be grilled lightly in the half-shell, or topped with breadcrumbs or garlic butter. They can also be gently cooked in soups.
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