Conventional wisdom says that any corn on the cob older than 10 minutes from being picked is no longer edible. But while it's true that enzymes in corn begin to convert from sugar to starch immediately after picking—and lose half the sugars in three days, according to Fine Cooking—the corn is obviously still good beyond 10 minutes. Corn that has spoiled or is no longer safe presents a few specific characteristics to look out for.
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Buy the freshest corn you can by choosing cobs that have been kept cold, remain in their husks and whose silk tassels appear fresh and light-colored instead of brown. Fresh corn cobs retain their quality for only 1 to 2 days in the refrigerator, wrapped in plastic or foil. Begin to look for signs that the corn is going bad after that time. You'll see the silk at the top of the cob turning brown and becoming slimy and the husks beginning to turn yellow and dry at the tips. The corn is still safe to eat at this point, even though it's less sweet and the kernels are less moist and juicy. Extend the life of corn on the cob for a few extra days by standing the cobs upright in a jar in your fridge with their stalks in a few inches of water. If you smell a moldy or rancid smell after you husk your corn, chances are that it has spoiled and you should throw it away. You may or may not see more signs of spoilage, including black, brown, greenish or white spots around or on the kernels and kernels that are opened and oozy. Toss the corn if you see any of these signs. If the signs of spoilage occur only at the very end of a cob, you might be safe in cutting off the cob a few inches below the bad spot. On the other hand, mold and bacteria are not always visible even if they're present. If you're not sure, throw it away. If corn has been in your refrigerator for two days and looks somewhat dried, but not spoiled, add it to corn muffins or corn bread or turn it into a corn relish or salsa to serve with pork, poultry or Mexican-inspired entrees, where spicy or buttery flavors compensate for the corn's lack of sweetness.