The link below explains...
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Tony Tantillo - Produce Tips - Fruits
Hard, unripe avocados should be stored at room temperature and usually take three to six days to ripen. Put avocados in a paper bag with an apple, banana, tomato, or pear, all of which give off ethylene gas, which speeds up the ripening process. Never put hard avocados in the refrigerator because they will never ripen. Ripe avocados will keep in the refrigerator for up to one week.
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