Grapes are surprisingly fickle fruits. If you don't store them correctly, they can shrivel up or even get moldy. But when you consider the fact that grapes are actually berries, and berries have a notoriously short shelf-life, it's kind of surprising that grapes last in your fridge as long as they do. In fact, if stored correctly, regular table grapes can stay fresh for long time. As Harold McGee explains in On Food and Cooking: The Science and Lore of the Kitchen, "Thompson seedless grapes," which are the most common type of table grape in the United States, "picked in the cool of the morning and treated with antimicrobial sulfur dioxide can be held for as long as two months at 32°F/0°C."
Chances are good, however, that the grapes you buy at the grocery store won't last a full two months, even if you do keep them at 32°F. You should store grapes in the refrigerator though, since grapes do best in the cold. As the experts from the California Table Grape Commission explain, the optimal storage conditions for grapes is 30-32°F with high humidity, about 90-95 percent. This is why the best place to store grapes is in the high humidity crisper drawer in your fridge.
But remember that high humidity is different than exposure to moisture. Keep your grapes away from direct contact with water, and store them unwashed, because the excess moisture will decrease their shelf life, according to the California Table Grape Commission. Your grapes should also be kept away from a "cooling unit’s direct air path," they write, because that'll speed up the dehydration process and cause your grapes to shrivel up.
You do, however, want to keep grapes well-ventilated. That's why our friends at Real Simple recommend storing grapes you buy at the grocery store in the original plastic packaging, which usually has holes in it to allow for some air flow. But don't keep that bag or box near vegetables or foods with a strong odor. (I'm looking at you, onions.) If you do, your grapes will start to take on those smells.
Once you're ready to eat the grapes that you've so carefully stored, all you have to do is wash them in cold water and enjoy.