We conducted a test to confirm (or disprove) this theory.
Rumor has it that dropping a raisin into an open bottle of flat champagne can bring back the bubbles; we ran a test to see if it was true. We dropped a raisin into a bottle that had been left open for four days and were hopeful when a stream of tiny bubbles appeared. We waited 2 minutes (the time recommended by most sources) and then tasted the “revived” sample alongside wine from another bottle that had also been open for four days; we also compared both to champagne from a freshly opened bottle.
The Verdict: Both four-day-old wines tasted less fizzy, less aromatic, and sweeter than the fresh champagne; furthermore, tasters couldn't tell which four-day-old wine had been “refreshed.”
So why the appearance of bubbles? The raisin's wrinkly surface coaxed the carbon dioxide dissolved in the wine to form bubbles, which merely made the champagne look effervescent. But the raisin can't actually add carbon dioxide to the wine, which is necessary to restore fizz.