Key lime and regular lime are different citrus fruits with distinct characteristics. Key limes are smaller, rounder, and more yellow-green than the larger, darker green Persian (Persian or Tahitian) limes commonly found in supermarkets. Key limes have thinner skin, higher acidity, and a more distinctive floral, slightly bitter flavor profile that many consider more complex and aromatic. They contain fewer seeds but require more fruit to produce juice.
Key limes originated in Southeast Asia and became famous in Key West, Florida, where they grow prolifically and are traditional in Key lime pie. Their juice has a higher acid content (around 6-7% citric acid versus 5-6% for Persian limes) and a unique terpene profile that gives them their characteristic tang. Persian limes dominate commercial production due to their larger size, higher yield, thicker skin for easier shipping, and longer shelf life—making them the standard "lime" in most grocery stores.
The flavor difference is noticeable: key lime juice tastes more complex and intensely tart with subtle sweetness, while Persian lime is cleaner and more straightforwardly sour. In cooking, they're somewhat interchangeable in most recipes, though key lime's distinctive taste makes it preferred for authentic Key lime pie and certain cocktails. However, Persian limes work as a substitute when key limes aren't available. Key limes are harder to find fresh outside Florida and specialty markets, though bottled key lime juice is widely distributed. The trade-off is clear: key limes offer superior flavor but inferior convenience and shelf life compared to their Persian cousins.