The bright green, seedless Persian lime most commonly found in US supermarkets is a subtropical fruit. The smaller, mottled yellow-green, heavily seeded Key or Mexican lime is a tropical fruit. All limes are harvested before they ripen. Limes are just less acidic than lemons and may be substituted for lemons in most dishes. Limes are the thinnest-skinned and most fragile of the citrus fruits; refrigeration causes the skin to brown.

BUYING TIPS:
Select limes that are heavy for their size with a glossy skin. Limes with a dull, dry skin are old and therefore less acidic than fresh limes. Key limes are less available, but buy them when you can; they have a very perfumed flavor, which is accentuated by cooking.