Cultivated for thousands of years in the Indo-Malayan region, the key lime (also known as "Mexican lime" and "West Indies lime") has long been treasured for its fruit and decorative foliage. Smaller and with more seeds than the more common Persian lime, its distinctive flavor makes it heavily sought after, especially for delicious desserts. Key lime is most often used for its juice, which is sweet and tart and a signature ingredient in key lime-based desserts, marinades, and cocktails.
What Are Key Limes?
A key lime is a hybrid citrus hybrid fruit that is spherical and measures about 1 to 2 inches across. It is green when picked but becomes yellow when ripe. A ripe key lime feels heavy for its size. Compared to a Persian lime, it is a smaller and has more seeds to remove, higher acidity, a stronger citrus aroma, and a thinner rind. The key lime is valued for its unique flavor and costs more too.
How to Use Key Limes
To select a ripe key lime, make sure the skin has turned yellow and is unblemished. It can be used for zest or juice. There is no preparation, just use a clean fruit. Some people believe rolling it prior to juicing will yield more juice. This is a myth that has been proven false. To zest, wash and dry the skin and remove the zest with a zester. To juice, wash and dry and cut in half before juicing.
The juice is used for syrups and, of course, key lime pie. Most commercially available key lime pies these days are made from the frozen concentrate of the Persian lime, not the key lime. If you've never had a true key lime pie, it's worth seeking out.
What Does It Taste Like?
The juice of a key lime is tarter and somewhat more bitter than a Persian lime, though some people find the taste very bitter and almost caustic. For this reason, key lime juice is often used in very sweet desserts. The bitterness of the key lime balances well with the sweetness of sugar.
Key Lime Recipes
Key limes are often used in sweet and tart desserts. Key lime pie is the best known, and there are plenty of variations on this classic treat. Key limes are also a welcome addition in sweet-and-sour cocktails and marinades.
Where to Buy Key Limes
It can be hard to find fresh key limes in most grocery stores. They only grow in certain regions of the United States and are in season for only a short time, from June through September in Florida and some parts of California. And they don't stay fresh long. Key limes are grown in Mexico year-round but can be finicky to ship to the United States because they are delicate. They're one of those fruits that don’t ripen after being picked, so when buying them, look for ones that are a yellowish-green color, heavy and firm but not hard, and that have skin that is smooth and intact. Bottled key lime juice can be found in specialty markets in the canned fruit aisle near the bottled lemon juice, and it's often sold online. Because key limes can cost upwards of two to three times as much as Persian limes, it can be cost-effective buying it bottled. Depending on the store, they might be sold loose or prepackaged in bags according to weight (e.g., one pound bag).
Because key limes grow in a tropical or subtropical climate, their growing season is limited. One way to get around this is to grow key lime trees at home indoors. They can be grown in planters, and trees as small as two feet high can produce fruit. For people who live in more tropical climates, key lime trees might thrive outdoors, or potted plants could be moved outdoors during warmer months.
It's best to store fresh key limes at cool room temperature. Do not store them in the refrigerator—this will speed their decay. Ripe key limes should be used within a day or two.
Key Lime vs. Persian Lime
Key limes are much smaller in size than Persian limes (about the size of a Ping-Pong ball or golf ball), nearly spherical, thin-skinned, and often contain a few seeds. Green key limes are actually immature fruits, prized for their acidity. As they ripen to a yellow color, the acid content diminishes greatly, resulting in a sweeter fruit. The juice of key lime is known for its tart flavor. While key limes are juicier than their Persian counterparts, the size difference means that a Persian lime has more juice than key lime. If you're substituting key limes in a recipe that calls for Persian lime (or just "limes"), you'll need to know exactly how much juice the recipe requires rather than going by the number of limes juiced. A good rule of thumb is that a Persian lime contains 2 to 3 tablespoons of juice while a key lime contains about 2 to 3 teaspoons of juice.