Kaffir limes are also called makrut limes. They belong to the citrus family and the leaves of kaffir lime trees contribute to the robust, distinctive citrus flavor found throughout Thai cuisine. The unique citrus and herbaceous notes are crucial to many Thai dishes and are responsible for much of authentic Thai cooking’s distinctive taste. Unfortunately, kaffir lime leaves can be quite difficult to find if you are in the US so you will probably need a substitute if you want to make an authentic-tasting Thai dish.
Make no mistake, the flavor of kaffir lime leaves is complex and difficult to replicate; however, you can create a facsimile with the right substitutes. Many of the alternatives are common and easy to find. If you cook regularly, you may have some or all of them on hand.
Your best bet: Bay leaf, lime zest and lemon thyme
Kaffir lime leaves are so complex that your best alternative requires that you use a combination of pungent ingredients. The leaves of the bay laurel tree bring a strong herbal flavor with floral notes and slightly bitter clove-like notes. The lime zest brings a sharp, clear citrus flavor and the lemon thyme adds a softer herbal note combined with its own citrus spark. Together, they provide the effect that you want from kaffir lime leaves.
When using this substitute, you will want to balance the constituent flavors carefully. Use half of a small bay leaf, combined with a 1/4 teaspoon each of lime zest and lemon thyme. This combination works best with soups, stir fries and other cooked dishes.
A decent second choice: Persian limes
Also known as Tahiti limes, Persian limes are easy to find and provide many of the desired flavor notes. Simply cut the limes in half and add them to dishes like tom yum soup and remove them before serving. Persian limes are usually seedless, which means that you will not have to hunt for and remove the seeds before serving. Another option is to use the zest of the limes as your kaffir lime leaf substitute. Simply add the zest as you would add sliced lime leaves and use as much of it as the recipe requires for lime leaves.
In a pinch: Other citrus leaves
The leaves of other citrus fruit like oranges, limes are lemons may be effective replacements for kaffir lime leaves; however, they may not be as fragrant as the lime leaves. To compensate, you can try adding more of them. Use 1 1/2 as much of the citrus leaves as your recipe requires for kaffir lime leaves. If necessary, you can add more.
Lemon zest an easy and effective option. By using zest, you can give your dish much of the acidity and lime intensity that you would get from kaffir lime leaves. Use the zest of one lime in place of every two lime leaves required by your recipe.
Curry leaves come from the sweet neem tree and can provide some of the citrus notes that you want from kaffir lime leaves; however, they are different from the lime leaves in that they should not be eaten. Remove them before serving just as you would a bay leaf.