Kaffir lime ( มะกรูด ) is a citrus fruit from the “makrut” lime plant, which is very common and widely used in Southeast Asia. It grows on a shrubby thorny bush, which can be cultivated both in a pot or in the open. It is such a key ingredient in Thai cooking that nearly every family living in the countryside has kaffir lime as a fixed part of their backyard.
Kaffir lime is easy to recognize by its distinctly hourglass shaped double leaves. You pick the darkest leaves on the bush first, and leave the lighter ones to mature a bit longer. The thick leaves are dark green and shiny on the front, and a paler green, matt on the back. The fruit itself is green and ripens to yellow, with an exterior that is bumpy and rough with brain-like looking textures. The fruit is about the size of a regular lime, but the surface makes it easy to distinguish from regular lime, which has a much more smooth peel.
The aromatic leaves are the most frequently used part of the plant and can be used fresh, dried or frozen. I highly recommend using fresh or frozen leaves, as the dried ones tend to be less aromatic and flavorful. The leaves freeze very well and can be stored for up to a year. Just place them in a zip-lock bag, take out a few leaves as you need them, and leave the rest in the freezer for next time.
The stem is not used, so to prepare the leaves for cooking, fold them in half and tear the stem off. Depending on the use, you then either leave them as is or slice them very thin. The whole leaves are used in soups and curries, and the sliced leaves as topping on dishes such as curries and salads. If you are using frozen leaves, you can thaw and soften the leaves by rinsing them in hot water. This also helps to bring out the flavor.Note that the leaves are used as a taste giver, similar to bay leaves. This means you do not eat it unless it is very finely chopped. If added to a soup or curry, just remove the leaf before serving. Or leave it in for the sake of appearance, and remove the leaves before eating.
Kaffir lime leaves can be purchased at any local market in Thailand, or at Asian food stores or larger supermarkets abroad. But you can also order them online.
If you are unable to find kaffir lime leaves for your cooking, I think, it is better to leave it out, than to substitute with another ingredient. There is really no other herb that has the wonderful aromatic citrus fragrance that kaffir lime leaves do.
How about the fruit itself? In Thailand, the kaffir lime is not consumed due to its bitter and tart juice. The zest is sometimes used in cooking, for example in curry pastes. But the fruits are mainly used to produce essential oils, household cleaning products, shampoo or for cleaning your feet before a massage. I have enjoyed this pleasure for years at my local massage place – they say it is good for your digestion and helps detoxify your blood….