The leaves and fruit of the kaffir lime tree (citrus hystrix) are intensely fragrant. The leaves provide a key flavor in Thai curries, soups, and stir-frys. You have probably tasted it in dishes such as penang curry, or tom yam. Often the leaves are chopped (chiffonade) finely in long strips and then go into the dish late in the cooking process. For soups the leaves can go in whole, and earlier in the cooking process. Recently La and I cooked with green peppercorns, and we tore the leaves into that stir-fry. I also used these leaves in an earlier recipe for tom yum plaa.
Less commonly used whole in cooking are the knobby kaffir limes themselves. One dish stands out in my memory with the fruit as a primary ingredient. The scrumptious gaeng tepo แกงเทโพ “tepo” curry, usually made with fatty pork, coconut-based red curry, and large-sized morning glory or water spinach (here is a link to a Thai language video on how to make this dish, and here is a link to a nice example image).
The rind of the fruit is, however, used to make Thai curry pastes which are used for a wide variety of purposes in Thai cooking. These are used to make coconut-based curries, like red, green, penang flavors, and these pastes are also used in stir-fry cooking (here is a recent example of one that we made), and they might be used to marinate meats before they are deep fried (fish cakes for example). Most people, even in Thailand, simply buy the paste pre-made from the market. In the US, of course our pastes are sold in handy pre-sized cans.
Nowadays kaffir lime leaves are readily available in well-stocked Cambodian, Vietnamese, Lao, Thai markets. The shape of these leaves are very unique. You know them by the double-leaf shape (see figures 1 and 2). In our Philly markets, we can get them fresh in little baggies (sometimes in the fridge section), and you can also find them frozen and dried. There was a time, however, about 4 years ago, when they could not be found in our region. We were living in Wisconsin then, and our area was apparently affected by a USDA restriction on the movement of these leaves to slow/stop the spread of a citrus greening disease.
During that time we found that we could buy young kaffir lime trees online. That’s what we did. So now, we no longer have to go to the market for these, we just cut some off when needed. For those interested, I’ve seen small kaffir trees also for sale occasionally at my North Philly Cambodian market, and also at the Big 8 supermarket on Washington Ave. Again, you can find these places via my maintained map of Philly Asian markets.
Kaffir limes are not found at markets (I have yet to see them anyway). That said, the majority of recipes use the leaves only–only occasionally the fruit or rind of the fruit is called for. I have seen dried kaffir lime peel at Southeast Asian markets. That can also be used as a substitute in dishes like the aforementioned “tepo” curry. If you are looking for this, South Philly’s 7th street would be the best bet.