Kaffir limes (makrut in Thai) are an important part of Thai cuisine. They are a separate species in the genus Citrus from the conventional lime. The leaves are two-lobed. The fruit is smaller than a Persian lime and knobbly, with thick rind and very little juice. Both the leaves and the fruit are very fragrant with a distinctive aroma. The fresh leaves (bai makrut) are used in many Thai soups and dishes. The rind is a key ingredient in Thai curry pastes. Fresh kaffir lime leaves are pretty easy to find; the limes themselves are hard to find in the US. There is a far larger market for the leaves and most growers pick off and discard the fruit after flowering to encourage the trees to produce more leaves. Most recipes for Thai curry pastes call for kaffir lime leaves rather than rind because the limes are so hard to find.
importfood.com, a grocery website specializing in Thai spices, sauces, condiments, and produce, sells fresh kaffir lime leaves. Fortunately they freeze well. They also get an allotment of whole kaffir limes once a year. I bought some once, but I found that they don't freeze all that well and, anyway, I've stopped making my own Thai curry pastes because there are commercial producers (especially Mae Pranom) who do a much better job than I do.
I'm a big fan of Moroccan-style preserved lemons. The thick rinds of kaffir limes got me thinking that the Moroccan salt-preserving technique could be applied to kaffir limes and one might be able to make dishes such as tagine that will get that wonderful kaffir lime scent and flavor. Yesterday I took delivery on the kaffir limes. Soon I'll be making Moroccan-style preserved limes from them. I will be adding conventional limes as necessary to get the appropriate amount of lime juice.
-Paul W.