Kaffir lime leaves are the leaves from the kaffir lime plant (Citrus hystrix), otherwise known as the makrut or leech lime. The plant produces a small wrinkly lime and aromatic leaves. These leaves are used extensively in Thai, Indonesian, and other South East Asian cooking to give a lemony aroma and taste.
Kaffir lime leaves are dark green with a glossy sheen. They are often more important in Asian cuisines than the fruits. They are mainly produced in Thailand, Malaysia, Indonesia, and India.
Most Asian markets in the West will stock dried kaffir lime leaves, which usually come in plastic bags that can be stored for months with little change to the leaves. It is sometimes possible to find frozen kaffir lime leaves, as well.
Frozen is better than dried, but fresh kaffir lime leaves are always preferable. Dried leaves do not retain as much flavor and aroma. As well, their quality owes much to the drying method used. Leaves dried using newer technologies, such as a dehumidified heat-pump, tend to retain their aroma better than traditional tray-dried leaves. In either case, if you must use dried leaves, it is best to crush them before use. Fresh leaves can be shredded, sliced, or torn, according to the individual recipe.
You can buy fresh kaffir lime leaves online.