Essential to the cuisines of Southern India and Sri Lanka this citrus leaf has no real substitute and is not worth much dried. Fortunately it is now grown in Southern California and can be had fresh from most Indian markets, at least around here. They keep only a few of weeks refrigerated. Freeze bagged in water to keep them from drying out. The tree bears pea sized dark purple fruits but I've not heard of them being used for cooking.
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In India, curry leaves are used in the cuisines of the south and east but not at all in the north. The southern cuisines are primarily vegetarian, so curry leaves are rarely used with fish, fowl or meat. In Sri Lanka, however, curry leaves are often used in chicken and fish dishes. The leaves also see some culinary use in Malaysia, and throughout the region they are much appreciated for their medicinal properties.
Note that some people do not like the resinous flavor of curry leaves, so it's best to be on the conservative side until you're experienced with them.
Buying: Enough curry trees are now grown in Southern California that most Indian markets will have fresh leaves. They are generally put up in small rumpled plastic bags, stems and all, and are found in the produce section. Commercially dried leaves are sold, but have lost most of their flavor and aroma. I have seen fresh leaves for sale on eBay, but the shipping cost can make them expensive.
Growing: If you live in a tropical or subtropical region the best solution is to grow your own tree - if you can find a nursery that gives you more than a blank stare. They are fairly small, growing only to about 5 feet and can be kept in a 5 gallon container. They want warm weather, full sun or part shade and plenty of water.
Unfortunately they grow slowly. I bought mine at 3 inches high (that would be about 8 months old). They're now 45 inches, but it's taken around 5 years to get there. They set fruit every year, and though I enjoy the sweet, slightly resinous berry size fruit, setting fruit does stunt their growth significantly. Seeds are easy to sprout indoors, but mortality seems quite high, so you need to plant plenty and tend them carefully.
Storing: Leave them on the stems or flavor will suffer. Kept in the plastic bag they came in, curry leaves will stay usable for seveal weeks in the refrigerator. They will keep longer if frozen, but freeze them in water or they will dessicate rapidly and lose flavor.
Another method I've used is to dry roast the leaves in a heavy pan over very gentle heat with plenty of stirring and making sure they don't brown at all. They are very tender so this will take only a couple of minutes. Dried quickly this way they stay quite green and hold their flavor well. In a sealed jar kept in a cool dark place they'll be useful for a couple of months.
Cooking: Many recipes call for them to be fried in hot oil at the beginning of the recipe. Fresh leaves will snap and splutter violently for a few seconds, then settle down. Other ingredients need to be added immediately before the leaves brown. If you have dried them as described above, add them later when there is liquid in the pan.
The leaves may or may not be removed from the finished dish before serving.
When making curry powders you need to dry roast the leaves as described above, then grind them with the other spices.
Substitutes: Well, there really aren't any. Bay leaves are not an acceptable substitute. The closest you can get is Salam Leaves (Daun Salam, used in Malaysia and Indonesia) which is less camphor and more citrus (even though Curry Leaf is a citrus and Salam is a myrtle). Salam leaves are usualy used dried.