Thai cooking utilises three type of basil: Thai basil, holy basil, and lemon basil. The latter is the most difficult to find outside of Thailand and the first in the list the most commonly available. Thais treat them as three separate entities and so these basils are generally not interchangeable with each other. The fresher the leaves, the more you’ll get out of them; the flavours will dull over time.
Types of Thai Basil
Thai basil (bai horapha) looks similar to your everyday Italian basil but with purple stems, and the flavour is of bold aniseed. If you can’t find this, you can substitute with Italian basil, although the flavour will be markedly different.
Holy basil (bai gaprow) is a milder variety with jagged green leaves and has a peppery flavour with a clove like undertone. This is a key ingredient in pat gaprow (kaprow/gaprao), which translates literally to fried holy basil. Always enquire at Thai restaurants when ordering this dish to see that they don’t use Thai basil – this is a good way to judge the reliability of the rest of a menu!
Lemon basil / Hairy Basil (bai maeng lak) is the most delicate and least utilised of the three, and needs to be used fairly quickly as the aroma will dull after a couple of days. In the dishes where it’s used (e.g. kanom jin nam yaa) it plays an important role in the overall flavour. I’ve found a worthy substitute to be Vietnamese lemon balm (cockscomb mint), which you can find in some Vietnamese shops in East London (try Star Night, among others). If you can’t find either, omit it from the recipe without replacement.
In London Thai basil is easily accessible in most well stocked East Asian supermarkets. You are likely to find it in Chinatown. Holy basil is a little trickier to find, but if you seek out a Thai supermarket and ring ahead, then you might have luck. Sometimes online retailers Thai Food Direct and Thai Food Shop have lemon basil in stock. They usually have Thai and holy basil as well.