Can I use Thai basil in pad kra pao?

You can, and it can taste good, but it becomes a different dish. Thai basil brings a sweet anise smell, not holy basil's hot, peppery one.

What is the difference between red and white holy basil?

They are the same species. Red is more pungent; white is used with seafood. One study found far more of the main aroma compound in red, but another concluded that leaf and stem colour do not reliably predict the aroma compounds.

Is the tulsi sold in Indian shops the same as kaphrao?

It is the same species, Ocimum tenuiflorum. Dried tulsi sold for tea, though, smells and behaves differently from the fresh leaves used in a stir-fry.

Holy basil vs Thai basil: two species, two smells, and how to get real kaphrao

Holy basil and Thai basil are not one leaf with two names. They are two species in the same genus. Holy basil (kaphrao, กะเพรา, Ocimum tenuiflorum) smells hot, of clove and pepper. Thai basil (horapha, โหระพา, Ocimum basilicum var. thyrsiflora) smells sweet, of anise and liquorice. Use the wrong one and pad kra pao becomes a different dish.

This article shows how to tell them apart by eye, touch and smell, what research found when it measured their aromas, and how to get real holy basil when eating outside Thailand. If pad kra pao itself is new to you, start with one-plate meals.

Four leaves from one genus

Thailand knows four Ocimum species: horapha, maenglak, kaphrao and horapha chang (tree basil). A Thai history magazine article citing Mahidol University’s Faculty of Pharmacy notes that kaphrao smells more pungent and is hairy, unlike horapha, which is not, though details vary by variety.

Leaf	Species	Main smell	How to spot it	In Thai cooking
Holy basil (kaphrao)	Ocimum tenuiflorum	Hot, clove and pepper	Hairy	Pad kra pao, stir-fried curries, jungle curry
Thai basil (horapha)	Ocimum basilicum var. thyrsiflora	Sweet, anise and liquorice	Not hairy, shiny leaves, square purple stem, pink-purple flowers	Green and red curries
Lemon basil (maenglak)	Ocimum citriodorum	Lemony	Clear lemon smell	Least used of the main three
Tree basil (horapha chang)	Ocimum gratissimum	Herbal, spicy, eugenol-led	Strong smell	Used little

Same genus, different species: check the hairs, the stem, then crush and smell

Red and white holy basil

Thailand uses two kinds of holy basil. Red is more pungent; white is used with seafood. Both are Ocimum tenuiflorum.

What the research measured

A 2018 study in the journal Foods extracted essential oils from five basils used in Thailand, measured the compounds with instruments, and had a panel rate the smells from 0 to 15.

Red holy basil: led by methyl eugenol at 683.90 µg/mL, then β-caryophyllene at 145.81 µg/mL
White holy basil: also led by methyl eugenol, but only 98.44 µg/mL
Thai basil: led by estragole at 452.80 µg/mL, then geranial and neral
Lemon basil: the strongest citrus smell of the five, 12.8 out of 15
Tree basil: led by eugenol at 408.00 µg/mL

The panel told red and white holy basil apart mainly by how strong the anise note was. Sweetness did not differ significantly among the five.

Worked example: how far apart are red and white?

Methyl eugenol in red holy basil oil: 683.90 µg/mL
In white holy basil oil: 98.44 µg/mL
683.90 ÷ 98.44 ≈ 6.9 times as much

That helps explain why red holy basil smells stronger, but it comes from one set of samples.

Where sources disagree

A 2022 Thai study in Frontiers in Plant Science grew many Thai holy basil lines in a plant factory with controlled light and temperature. A red commercial cultivar had the most methyl eugenol, 3,357.27 µg/g dry weight, and very little eugenol, 419.48; the more of one compound a plant had, the less of the other. Crucially, the authors concluded that leaf or stem colour did not predict the proportion of the main aroma compounds.

Wikipedia, meanwhile, collects two quite different profiles from studies elsewhere: one led by eugenol at 70%, another by camphor at 32%. The takeaway is that holy basil from different places and lines can smell quite different. Studies of holy basil in Thailand point to methyl eugenol as the usual lead compound in holy basil grown in Thailand, but that is not a fixed rule.

To tell holy basil from Thai basil for sure, use three checks together: hairs on leaves and stem, Thai basil’s square purple stem, and the smell of a crushed leaf. Never go by colour alone.

How to get real holy basil

In Thailand, pad kra pao is made with holy basil. If in doubt, crush a cooked leaf and smell it: hot and peppery is holy basil, sweet and anise-like is Thai basil.

Abroad, Wikipedia notes that Western kitchens commonly use Thai basil as the substitute. If a menu says “Thai basil”, that is horapha, not kaphrao. Ask: “Is this holy basil or Thai basil?” To cook it yourself, some Asian grocers and Indian shops sell it as holy basil or tulsi, the same species as kaphrao. The 2018 study above used the Shyama type for red holy basil and the Rama type for white.

If you have food allergies, the leaf is rarely the issue in pad kra pao, but the sauces can contain soy, wheat or oyster. Ask the kitchen every time.

Common mistakes

Thinking “Thai basil” is the leaf in pad kra pao. Thai basil is horapha; the leaf in pad kra pao is holy basil.
Going by colour alone. Thai basil has a purple stem too; check the hairs and the smell.
Assuming red is always stronger. Usually it is, but the 2022 study found colour does not reliably predict aroma.
Using dried tea tulsi instead of fresh leaves. The smell and texture are quite different from fresh leaves in a stir-fry.

Summary

Holy basil and Thai basil are relatives with opposite smells. Holy basil is hot and peppery with clove notes and hairy leaves; Thai basil is sweet and anise-like, smooth, with a square purple stem. Research found about seven times more of the main aroma compound in red holy basil than in white in one sample set, but colour does not tell you everything. When eating abroad, ask: “holy basil or Thai basil?”

Sources

Ingredients and heat vary by kitchen and recipe. If you have a food allergy, ask before you order, every time.