Genovese Basil vs. Thai Basil: One Species, Two Very Different Oils

Genovese basil and Thai basil are the same species, Ocimum basilicum, but they smell like different herbs. Genovese is the large-leaved, sweet Italian pesto basil; Thai basil, a cultivated form sold in Thai as horapha, tastes of anise. When Purdue University researchers grew both side by side, the Genovese oil was led by linalool, while most of the Thai basil lots were dominated by methylchavicol, also called estragole, the compound that makes up 60 to 75% of tarragon oil.¹² That one difference explains most of what changes in the pan. Even so, one of the four Thai lots in that trial looked more like the sweet basils.

Side by side

The chemistry below backs up the anise half of the flavor row. For each herb in depth, see the Genovese basil and Thai basil profiles.

The difference in the oil

Both herbs were grown in the same 1999 Purdue field trial, from seed sold by North American suppliers and distilled the same way, so their oils compare directly:¹

Three of the four Thai lots were dominated by methylchavicol. The fourth, the only one with green stems and white flowers, looked more like the sweet basils. The Purdue authors also describe a basil oil type, produced more recently in Madagascar, Thailand and Vietnam, with high concentrations of methylchavicol.¹ So in this trial, a seed packet labeled “Thai” didn’t guarantee the methylchavicol-rich profile.

Two other studies point the same way, on a different scale. A 2018 Chiang Mai University study found Thai basil oil from plants grown in northern Thailand led by estragole, followed by geranial and neral, and linked it to anise, lime-like and fresh aromas.⁷ On the Genovese side, Italian researchers found methyleugenol and eugenol as the main components of young ‘Genovese Gigante’ plants.⁸ Those studies used different plants, ages and units, so we don’t set their numbers against the Purdue table.

How to tell them apart

Across O. basilicum, PROSEA records stems that are smooth or hairy when young and light green to dark purple, so color alone won’t tell you which basil you’ve got. PROSEA’s mark of the Thai type, var. thyrsiflorum, is the flower head: densely branched, with loose whorls of flowers.⁹ In the Purdue trial, most Thai lots had purple stems and pink flowers, against Genovese’s green stem and white flower, and the Thai plants were shorter (35 to 41 cm against 49 cm).¹

Andy Ricker warns that the label “holy basil” sometimes ends up on purple-stemmed Thai or sweet basil.¹⁰ Holy basil is a separate species; see Thai basil vs. holy basil. The surest test is to rub a leaf: Thai basil smells plainly of anise.

Differences that change the dish

When it goes in. LSU AgCenter groups basil with the delicate herbs best added a minute or two before cooking ends, because long heat costs flavor and aroma,¹¹ and the Herb Society says to add basil in the last few minutes.⁴ That suits Genovese. Sherri Pengjad of Thai Foodie says Thai basil is sturdier and holds up much better in hot dishes, and she stirs it into green curry right before serving.¹² So both go in late. We found no test of how long either one survives a simmer.

Pesto. Genovese is the pesto basil; the Herb Society calls it “an ideal pesto basil.”⁴ Going by the chemistry, a methylchavicol-rich Thai basil would put anise front and center in a pesto, though we didn’t find anyone who’s run a tasting.

Raw on the plate. For Vietnamese cooking, Andrea Nguyen lists Thai basil on her pho garnish plate, adding it when it’s in season and telling readers to add or subtract herbs as they like.¹³ Genovese leaves are just as edible raw, but they taste Italian.

Italian and Thai classics. Pizzas, pasta, chicken and cheese dishes, and pesto are classic Italian uses of sweet basil, and sweet basil, under the Thai name horapha, is an important ingredient in Thai cuisine.

Can one stand in for the other?

The amounts below match our Thai basil substitutes page.

• Genovese for Thai basil: same amount (Attributed). Nagi Maehashi of RecipeTin Eats calls normal basil the best substitute for Thai basil, while noting it lacks the slight aniseed flavor.¹⁴ Pengjad also names Italian basil as the best substitute.¹² Pailin Chongchitnant’s pad kra pao recipe lists holy basil or regular basil in the same 1 1/4 cups and says Thai basil will also work, so she’s treating them as interchangeable by volume.¹⁵ Add it at the very end.
• To get closer to Thai basil. Pengjad suggests mixing Italian basil with a little mint.¹² Start with about four parts basil to one part mint, in the same total amount, and taste.
• Thai basil for Genovese. In a cooked tomato sauce it works if you’re fine with a licorice note; start with a little less than the recipe’s basil and taste. For pesto, Caprese salad and other raw Italian uses, the anise takes over, so we wouldn’t swap. If you have to use Thai basil raw, pick the leaves off the stems and slice them fine.

What isn’t known

• The like-for-like comparison is one 1999 field trial with one seed lot per cultivar.¹ No newer study grows Genovese and Thai basil side by side.
• None of the sources we opened describes Thai basil’s leaf shape or edge next to Genovese’s.
• The idea that Thai basil handles long cooking better than Genovese comes from cooks’ experience. We found no test.

Safety

The anise compound that sets Thai basil apart, estragole, is the one regulators have looked at hardest in basil. The EU Scientific Committee on Food concluded in 2001 that estragole is genotoxic and carcinogenic, based mainly on studies in mice, and couldn’t set a safe exposure limit.² Germany’s BfR said in 2022 that the risk from estragole and the related methyleugenol in food can’t yet be conclusively assessed; it named basil-containing pesto among foods that may contain high amounts and made no consumption recommendation for consumers.⁶ Neither body tells people to stop eating basil or singles out either type.

Related

• Genovese basil and Thai basil profiles
• Basil vs. Thai basil and Genovese basil vs. sweet basil
• Thai basil substitutes and Thai basil vs. holy basil
• Cinnamon basil vs. Thai basil, holy basil vs. basil
• Thai spices, how to store basil