One of the most important aspects about choosing the best Thai basil is to be absolutely certain that the herb actually is Thai basil and not a variety that has a stronger licorice flavor. Thai basil can be easily identified by its purple stems and, sometimes, purple buds on the top of the stem. When choosing the basil from a grocery store, it should look healthy, with leaves that are vibrant and thick, instead of slimy, wilted or full of holes. The basil also should have a fairly robust scent, and any bunches that have no aroma at all should be avoided, because they might not be basil or might have been sitting on the shelf for too long. The best basil, along with being fresh and vibrant, should not have a large number of large buds on it, because this could indicate that the plant has gone to seed and changed the flavor of the leaves.
When choosing fresh Thai basil, the first step is to ensure that the herb actually is Thai basil. There are a number of types of Asian basils and sweet basils that sometimes are mistakenly advertised as the Thai variety. True Thai basil will have a reddish-purple stem, with the color sometimes creeping up to the leaves, turning them partially purple or reddish. Some varieties that are not the Thai form will have green stems or might have a thick, brown, woody stem that looks more like the branch of a tree.
When examining Thai basil, there should not be any large groupings of buds, flowers or blossoms. This would indicate that the plant has gone to seed, which can happen even after it has been harvested. After the seeds begin to form, the taste of the leaves changes and becomes increasingly bitter.
The appearance of Thai basil is important to ensure that it is fresh, tastes good and will keep once purchased. The leaves of the plant should be full, free from large black spots and supple to the touch. Leaves that have a slick texture on their surface or are wilted are not in good shape and, while they can be eaten, are not going to be the best.
If possible, pressing the leaves of the basil firmly between the fingers and then rubbing gently should cause the plant to release oils. The oils create a strong aroma on the fingers that should have a distinct sweet licorice smell. If no odor comes from the leaves, then the basil might be old or overly dry, meaning it might not have a strong taste when used in a recipe.