When making Pasta Salad with Pesto, storing leftover basil won't be an issue—the large bunches available at the supermarket will be easily used up. But what about those occasions when only two or three leaves are needed to season a dish? We wondered how long we could keep leftover store-bought basil and what would be the best way to store it.
Since leaving basil out on the counter wasn't an option (it wilted within hours) we were stuck with refrigerator storage, which is about 15 degrees colder than the recommended temperature for basil. We tested storing basil in unsealed zipper-lock bags (to prevent build-up of moisture, which can cause basil to turn black), both plain and wrapped in damp paper towels (our preferred method for most leafy greens). After three days in the refrigerator, both samples were still green and perky. But after one week, only the towel-wrapped basil was still fresh looking and tasting. Don't be tempted to rinse basil until just before you need to use it; when we performed the same tests after rinsing, the shelf life was decreased by half.
Source: Published July 1, 2006, in Cook's Illustrated