Why Do Basil Leaves Turn Dark?

Basil leaves darken because an enzyme released from cut or bruised cells reacts with oxygen in the air. It's a similar process to what makes a cut apple turn brown; compounds inside the leaf react with oxygen and form dark, sometimes nearly black, spots. Basil is especially prone to this compared to many other herbs, since its leaves are thin and delicate; even the slightest bruise or cut exposes the cell walls quickly. That's why the edges of chopped or knife-cut basil start darkening within a short time. How do temperature and moisture affect darkening? Basil also darkens faster when stored cold in the fridge; the plant is tropical in origin and sensitive to low temperatures. Cold damages the leaf cells and speeds up darkening, which is why basil kept in water at room temperature usually stays green much longer. Excess moisture left on the leaves also speeds up darkening; wet leaves are more prone to both decay and oxidation. Are there ways to slow down the darkening? Standing basil in a glass of water, stems submerged, at room temperature away from direct sunlight keeps the leaves noticeably fresher for longer. The water should be changed every couple of days. Using a sharp knife when chopping basil, instead of a dull one that crushes as it cuts, reduces cell damage and helps the cut edges darken more slowly. Does freezing protect basil? Freezing basil leaves directly makes them darken and turn mushy the moment they thaw, so freezing isn't recommended for leaves you plan to use fresh. But basil blended with olive oil and frozen in ice cube trays holds both its color and aroma far better. With this method, basil leaves are blended with a bit of olive oil and poured into cube trays; the frozen cubes can then be dropped straight into a sauce, soup, or sauté. The oil cuts off the leaves' contact with oxygen and largely prevents darkening. Dried basil, by contrast, keeps far longer than fresh, but its flavor is noticeably different from fresh basil; some of the volatile aroma compounds are lost during drying, so dried basil isn't a direct one-to-one substitute for fresh. Soft-leaved herbs like mint, cilantro, and parsley go through a similar oxidation process, but they don't darken as fast or as noticeably as basil; the difference comes down to each plant having its own enzyme and compound levels. Sturdier herbs like thyme or rosemary barely show this reaction at all. Cutting soft herbs with a sharp knife and as few strokes as possible before chopping is a shared rule that slows darkening in a similar way across all of these herbs. Adding basil at the very last moment of cooking also prevents darkening; basil that simmers in a dish for a long time loses both its color and its aroma quickly. Washing basil right before chopping it, rather than ahead of time, also keeps the leaves from meeting moisture too early and darkening.