Chopped fresh herbs darken quickly mainly because enzymes released from the cut cells oxidize on contact with air; the most effective way to slow that down is chopping with a sharp knife for a clean cut and keeping the chopped herbs exposed to air for as little time as possible.
Why do herbs darken so fast?
When plant cells are cut, enzymes and phenolic compounds that normally sit in separate compartments come into contact, and that mixture triggers a browning reaction on exposure to oxygen — the same reaction behind a cut apple or avocado turning brown. Delicate leafy herbs like parsley, dill or mint, with their more fragile cell walls, go through this reaction much faster than sturdier herbs like thyme.
Chopping with a dull knife speeds this up too, since a dull blade crushes cells instead of cutting them cleanly; crushed cells release more enzymes and liquid, which widens the surface exposed to oxidation.
What are the best ways to slow the browning down?
Keeping the knife sharp lets it slice through herbs in one clean motion instead of crushing them, which noticeably cuts down on the enzymes released and slows browning. Washing herbs right before chopping and drying them thoroughly matters too, since leftover moisture on the leaves both makes chopping harder and can add to how fast the cut surfaces darken.
Using chopped herbs right away is the most reliable fix; chopping just before they are needed, rather than well ahead of time and leaving them exposed to air, keeps the browning risk to a minimum.
How should already-chopped herbs be stored?
If chopped herbs will not be used right away, covering them with a damp paper towel and keeping them in a tightly sealed container in the fridge cuts down on air exposure and slows the darkening. Mixing the herbs with a little olive oil before storing also isolates the surface from air in a similar way.
For longer storage, freezing chopped herbs in ice cube trays topped with a little water or olive oil is a method that noticeably delays both browning and the loss of freshness.
Do all herbs brown at the same speed?
Delicate, water-rich leaves like mint and basil can darken faster than parsley or dill, since their cell walls are even more fragile and crush more easily when cut. Woody, sturdier herbs like thyme or rosemary go through this browning much more slowly, so those can sit chopped for a bit longer without much visible change.
Pulling the knife through in one direction rather than sawing back and forth is a small technique detail that reduces cell crushing too; running the blade over the same spot repeatedly instead of moving to a fresh section each time slows browning down a little further.