How Can You Keep Chopped Fresh Herbs From Darkening Quickly?

Fresh herbs darken quickly mainly because enzymes released from cut cells oxidize when exposed to air. The most effective way to slow this down is to make clean cuts with a sharp knife and expose the chopped herbs to air for as little time as possible.

Why do herbs darken so quickly? When plant cells are cut, enzymes and phenolic compounds that are normally in separate compartments come together. When exposed to oxygen, this mixture starts a darkening reaction, the same kind that makes a cut apple or avocado turn brown. Delicate leafy herbs such as parsley, dill, and mint have more fragile cell walls, so this reaction happens much faster than in sturdier herbs such as thyme. Chopping with a dull knife also speeds up the process because a dull blade crushes cells instead of cutting them cleanly. Crushed cells release more enzymes and liquid, increasing the surface area exposed to oxidation.

How can you prevent darkening? Keep the knife sharp so it cuts the herbs cleanly in one motion instead of crushing them. This releases fewer enzymes and noticeably delays darkening. It is also important to wash the herbs just before chopping and dry them thoroughly. Moisture left on the leaves can make chopping more difficult and may contribute to faster darkening of the cut surfaces. Using chopped herbs right away is the most reliable precaution. Chop them as close as possible to when you plan to use them, rather than chopping them in advance and leaving them exposed to air, to minimize the risk of darkening.

How should you store chopped herbs if you are not using them right away? Cover them with a damp paper towel and keep them in a tightly sealed container in the refrigerator. This reduces air exposure and slows darkening. You can also mix the herbs with a little olive oil before storing them; this similarly isolates the surface from air. For longer storage, freeze chopped herbs in ice cube trays covered with a little water or olive oil. This can significantly delay both darkening and loss of freshness.

Do all herbs darken at the same rate? Delicate, water-rich herbs such as mint and basil may darken faster than parsley and dill because their cell walls are more fragile and are crushed more easily during cutting. Darkening progresses much more slowly in woody, sturdy herbs such as thyme and rosemary, so these herbs can sit chopped for a little longer. Cutting by pulling the knife in one direction rather than moving it back and forth also reduces cell crushing. Instead of repeatedly cutting through the same spot, move the knife to a new area each time; this can delay darkening somewhat and help the leaves keep a fresh appearance. Some cooks also believe that using a wooden cutting board instead of a plastic one helps crush the leaves less because the wooden surface reduces blade slippage.