Why Mint Leaves Turn Black in Water|How To Store Properly

Have you ever noticed that adding mint leaves to water can sometimes cause them to turn black? This can be a frustrating problem, but have you ever wondered why it happens and how to prevent it?

Mint leaves turn black in water due to the process of oxidation in which oxygen reacts with enzymes causing leaves to darken. Also, the combination of water and cold temperatures hastens the loss of chlorophyll, but bacterial growth could also be to blame as it causes the plant tissues to break down.

In the following, we’ll explore why mint leaves turn black in water and provide some tips on storing them properly so your mint will taste and look fresh for more than a few hours.

So read on to learn more about why mint leaves turn black in water and how to store them correctly!

Why Mint Leaves Turn Black in Water

Going to the trouble of picking fresh mint, washing it carefully, and storing it in what you assumed was the correct way only to find that pretty green has been replaced by an ugly black can be quite frustrating!


Let’s explore the reasons why this happens so you can prevent it in the future.

1. Oxidation

Oxidation is a complex chemical process where oxygen atoms take electrons from other molecules, altering their structure and function in the process.

It’s at work any time you slice an apple and the pieces turn brown, when metal rusts away, and when fresh herbs turn black. Simply put, oxidation is nature‚Äôs way of breaking things down!

2. Loss of Chlorophyll

Chlorophyll is the green pigment found in plants. It plays a crucial role in the process of photosynthesis, which is how plants produce energy from sunlight.

When fresh herbs are harvested, they continue to respire, meaning they take in oxygen and release carbon dioxide.

As the herbs respire, their cells start to break down, and the chlorophyll begins to degrade.

As chlorophyll degrades, the green color of the herbs starts to fade, and the herbs may start to turn yellow or brown.

Eventually, the chlorophyll breaks down completely, and the herbs turn black.

3. Bacterial Growth

When herbs are harvested, they are no longer connected to the plant’s root system, and their natural defense mechanisms are weakened.

This makes them more susceptible to bacterial growth, which can occur when the herbs are exposed to moisture.

When bacteria start to grow, they consume the nutrients in the leaves and release waste products. These waste products can be toxic to the herb’s cells, causing them to break down and turn black.

In addition, the bacteria can produce enzymes that break down the herb’s cell walls, which can lead to further degradation of the leaves.

Bacterial growth can be particularly problematic when herbs are stored in water as this provides a moist environment that is ideal for bacterial growth.

How To Prevent

There are several ways to prevent mint leaves from turning black when stored in water and keep your mint fresh for longer:

  • Use distilled or filtered water: Tap water can contain minerals and other impurities that can promote bacterial growth and oxidation. Using distilled or filtered water can help prevent these issues.
  • Change the water frequently: Bacteria can quickly multiply in stagnant water. Changing the water in your mint container every day can help prevent bacterial growth and keep the leaves fresh.
  • Keep the mint leaves cold: Mint leaves will wilt and degrade if exposed to warm temperatures after harvesting. Keeping your mint container in the refrigerator can help slow this process and preserve freshness.
  • Add some acid to the water: Adding a small amount of lemon juice or vinegar to the water can help prevent oxidation and preserve the green color of the mint leaves.
  • Don’t overcrowd the container: Overcrowding the container with too many mint leaves can make it difficult for air to circulate and promote bacterial growth.
A fresh mint sprig resting on a counter backlit by the sun.

Can You Use Black Mint Leaves?

Blackened mint leaves may or may not be safe to consume depending on the cause. If bacterial growth is to blame, it is best not to use them.

If oxidation or chlorophyll breakdown is the reason, the leaves may still be used, but nutrient values and taste may be negatively affected.

It’s best to err on the side of caution, and throw away blackened mint leaves or compost them.

Mint Leaves Turning Black in Fridge

Mint leaves can turn black in the fridge for the same reasons they turn black in water: oxidation, loss of chlorophyll, and/or bacterial growth.

To avoid this problem, do not submerge the mint in water. Rather, provide just enough moisture to keep them hydrated (wrap them in a damp paper towel before placing them in a container), and do not seal the container.

How To Store Fresh Mint

Wrap fresh mint leaves or bunches of mint loosely in a slightly damp paper towel and keep it in a container or plastic bag in the refrigerator.

You could also keep your mint bunches in the fridge standing in a jar with a small amount of water (about 1 inch) at the bottom to prevent wilting. Place an inverted plastic baggie over the top, and it should last for up to five days.

How Long Does Fresh Mint Last?

The shelf life of fresh mint can vary depending on a few factors, including how fresh it was at the time of storage. Here are some guidelines on how long fresh mint can last:

  1. Whole mint leaves: If stored properly, whole mint leaves can last up to two weeks in the refrigerator. Keep them away from other fruits and vegetables that release ethylene gas, which can cause the mint leaves to deteriorate more quickly.
  2. Chopped mint: Chopped mint can last up to one week in the refrigerator when stored properly. To extend its life, you can also store chopped mint in the freezer for up to six months.
  3. Mint sprigs: If you have mint sprigs with the stem still attached, you can store them in the fridge in a glass with a small of water and a plastic bag loosely covering the leaves. This method can keep the mint sprigs fresh for up to one week.

Related Questions:

Can Mint Be Frozen?

Mint can be frozen easily for future use. Simply chop the leaves, and distribute them evenly in an ice cube tray. Fill each compartment with water before carefully placing the tray in the freezer.

Can You Use Dried Mint Instead of Fresh?

You can substitute dried mint for fresh mint in many recipes. It’s best to crumble the leaves first and add them sooner than you would when using fresh to allow them to rehydrate.

Use only one-fourth of the amount called for in the recipe.

Final Thoughts

Now you know how to avoid ugly blackened mint leaves and some tips for keeping your fresh mint looking and tasting its best for as long as possible.

The only question left to consider is how many delicious concoctions can you come up with to highlight mint’s refreshing flavor!