Food Science: Why Blanched Vegetables (Sometimes) Turn Brown (2024)

Emma Christensen

Emma Christensen

Emma is a former editor for The Kitchn and a graduate of the Cambridge School for Culinary Arts. She is the author of True Brews and Brew Better Beer. Check out her website for more cooking stories

published Jun 17, 2008

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Food Science: Why Blanched Vegetables (Sometimes) Turn Brown (1)

A few weeks ago, we talked about blanching as a great way to prepare vegetables for salads and crudite platters.

Cooking them quickly in boiling water brings out both their flavor and their bright colors.

That is to say, except for when they turn brown! Why does this happen? Read on!

The most likely reason our veggies turned brown is because we covered the pot with a lid after adding the vegetables.

Yes, covering the pot certainly seems like the logical thing to do! After all, a covered pot conserves energy, brings the water back up to a boil, and keeps the steam inside the pot–all things that would theoretically help our veggies cook more quickly.

Here’s what is really going on in that pot:

As the veggies hit the boiling water, volatile acids are released into the water and are carried away in the steam.

When the pot is covered, the steam and the acids it contains are forced back into the water. Once there, the acids react with the chlorophyll in the vegetables, turning them an unsightly shade of brown.

A similar reaction will take place if there’s too little water in the pot (thus concentrating the acids in the water) or if you overcook the vegetables (thus prolonging the exposure time to the acids).

Solutions? Use a large amount of boiling water for blanching, test the vegetables frequently to check their doneness, and leave the pot uncovered!

Got a kitchen question tickling your noggin? Ask away!

Related: Why Sliced Fruit Turns Brown

(Image: Flickr member darwinbell, modified from original by Emma Christensen under Creative Commons license)

Food Science: Why Blanched Vegetables (Sometimes) Turn Brown (2024)


Food Science: Why Blanched Vegetables (Sometimes) Turn Brown? ›

Enzymic browning is an oxidation reaction that takes place in some foods, mostly fruit and vegetables, causing the food to turn brown. Oxidation reactions occur in food and non-food items.

Why do vegetables turn brown? ›

Once the fruit or vegetable is cut, some of the cells are opened up to the air. The enzyme polyphenol oxidase then has access to oxygen in the air and a reaction occurs, which results in the fruit or vegetable turning brown.

Why does vegetable soup turn brown? ›

Many green ingredients contain enzymes that can turn them brown, either by degrading the structure of chlorophyll or by catalysing the oxidation of phenolic compounds.

Why do cut vegetables take up a brown color when exposed to air? ›

The enzyme polyphenol oxidase (also known as tyrosinase) located in the cells gets exposed to and reacts with oxygen in the air when fresh fruits and vegetables are peeled or cut open. The browning of fruits and vegetables is caused by a chemical reaction known as oxidation.

Why do some fruits and vegetables after cutting turn brown? ›

When fresh fruits and vegetables are peeled or cut open, the enzyme called polyphenol oxidase (also called tyrosinase) contained in the cells is exposed to and reacts with the oxygen in the air. The reaction that occurs, which is called oxidation, is what turns the fruits and vegetables brown.

How does blanching prevent enzymatic browning? ›

Physical methods to regulate enzymatic browning include thermal treatment, prevention of oxygen exposure, use of low temperature, and irradiation. Heat treatment, such as blanching, can easily inhibit the enzymatic activity because enzymes, which are composed of proteins, are denatured [7,8].

What vegetables turn brown? ›

Apart from potatoes and apples, other fruits and vegetables turn brown when cut, too, that should be treated in an almost similar manner. These also include eggplants, bananas, mushrooms, lettuce, pear, cauliflower, and especially avocados. Don't sit and watch these produce turn brown!

What causes broccoli to turn brown? ›

A brown discoloration of the internal hollow stem tissue may be associated with a boron deficiency. Some recently developed varieties are resistant to hollow stem.

How do you keep vegetables from turning brown? ›

Bottled lemon juice can be made into a holding solution to prevent enzymatic browning by mixing ½ cup lemon juice with 2 quarts of water. Note: Citric acid and lemon juice are not as effective in preventing browning as ascorbic acid solutions.

Why do beans turn brown when cooked? ›

The beans turn brown because of something called the Maillard reaction. This is a fancy scientific term that basically means: heat causes the sugars and amino acids in the beans to react and create new flavors and aromas.

What is the largest fruit crop in the world? ›

Global fruit production in 2022, by selected variety (in million metric tons)*
CharacteristicProduction in million metric tons
9 more rows
Feb 13, 2024

Why do vegetables turn black? ›

It is an oxidation reaction that takes place due to exposure in oxygen (present in air). Visit the link to know oxygen is required for oxidation. Peeled or sliced potatoes, brinjals, bananas, apples get brown or black upon exposure to air.

What process turns some produce brown when cut? ›

This unappetizing phenomenon is actually due to a chain of biochemical reactions known as “enzymatic browning.” When an apple is injured (or cut into pieces), the plant tissue is exposed to oxygen.

What factors affect enzymatic browning? ›

The main factors that determine the rate of enzymatic browning of fruit and vegetables include the concentrations of active PPO and phenolic compounds present, the temperature, the pH, and the oxygen availability of the tissue. Antibrowning agents may be used to delay the enzymatic browning in fruit and vegetables.

Why does enzymatic browning happen? ›

When fruit tissue is damaged because of heat, cold, age or mechanical stress, its cells break open and the phenolic compounds and the enzyme are released and mix with oxygen in the air. As a result the damaged tissue turns brown almost immediately.

Why does food turn brown? ›

Basically, it is a chemical reaction between amino acids and simple sugars in food which then leads to the formation of brown colour. During the process, simultaneous chemical reactions occur when amino acids and simple sugars in the food are transformed by the heat applied.

Is it OK to eat brown vegetables? ›

But just a browning of a fruit and vegetable isn't necessarily a sign that it has gotten bad, but it's a sign that we should eat it right away or prepare it right away and eat it as soon as possible.

How to prevent veggies from browning? ›

Squeeze some lemon juice onto the cut surfaces of the fruits or vegetables before storing them. The acid in the lemon juice helps slow down the browning process.

Is it OK to eat oxidized vegetables? ›

Think of an apple, even though it may start to brown right away, it takes a long time for the entire apple slice to change color. Even then, you cut off the brown and the inside remains unaffected. Fruits and veggies are also so rich in nutrients that the body will still profit if a loss occurs. So drink up!

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