Why Do Insides of Apples Turn Brown After Slicing?

By Dr. Mercola

Apples are rich in phenolic compounds called polyphenols, which play a role in the color, flavor and nutritional quality of the fruit. They’re also the reason why apples turn brown soon after you cut them.

Phenolic compounds are located in the vacuoles of plant cells, while an enzyme known as polyphenol oxidase (PPO) is located in cell structures called plastids. When you cut the apple, the damage from the cut allows contact between PPO and phenolic compounds, which triggers a reaction known as enzymatic browning.¹

Cutting an apple exposes the cells to oxygen, which allows the PPO enzymes to rapidly oxidize the phenolic compounds in the apple tissues into ortho-quinones (o-quinones). O-quinones form a natural antiseptic that helps protect the apple from bacteria and fungi.

While o-quinones have no color, they react with oxygen and amino acids to produce melanin, which turns the apple brown. Put another way by a study published in the journal HortScience:²

“The enzymatic browning is a consequence of the oxidation of polyphenols to their corresponding quinones by PPO. These quinones are then polymerized with other quinones or phenolics, originating brown pigments.”

Many Factors Affect How Fast an Apple Browns

An apple’s PPO level and phenolic compound concentration, and thus its propensity for browning, varies between varieties (such as Fuji and Granny Smith) and is also influenced by growing conditions.

Apples with higher levels of phenolic compounds are best for your health, but also tend to brown faster than apples with lower levels. One study suggested that among the apple varieties studied, Fuji is the best for fresh consumption because of its higher phenolic content at harvest time.

However, they suggested a variety known as “Aori27” is best for processing, as it had the lowest PPO activity and the lowest polyphenol content, and therefore the lowest potential for enzymatic browning.³

You’re certainly better off choosing polyphenol-rich apples for your health and using simple methods to stave off browning (if you’re not planning to eat the whole apple at once, that is). Among them:

• Put cut apples in the refrigerator. This will slow down the chemical reactions and oxidation process that leads to browning.
• Spray exposed areas of cut apples with pineapple juice or lemon juice, which will slow enzymatic browning.
• Blanch apples in boiling water for four to five minutes (this should only be used for apples you plan on cooking, as it will affect the texture).

FDA Approved Genetically Engineered (GE) Arctic Apples in 2015

In 2015, the U.S. Food and Drug Administration (FDA) approved so-called Arctic Apples, which are genetically engineered to resist browning when sliced or bruised. Developed by Okanagan Specialty Fruits, the apples are engineered to suppress the production of the enzyme that causes browning.

Although the GE apples have been approved for commercial planting, it will be a few years before they’re widely available because the trees first have to be planted and then mature enough to grow fruit.

They will eventually be available in two varieties — Granny Smith and Golden Delicious. To avoid these “Frankenfruits,” choose organic apples. As Wenonah Hauter, executive director of Food & Water Watch, said in a statement:⁴

“This GMO apple is simply unnecessary … Apple browning is a small cosmetic issue that consumers and the industry have dealt with successfully for generations.”

Apples Are Antioxidant Powerhouses

Berries, pomegranates and other flashier fruits often make headlines for their antioxidant levels, but apples are also a phenomenal source.

Compared to other commonly consumed fruits in the U.S., apples ranked second for highest antioxidant activity. However, they ranked highest for the proportion of free phenolic compounds, which means they are not bound to other compounds in the fruit and therefore may be more easily absorbed into your bloodstream.⁵

Notably, much of apples’ antioxidant power is contained in the peel, where you’ll find antioxidants like catechin, procyanidins, chlorogenic acid, ploridizin and more. According to the New York Fruit Quarterly:⁶

“Since apples are so high in antioxidants, it is no surprise that apples, specifically, are associated with a decreased risk of chronic disease.

Three studies have specifically linked apple consumption with a decreased risk for cancer … [and] a study has shown that apple and pear consumption has been associated with a decreased risk of asthma.

Apple consumption has also been associated with a decreased risk of coronary heart disease … [and] a reduced risk of Type II diabetes was associated with apple and berry consumption in another major Finnish study.

Five Major Health Benefits of Apples

It’s said that an apple a day keeps the doctor away, and here are some reasons why:

1. Brain Health: Apples have been found to protect neuron cells against oxidative stress-induced neurotoxicity and may play an important role in reducing the risk of neurodegenerative disorders such as Alzheimer's disease.⁷

2. Stroke: Eating apples is linked to a decreased risk of stroke.⁸

3. Diabetes: Three servings of apples (and other fruits such as blueberries and grapes) are linked to a 7 percent lower risk of type 2 diabetes.⁹ This may be due to their beneficial role in blood sugar regulation, as apples contain compounds that may:¹⁰

• Lessen absorption of glucose from your digestive tract
• Stimulate beta cells in your pancreas to secrete insulin
• Increase uptake of glucose from your blood by stimulating insulin receptors

4. Cancer: Apples have a number of properties that may help reduce the risk of cancer, including antimutagenic activity, antioxidant activity, anti-inflammatory mechanisms, antiproliferative and apoptosis-inducing activity, as well as “novel mechanisms on epigenetic events and innate immunity. According to the journal Planta Medica:¹¹

“Apple products have been shown to prevent skin, mammary and colon carcinogenesis in animal models. Epidemiological observations indicate that regular consumption of one or more apples a day may reduce the risk for lung and colon cancer.”

5. Heart Disease: Eating apples is associated with a lower risk of death from heart disease, an association that may be related to their content of antioxidant flavonoids.¹²

Apples Are Heavily Contaminated With Pesticides: Choose Organic

Apple earned the second highest spot on the Environmental Working Group’s 2016 “Dirty Dozen” list for most pesticide-contaminated fruits and vegetables (only strawberries were more contaminated).¹³

Further, much of the antioxidant content of an apple is found in its skin, so you’ll want to leave the peel on when you eat it. For this reason, look for organic apples whenever possible to minimize your exposure to pesticides.

If you purchase conventional apples, briefly soaking them in a solution of 10 percent vinegar to 90 percent water may help to remove some pesticides (and bacteria). When it comes to choosing an apple, the Washington State Apple Commission recommends:¹⁴

• Choosing an apple with shiny, not dull, skin (dull apples will not be crisp)
• Firm apples free from bruises and punctures
• Refrigerating apples at 32 degrees F to maintain crispness
• Protecting cut apples from browning by dipping them into a solution of one part citrus juice to three parts water

If you’re looking for even more apple facts and nutrition information, be sure to check out our apple food facts page.

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