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).