Many believe that the iron in the apple is the reason it turns brown, as it oxidizes when exposed to air, just like what happens to a nail, for example.
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However, apples contain a small amount of iron, about 0.9 mg per 100 grams, which is less than beans or nuts. The brown color is caused by a completely different process. So what happens in the first few minutes after cutting an apple?
According to experts, when an apple is whole, its cells function as small, enclosed chambers. Inside each cell is a vacuole, a bubble of cell sap that stores polyphenols. These substances contain phenolic rings in their molecules and give the apple its tart flavor and help protect it from pests.
In another part of the cell, inside the plastids, there is an enzyme called "polyphenol oxidase," and this enzyme does not react with the polyphenols in the whole apple.
But when the knife cuts through the tissue and tears the cell walls, the vacuoles burst, releasing polyphenols. The plastids are also damaged, and atmospheric oxygen then penetrates the damaged cells, where the three components—enzyme, polyphenols, and oxygen—come together. The reaction begins almost immediately.
The enzyme polyphenol oxidase contains two copper ions at its active site. Copper acts as a carrier, helping the enzyme remove electrons from polyphenol molecules and transfer them to oxygen. During the first stage of the reaction, quinones are produced—unstable compounds with high chemical reactivity. Therefore, they react almost immediately with each other and with amino acids, forming long chains. These chains are the dark pigment we see on a cut apple.
Of course, the rate of the reaction depends on the temperature and acidity of the surrounding environment. If the apple is cold, the process slows down, and the reaction rate decreases if the apple pulp is acidic.
The browning of apple flesh is a defense mechanism, as quinones are toxic to many microorganisms and insects. When the fruit is damaged, the tree cannot heal the wound immediately, but it forms a chemical barrier of active compounds at the site of the damage.
These pigments are harmless to humans, meaning that an apple that has turned brown has not spoiled, but only changed color, and retains its fiber and most of its nutrients.
The easiest way to prevent cut apples from turning brown is to spray them with lemon juice to lower the pH to below 3. The ascorbic acid in lemons also binds to oxygen. Cold water prevents air from reaching the apple's flesh, so temporarily soaking it in water has a similar, though weaker, effect to that of the acid.
In addition, apples can be blanched, meaning the slices can be heated in boiling water for a few minutes. After this treatment, the apples will not darken, even without using acid. For salads and grilled dishes, this method is more effective than sprinkling the apples with juice.
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