Apples, as well as other fruits and vegetables such as potatoes, start to turn brown shortly after they are cut. Polyphenol oxidase compounds, which are enzymes found in the apple's flesh, react when exposed to air, causing the browning process. A variety of factors can impede or speed up this process.
Contact with metal
The experts at the University of California Science & Health Education Partnership say that contact with iron and copper speeds up the browning process. Cutting apples with a rusted knife or storing them in a copper bowl will cause the chemical reaction causing the brown colouration to occur faster. Clean steel knives and plastic or glass bowls will not have the same affect.
Cutting an apple exposes the polyphenol oxidase to oxygen. The enzyme reacts to oxygen, so preventing oxygen from reaching the apple will prevent browning. You can accomplish this by placing the apple slices in a bowl of water. Covering them tightly may slow the oxidation process, but the air inside the bowl will still contribute to some amount of browning.
Different apples contain different amounts of the crucial enzyme. Growing conditions, breeding strains, and natural acidity levels in the fruit all affect the process. Some apples naturally turn brown faster, while others may not turn brown at all.
Heat stops the oxidation process that turns cut apples brown, but it also affects the texture of the fruit. The experts at Scientific American explain that cooking apples in hot water for a few minutes will destroy the enzymes responsible for this process. Cooking apples softens them, and they may be unsuitable for recipes calling for freshly cut fruit.
Levels of pH
The pH level on the surface of cut fruit and vegetables also affects the browning process. Acids like lemon juice are commonly recommended as a coating to keep apples white. Covering the fruit in an acid prevents the chemical reaction from taking place.
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