How Does an Apple Decay?
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Apples Are Fruits
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Apples are an intermediate stage of the apple tree's reproduction processes called fruit. They appear between flowering (inflorescence) and the germination of new seed. Apples are the seed-pods of the fertilized apple blossom. They grow until they are mature (ripe) and die. To help the fruit ripen, apples produce a hormone called ethylene oxide. As long as the apple grows, this hormone is beneficial but when the apple is mature and the seed fully developed, the hormone helps break down the apple's tissue. In short, the apple's prime purpose is to protect the ripening seed and to rot away when the seed is mature. That humans find the fruit tasty and nutritious is incidental.
Chemicals and Proteins
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Ethylene oxide helps ripen fruits by softening its flesh and turning its skin rosy and soft. Store a bunch of green apples, tomatoes or any other fruit in a paper sack and they will ripen naturally because of the emission, or off-gassing, of ethylene oxide. While the ethylene oxide is doing its job, the fruit is also producing carbohydrates (sugars) to nourish the growing seeds and enzymes (amylases and proteases) to begin the process of softening of the tissue and seed casing around the seeds. Once the seed is mature, the carbohydrates and proteins produced by the fruit as it ripens are no longer consumed and build up in the flesh. When the balance between maturation and decline is tipped, the apple's tissues begin to break down. They begin to soften and are easily damaged and bruised.
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Bacteria and Fungi
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Bacteri, live on the surface of apples but cannot penetrate the surface until the skin is broken. Once the skin begins to soften, though, bacteria are able to enter the apple and feed on the carbohydrates in its flesh. Sugar-rich tissues turn brown as they are "oxidized" by contact with the air. Air or insect-borne mold and fungus spores attach themselves to porous surfaces like stems, blossom ends and exposed flesh. These microscopic parasites and predators feed on the soft, sweet flesh of the ripened apple, turning it to mushy waste as they consume the carbohydrates and proteins no longer required by the mature seeds. When they reach the seeds, the hard shell halts their progress and they abandon the exposed seeds to grow where they lie or be carried elsewhere in the digestive systems of birds or on the backs of insects.
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References
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