Fruits like apples can rot and decay over time. Mold and fungus cause the quickest degradation, but even exposing an apple to heat can cause it to oxidize. Bruises and punctures offer the quickest route for mold and fungus to take hold. Different pathogens have different effects on rotting apples.
Oxidation
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Apple flesh has chemical compounds known as phenolics that react with oxygen when exposed to air. Once the skin of the apple is broken, the compounds are exposed to oxygen and enzymes in the apple known as polyphenol oxidases cause the browning reaction. One way to keep sliced apples from browning is to store them in acidic water--add one part lemon juice to five parts water to prevent apples from browning. Refrigeration also slows the browning process.
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Apple Rot
Fungi can take hold of apples, particularly through a puncture or other wound that breaks the skin of the apple. The most common causes of apple rot are from the fungi Penicillium expansum and Monilinia fructigena. These fungi feed on and kill the cells that make up the apple. The fungi produce pectic enzymes that break down apple pectin to expose the nutrients of the cells to the fungi. Different fungi and molds produce different patterns of rot damage.
Humidity, Light and Heat
Environmental factors can affect how quickly an apple will rot. Storing apples in a cool place will slow the onset of rot but will not decrease the extent of damage over long periods of time. Moderate humidity does not accelerate rot. While apples require light to ripen, the fungi and molds that cause rotting do not require light. Light that is not accompanied with heat should not accelerate apple rotting.