|MadSci Network: Agricultural Sciences|
What you have observed with your apple and banana can be explained by physical and/or chemical principles. First, it is possible the during transporting of your lunch you intiate bruising of the apple by physical means. It could be from contact with the banana, another item in your lunch or with a hard external surface. Once you have initaited the bruising process, the ethylene gas being produced by the banana will speed up the chemical reaction producing a more pronounced bruise. Some fruits, like bananas, produce ethylene after picking which promotes ripening. Over time, the fruit will become over ripe as a result of chemical and biochemical reactions (when banana peels turn brown and the banana becomes soft and darker in color).
Hope this helps answer your question.
Professor of Food Science
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