On one level, the answer is simple: a single chemical called ethylene, a gas produced by the fruit itself, is responsible for ripening process.
Some fruits ripen best on the vine or tree; others, such as avocados, don’t ripen until they fall. Most fruits soften as they get ripe. A few, though including coconuts, become harder. Ripening is heralded in many fruits by a marked increase in ethylene production. This seems to affect the fruit’s physiology. It begins to respire, to breathe ‘oxygen’, a process that rises its internal temperature slightly.
Respiration can increase three, four, or even five-fold, providing extra energy for the work ahead. Chlorophyll is broken down and the fruit loses its green, unripe color. The starch present in it gets converted into sugar. Acids (and thus sourness) decreases. The pectin that cements cell walls together begins to disintegrate, softening the fruit. A corky layer may firm at the base of the fruit’s stalk, causing it to fall. Finally, many fruits produce aromatic chemicals that impart to them an enticing aroma.
But why do many plants expend so much energy and effort surrounding their seeds with tasty fruit, only to have it fall to the ground and rot or to be carried away by bird, animal or man?
The key seems to be dispersal. Plants, forever rooted in one spot, take advantage of the mobility of animals. Fruit is bait, dangled temptingly for all to see. Once ‘taken’ by some hungry forager, the seeds within the edible exterior pass unscathed through the digestive tract and are deposited far from the plant that produced them.