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Why does ripe fruits have sweet but raw fruits are sour?
 Fruits ripen because, there is commonly a ripening signal, it is a burst of ethylene production. Ethylene is a simple hydrocarbon gas (H2C=CH2) that ripening fruits make and shed into the atmosphere. (Sometimes a wound will cause rapid ethylene production...thus picking a fruit will sometimes signal it to ripen...as will an infection of bacteria or fungi on the fruit). This ethylene signal causes developmental changes that result in fruit ripening.

New enzymes are made because of the ethylene signal. These include hydrolases to help break down chemicals inside the fruits, amylases to accelerate hydrolysis of starch into sugar, pectinases to catalyze degradation of pectin (the glue between cells), and so on. Ethylene apparently "turns on" the genes that are then transcribed and translated to make these enzymes. The enzymes then catalyze reactions to alter the characteristics of the fruit.

The action of the enzymes cause the ripening responses. Chlorophyll is broken down and sometimes new pigments are made so that the fruit skin changes color from green to red, yellow, or blue. Acids are broken down so that the fruit changes from sour to neutral. The degradation of starch by amylase produces sugar. This reduces the mealy (floury) quality and increases juiciness. The breakdown of pectin,  to pectinase, between the fruit cells unglues them so they can slip past each other. Also enzymes break down large organic molecules into smaller ones that can be volatile (evaporate into the air) and we can detect as an aroma.
In pears, the ethylene signal causes the fruit to change from green to yellow, from hard to soft, from mealy to juicy, from tart to sweet, from odorless to fragrant.


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