There is a process called ripening, which makes the fruit sweeter and tastier by involving the induction of a variety of metabolic processes. This process is enigmatically regulated. Moreover, the signals that are sent by plants are also unique. They send it by using a special kind of hormones.

 In fact, ethylene is a gaseous hormone produce by a plant that helps in the process of fruit ripening. The plants that grew faster usually produce this ethylene gas. Usually, the ripening of fruit, damaged tissues, growing of flowers, etc. helps in the release of ethylene hormone. Therefore, the processes by which green fruits are picked depending upon the activation of ethylene.
The expression of several genes involved in fruit ripening is said to be regulated by phytohormone so as to activate various enzymes involved in the process. These ethylene signals are made because of these new enzymes and the characteristics of these fruits are alter by the catalyst reaction. These enzymes include the process of hydrolysis. It helps in breaking down chemicals inside the fruit, starch into sugar which help in catalysing the degradation of pectin.
In short, the ripening responses are caused only because of the action of these enzymes. Sometimes new pigments are made by breaking down chlorophyll so that the fruit skin is able to change its colour. The simple sugar is produced by degradation of starch by amylase. The breaking down of pectin results in a softer fruit. This breakdown is caused due to pectinase. In order to detect an aroma, these pectinase is broken down. However, this breaking phenomenon occurs only with climatic fruits such as mango, apple, bananas, guava etc. The process, which is accelerated by ethylene gas continues to ripe after they are being picked. Some fruits, such as watermelon, strawberries, and orange, do not become ripe after they are being harvested because they are non-climatic fruits.