Ethylene is the name given to a hormone that ripens naturally . Ethylene is especially responsible for fruit ripening . At normal temperatures ethylene is a gas whose molecular structure is CH2 = CH2 . This is called phytohormone. If the raw fruit is picked from the tree and left in contact with ethylene, it will be seen that the fruit will ripen. Carbide and water vapor in the air produce a gas called acetylene , which has many properties similar to ethylene. For which litchi ripens. Raw fruit contains a lot of chlorophyll. And there is a little xanthophyll and carotene. When the fruit begins to ripen, the production of new chlorophyll stops and the previous ones are destroyed. On the other hand, the amount of colored carotene , xanthophyll , is increasing. As a result, green color changes to yellow (if xanthophyll is high), orange (if carotene is high), red (if lycopene is high) etc. Even if the fruit is picked from the tree and kept at home, the production of chlorophyll stops. And the color of the fruit according to the characteristics is yellow , orange or red.