Plants produce a gas called ethylene. This gas causes the plant to produce chemicals called enzymes, which do several things. They cause the starch and acids in the fruit to change into sugar. This makes the fruit more sweet and less sour. They also weaken the cell walls, causing the fruit to soften.
Apples produce a lot of ethylene gas as they ripen.
By placing the unripe plums into the bag with the apple, you are adding extra ethylene, making them ripen faster. As the fruit gets riper, it generates more and more ethylene. This is where we get the saying that one bad apple spoils the whole barrel. One overripe apple stored with the rest of your fruit can quickly cause it all to over ripen very quickly. This page 1998 Edgerton Explorit Center all rights reserved.