As fruit ripens it releases a hormone called ethylene and it is that hormone that really ripens fruit when you have it shut up in a brown paper bag in the refrigerator.
The paper bag traps the released hormone next to the fruit in the brown paper bag and it ripens the fruit in a hurry. Be sure you watch your fruit carefully because bananas, peaches and pears can go from hard to mushy soft in only a few days. So be sure to watch your fruit carefully.
If you want to get a little more technical the ethylene causes the creation of enzymes which cause the break down of of starches and acids into sugar.
Certain fruits including apples contain a lot of ethylene so its important to store apples away from other fruits and vegetables. You can add a apple to a bag of tomatoes and it will speed up the process of the ripening.
Fruit Does Get Ripe Faster In A Brown Paper Bag
Fruit normally gives off ethylene gas which hastens the ripening process. When you place a apple and tomatoes in a brown paper bag in the refrigerator the apple will give off the ethylene and the tomatoes will be ripened in a hurry. Have you ever noticed that when a apple is put in a fruit bowl with oranges and tangerines that the citrus will ripen very quickly. Yes it does indeed happen so keep the apples out of the bowl with oranges or tangerines.
The ethylene gas is trapped inside a brown paper bag and it causes the fruit in the bag to ripen quickly.