Scientific reason on why fruit ripen in paper bags? What causes it?!
Answers: I heard that if you put certain fruit in a paper bag, it speeds up the process of ripening that fruit. I think I heard that putting a banana in with the fruit will increase the speed even more. What is the scientific reason behind this? And which fruit is affected by this (i'm sure not all)? And does a banana in the bag as well help? If so, why?
The reason some fruits ripen when placed in a bag is because they give off ethylene gas, which also ripens the fruit via various hormonal activity (ethylene is a hormone in some fruits and other plants).
Ethylene is also responsible for the ripening of tomatoes, peaches, apples, and the sprouting of potatoes. The effects of ethylene on plants is actually very complicated, and I'm not botanist. Ethylene causes literally dozens of effects on plants (other than ripening). Answering your question is sort of like answering the question "How does testosterone cause 17 year olds to grow beards?"
You can help along ripening by placing various fruits together. Bananas give off a lot of ethylene. Try ripening pale tomatoes by placing them in a bag with a couple bananas for a day or 2. I have also forced a pineapple plant to bloom by placing bananas around it, then covering the entire plant and fruit loosely with a bag for a couple days--ethylene causes some plants to bloom.
Commercial fruit/vegetable producers often gas large amounts of bananas/tomatoes with ethylene to ripen it after shipping. For ease of shipping, underripe (hard) tomatoes are shipped to their destination city. There, at a produce wholesaler, the tomatoes (or bananas or whatever) is placed in a chamber, which is filled with synthetic ethylene gas, which quickly "ripens" the produce, which is then sold.
Fruit ripens as it decays, and it decays faster with heat. When fruit decays, it releases heat, as well as chemicals. The heat given off by the fruit is trapped in the paper bag, and adds to the decay process.
Banana's are a bit special, the chemical released when they decay causes bacteria to replicate faster, causing more heat, and more ripening.
This may sound a bit gross when you think of eating it, but we need that bacteria, which is why we like fruit. Just dont forget the fruit in a paper bag for too long. I've used the trick on apples, oranges, pears, peaches, nectarines, etc and it's worked on all of them. Generally 1 day in the bag will ripen them fully, but it depends on how ripe they were to begin with.
I don't know about just putting the fruit in a paper bag but I do know why you add the banana.
Bananas release an enzyme which speeds up the ripening process
I agree with Chuck except I was told that it is apples that cause others fruits to ripen faster, that is why I never put them anywhere close to bananas or other fruits I do not want to ripen too fast.