system
Given the following post, respond as if you're commenting on Reddit:
human
Why is a frozen and thawed banana so much sweeter, and how does this change its nutritional value?
gpt
Putting together the information here

One of the main processes in bananas (and all fruit) ripening is the amylase dependent conversion of savoury or flavourless starches in to sugar (specifically glucose). Amylase is a common enzyme (also present in your saliva) which converts starch to sugar and is an important part of your digestion.

There are essentially 2 ways something will taste sweeter.

1) There is more sugar present 

or

2) Your tastebuds can access the sugar more rapidly

Freezing and then defrosting fruit essentially lets both of these things happen. Freezing causes water in the fruit cells to crystallise and expand. This destroys the cell walls and is the principal reason defrosted fruit is soggy and limp. However it also means that the cell contents (all those sugars) are now in the juices that are running off the fruit and if you taste the juice you'll find it is very sweet. You can experience this at the most extreme if compare the difference in sensation between holding a mouthful of orange juice in your mouth or holding a slice of orange (without chewing).  In the case of a banana there isn't much excess of liquid to run off so those exposed cell contents will largely stay within the fruit pulp/body rather than running off.

The other thing that happens while the fruit is defrosting is that all the amylase and starches in the cells are now able to diffuse (a little) through the defrosting fruit pulp. The amylase is no longer confined to the cell it started in, where it may have completed its starch converting job, and is free to find any remaining starch that may have come out of other nearby cells. This means that some of the remain starches will be converted to some extra sugars.