How ripe is too ripe for banana bread?

Still, as long as you don’t see evidence of microbial growth (visible mold, seeping liquid, sliminess, or a gash in the peel that could have allowed microbes in), quality, rather than safety, will be the issue. 

If your banana is on the edge, though maybe in slightly better shape than the banana depicted here, look out for these bad banana flags: 

• Any fuzz, sliminess or an obviously advancing rotten area (scroll down on this post to see what that looks like), which all indicate microbial growth.
• Foul odors aside from the natural banana odor, which may be quite intense.
• Anything leaking or seeping from the banana.
• A break in the peel. If the peel’s been busted open and the banana looks wrecked, all manner of environmental microbes could have dropped in and it’s best to direct the banana to the compost. 

Bananas are programmed to eventually break down and blacken

“This is just normal aging,” Brecht explained. “This is the banana dying, basically.”

Many things happen as bananas ripen and then break down (a process known as senescence) with varying effects on taste, appearance and texture. Here’s what you may notice:

• They taste sweeter. Bananas turns starches into sugars as they ripen.  
• They develop brown spots on their peels. Not to be too blunt about it, but those cute banana freckles are places where cells gave up the ghost and oxidized to that brown color. Wait long enough and the brown spots will overtake the yellow areas; the banana will simply look brown. 
• They shrink. Moisture transpires away, into the surrounding air. With less water plumping up their cells, bananas shrink. (Interestingly, some water also migrates from the peel into the flesh; it’s drawn there by the increased sugar concentrations, but the banana still loses water overall through evaporation of water from the peel to the air.)
• Their flesh goes translucent. As a banana’s cells physically break down, liquid leaks into the spaces between the cells, creating a “water-soaked” look. 
• They develop alcohol-like odors and flavors. Those flooded intercellular pathways we referenced above? They make it hard for oxygen to reach the banana’s cells. Even after harvest, banana cells still take in oxygen and release carbon dioxide. With their oxygen supply blocked, the cells resort to anaerobic respiration, which produces ethanol as a byproduct. 
• Their flesh blackens. Wait long enough and the membranes keeping various compounds separate will leak. When their contents mix with oxygen, they’ll react and turn various shades of black and brown.

All of these goings on will impact the taste and flavor of the banana. Personally, when it comes to evaluating bananas for banana bread (or much easier and less time consuming banana cookies) I’m comfortable with some translucency, but I can’t deal with a banana that’s covered in odd black and brown spots and smells like an ill-conceived cocktail.