As you bakers know well, for banana bread (and muffins) we generally turn to mature bananas. Sweeter and smoother than their greenish-yellow, unripened counterparts, over-ripe bananas are perfect for baking recipes.
The food science secret behind browning bananas? A process called enzymatic browning. An enzyme like polyphenol oxidase (PPO) catalyzes oxidation, eventually turning the fruit brown. Most of the time, producers prefer to delay the enzymatic browning process and preserve the quality of a perfectly-ripe fruit as long as possible – after all, most of us use color as the primary indication of produce quality. Thus, factors like temperature, pH, and exposure to oxygen (all catalysts of the browning reaction) are carefully monitored on a commercial scale when bananas and other fruits are transported and stored.
However, if you’re in need of a banana ripe for baking, toss your yellow fruit in a paper bag and store it in a warm place. The bag serves a few purposes that expedite enzymatic browning: oxygen permeates the paper, and the bag traps ethylene gas that the banana releases during ripening.