According to Dr. Amal Ghosh, Clinical Tutor (Rtd) , N.R.S. Hospital, Kolkata, the cold temperature of the fridge encourages an enzyme called polyphenyl oxidase that is naturally found in banana to polymerise phenols in the banana skin into polyphenols . Polyphenols are similar to melanin, the pigment responsible for the colour of our skin. These polyphenols therefore blacken the skin of the bananas.
Despite the colour, the cold temperature will keep the bananas firmer than those left at room temperature for the same amount of time. The enzymes that break the starch into sugar (which makes the banana soft and ripe) work better at room temperature. Or in colder months, you can find a shaded corner and store the bananas.
