ANSWER I: Banana peels can turn brown or black when refrigerated. The cells on the peels of tropical fruits like banana get damaged when stored under cold conditions. This releases some phenolic compounds that get oxidized by enzymes like polyphenoloxidase leading to the darkening of the peel.
If you put a banana peel in the fridge or freezer for a few hours and then place it at room temperature, it will turn brown quicker than a peel that was kept at room temperature all the time. However, the quality of the edible part of the banana fruit is not affected by refrigeration. In fact, refrigeration may delay the ripening of banana fruit for a few days. Temperate fruits like apple and pears do not get their cells damaged due to cold conditions and hence they can be refrigerated.
ANSWER II: While many fruits are stabilised by refrigeration, most tropical and subtropical fruits (bananas in particular) exhibit chilling injury. Chilling injury can result in great losses in the quality and shelf life of the fruit. Fruits and vegetables subject to chilling injury includes banana, lemon, lime, eggplant, tomato, avocado, cucumber, pineapple, jackfruit and papaya.
Banana is one of the most sensitive fruits because it develops injury below about 12 degrees C. Apparently, cold hardy fruits, such as apple and pear, are not affected as they evolved tolerance to chilling temperatures. The blackening of the banana skin is caused by the release of enzyme, polyphenoloxidase (PPO). This is an oxygen-dependent enzyme, which polymerises naturally occurring phenols in the banana skin into polyphenols similar in structure to melanin formed in suntanned human skin.
Like all living organisms bananas adjust the composition of their cell membranes to give the right degree of membrane fluidity for the temperature at which they normally live.
Below a certain temperature the physical state of cell membranes changes allowing leakage of enzyme, polyphenoloxidase. The enzyme polyphenoloxidase can oxidize phenolic compounds that leak from vacuoles and produce the dark colour. Also the low temperature causes dissociation of enzymes and proteins into subunits, which affects enzyme activity and cell structural proteins such as tubulin.
Bananas still can be stored in the refrigerator for few days. Even, the peel of banana will darken, but inside will remain firm and delicious. Coating the banana with wax to exclude oxygen can also slow blackening of the skin. Bananas can be frozen once the banana is peeled and stored in a freezer bag. A touch of fresh lemon juice may help to reduce browning.
Courtesy : The Hindu