Bananas often turn brown and soft within a few days. An American food expert has a simple trick. Cut a banana in half, but leave the peel on. Put both halves in an airtight glass jar with a metal lid. Then keep the jar in the refrigerator.
The expert tested this with three bananas of similar ripeness. Two stayed on the counter, one went into the jar. After 16 days, the two on the counter were black and mushy. The refrigerated banana's skin was still mostly yellow. The sealed jar helps limit contact with a gas called ethylene, which ripening fruit releases. The peel also keeps oxygen away from the soft inside. Bananas that have just turned evenly yellow work best.