Bananas

Bananas ripen on the counter and should be kept there — about 3 days once ripe, longer if you buy them green.

To slow them down, separate the bunch and wrap the crown of each stem in plastic or foil. Most of a banana's ethylene comes from the stem, so covering it genuinely slows ripening by a day or two.

To speed them up, put them in a paper bag, ideally with an apple.

Refrigerating a ripe banana is fine and useful. The peel turns black within hours, which alarms people, but the flesh inside stays firm and good for several more days. Refrigerating an unripe banana, however, stops the ripening permanently — it will never develop properly.

Peeled bananas freeze well for 2–3 months and are ideal for smoothies and baking. Freeze them peeled and in chunks; frozen in the skin they are unpleasant to deal with.

Bananas are the most ethylene-driven fruit in common use. They are harvested completely green and ripened commercially in rooms flooded with ethylene gas, which is why they arrive at the shop at a predictable stage. Once ripening starts they produce large amounts of ethylene themselves, creating a self-accelerating process — which is why a bunch goes from perfect to overripe in a day.

The ripening itself is a starch-to-sugar conversion. A green banana is mostly resistant starch; a yellow one is mostly sugar. That is why the flavor, texture, and even the nutritional profile change so dramatically.

Refrigeration causes chilling injury to the peel, damaging cells and releasing polyphenol oxidase, which turns the skin black through the same enzymatic browning that darkens a cut apple. The flesh is unaffected. A black-skinned refrigerated banana is fine inside — this is one of the most common cases of good food being thrown away for cosmetic reasons.

For an unripe banana, chilling injury does more: it disrupts the enzymes ripening depends on, so the fruit stalls permanently.

Brown spots on the peel are normal ripening. Discard when the flesh itself is brown and mushy, there is a fermented alcoholic smell, or mold has appeared — usually first at the stem.