Why are bananas sold green, and how does fruit ripening work?

Bananas are harvested and sold while still green primarily for logistical and shelf-life reasons. Because they are climacteric fruits—meaning they continue to ripen even after being picked—transporting them in a firm, green state prevents them from bruising, rotting, or becoming overripe during the long journey from tropical farms to your local grocery store.

Here is how the ripening process works:

The "Climacteric" Process

Most fruits, like apples and bananas, are climacteric. They have a specific trigger mechanism that allows them to undergo a burst of respiration and ethylene production once they reach a certain stage of maturity.

1. Ethylene Production: Ethylene is a natural plant hormone in gas form that acts as the "ripening signal." As a banana matures, it begins producing more ethylene, which travels through the fruit and triggers a chain reaction of chemical changes.
2. Starch to Sugar: Green bananas are packed with complex starches, which give them a hard texture and a starchy, slightly bitter taste. The ethylene signal activates enzymes (specifically amylase) that break these long starch chains down into simple sugars like glucose, fructose, and sucrose. This is why bananas get softer and sweeter as they ripen.
3. Color Change: Inside the peel, the green chlorophyll—the pigment responsible for photosynthesis—begins to break down. As the chlorophyll fades, other pigments called carotenoids become visible, turning the peel yellow.
4. Softening: Other enzymes, such as pectinase, break down the cell walls of the fruit (specifically the pectin that acts like "glue" between cells). This causes the internal structure to soften, turning the fruit from firm to creamy.

Why they turn brown

Once the banana reaches peak ripeness, the process doesn't stop. The cell walls continue to break down, and the enzymes interact with phenolic compounds in the peel. This results in the formation of dark pigments called melanin—the same compound responsible for skin and hair color in humans. This is why the spots appear, and eventually, why the entire peel turns dark.

How to manage it at home

If you buy green bananas, they will continue this process at room temperature. If you want them to ripen faster, you can place them in a paper bag with another ripe fruit (like an apple or another banana), as the ethylene gas will be trapped and speed up the process. If you want to stop or slow them down, placing them in the refrigerator will stall the ripening enzymes, though the cold may cause the peel to darken even if the fruit inside remains firm.