What Causes Bananas to Ripen?

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What Causes Bananas to Ripen?
(Images: M. J. Doran)

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Bananas are the world's fourth-largest fruit crop. The sweetness of the desert banana and the versatility of the plantain as a staple food account for the banana's popularity. But ripe bananas are very perishable and don't ship well. This begs the question--how do ripe bananas get to our grocery stores and our kitchen tables without spoiling? To understand this, you need to know a little about what makes a banana ripen.

Bananas Begin to Ripen as Soon as They Are Harvested

Commercial bananas are harvested when they have become plump, but are still green. Once the bananas are picked, hormones in the fruit convert certain amino acids into ethylene gas. Ethylene gas, in turn, stimulates the production of several enzymes that change the colour, texture and flavour of the banana.

The Banana Becomes Sweet and Soft

The enzyme that makes bananas sweet is called amylase. Amylase breaks down the starch in the banana fruit. When the starch is broken into its smaller sugar components, called glucose, the banana tastes sweet. The enzyme that softens the banana is called pectinase. Pectinase breaks down the cell walls in the banana fruit so that it is less firm. The peel of the banana also becomes softer as it ripens.

What Causes Bananas to Ripen?
Enzymes sweeten and soften the banana, and change the colour of the peel.

The Banana Peel Changes Color

Other enzymes break down chlorophyll molecules in the banana. The green pigment of chlorophyll is destroyed and replaced by yellow, red or blue pigments. Depending on the type of banana, the result is either the golden yellow colour of our favourite dessert banana, or shades of red or purple for other banana varieties.

Eventually, Oxygen Turns the Banana Peel Brown

Once the banana peel has been softened, it bruises much more easily. Bruising causes yet another enzyme called polyphenoloxidase to speed up oxidation, which turns a banana peel brown. Eventually the entire peel will turn almost black. Even when a fresh banana is bruised, a brown spot will often develop. The flesh of the banana will also turn brown if the bruising is deep enough.

Green bananas are tougher and therefore easier to ship. During shipping, the environment of the bananas is carefully controlled so that ripening is slowed. The temperature is kept low, and the concentration of ethylene is controlled so that enzyme activity is minimised.

What Causes Bananas to Ripen?
Bananas fresh from the market are often not fully ripe.

Banana Ripening Continues at Home

The ripening process continues after you bring your bananas home. You can speed ripening by loosely covering your unripe bananas to concentrate the ethylene gas emitted by the fruit. If the bananas were shipped to the grocery store still green, they'll reach their best balance between sweetness and firmness when they're generously speckled with brown. Sometimes, though, bananas are given an extra dose of ethylene gas just prior to delivery, to hasten ripening. In this case the bananas will already be light yellow in the store. At home, they'll be most enjoyable when the peel colour deepens to a golden yellow with just a few brown spots.

What Causes Bananas to Ripen?
These bananas were green in the store and now are just right for eating.

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