Many of us avoid eating bananas when they start to form tiny brown spots, leading to costly food waste.
Now scientists say the formation of these spots can be slowed down by reducing the oxygen levels available to microscopic skin cells.
Scientists have developed a new method to simulate blotch patterns on bananas, providing new insight into how the world’s most famous fruit turns brown over time.
The study authors said that preventing banana peels from turning brown may be the key to reducing food waste.
And each year, 50 million tons of bananas end up as food waste, according to lead study author Oliver Steinbock of Florida State University.
“For 2019, total banana production was estimated at 117 million tons, making it the world’s leading crop,” said Steinbock. When bananas are ripe, they form many of the dark spots most people are familiar with and often used as an indicator of ripeness. However, the process of formation and growth of these spots and the resulting pattern remain poorly understood to date.”
And the pulp of many fruits turns brown when sliced and isn’t eaten right away — not just bananas, but apples too.
This happens because the enzymes in the fruit react with the oxygen in the air — a process known as enzymatic browning. But the banana peel also gradually turns brown. This is because banana peels contain a gas called ethylene that breaks down chlorophyll, the chemical that keeps plants green.
Brown comes from dark pigments, including melanin, which is found in people’s hair and skin.
These pigments form in bananas when oxygen in the peel reacts with natural chemical compounds called phenols.
When bananas brown, much of the starch is converted to sugars, making them an excellent natural source of sweetness.
Despite this, the public tends to throw away brown bananas, although they are still edible and ideal for use in baking recipes.
In the study, the researchers used a combination of time-lapse video clips and a computer model to show how banana brown spots form.
The team’s computer model was used to look at the oxygen concentration and browning of the skin in the videos.
Oxygen from the air enters the skin through tiny holes called stomata, which explains why tiny spots form and spread, rather than the whole skin turning a uniform brown.
“Banana peels have a waxy outer layer that — when intact — does a good job of keeping oxygen out,” Steinbock said. “The result is brown spots on a yellow skin. But very slowly oxygen eventually gets into the skin everywhere, so that the old bananas are really well built.”
The researchers found that the spots appear and expand rapidly before their growth mysteriously stops — all within two days.
“Our results strongly suggest that the small holes in the center of the brown spots are collapsing,” Steinbock said. It re-closes the cortex, cutting off the flow of oxygen, thus stopping the growth of the macula.”
Browning can be prevented by genetic modifications or better storage conditions, such as special containers with a low oxygen content.
Another mechanism to protect bananas is a thin film to prevent oxygen from the air from entering the peel into the small openings.
Contrary to popular belief, the refrigerator is generally not a great place to store bananas after they have been taken from the store.
If we put unripe bananas that are still a little green in the fridge, the flesh will not ripen at all, but the skin will turn black.
However, placing a ripe banana in the refrigerator will keep it ripe (rather than overripe) for a few more days.
The new study was published in the journal Physical Biology.