You may have noticed that to keep bananas fresh, you end up making them brown and soft. Most things that go in the fridge last longer, so why do bananas seem to age more quickly?
Lower temperatures are typically associated with a reduction in ripening processes, and the skin color of the fruit is commonly an indicator of its stage of ripeness. However, when bananas are put in the fridge, they ripen slowly and turn brown quickly, which seems a bit backward.
It turns out that this is because the browning of skin is different from the ripening of fruit.
So why do bananas turn brown in the fridge? Bananas turn brown in the fridge because they’re tropical fruit, sensitive to low temperatures and humidity. In the cold, dry air of the fridge, enzymes responsible for ripening are inhibited, but so are structural processes in the skin, which then becomes damaged and releases browning enzymes.
According to Dr. Svenja Lohner, Scientist for STEM Education Technology, the cold temperatures in the fridge damage cells in the skin of the bananas, releasing an enzyme called polyphenolox oxidase. This enzyme is one of the reactants in the formation of melanin, the dark pigment that colors human skin.
The ideal temperature for a banana plant is 78-86F, or 26-30C. The standard temperature in your refrigerator is closer to 40F or 4C. This massive drop in temperature is too much for a banana plant’s delicate cellular structures and chemical processes and will damage it.
But keeping them in the living room or kitchen seems to bring fruit flies, and they don’t last long. So, is there a way to keep them in the fridge without them turning brown?
How to Keep Bananas from Turning Brown in The Fridge
To keep bananas from turning brown in the fridge, use methods that inhibit the effect of the polyphenolox oxidase or PPO enzyme. These include wrapping the whole banana to prevent dehydration and contact with oxygen or adding an acid like citric or ascorbic acid (vitamin C) to reduce the pH.
There are so many myths around this fruit that it’s difficult to know where to find the truth. Many of these myths revolve around the misconception that the same mechanism causes the browning process and the ripening process.
This fundamental misconception has resulted in a series of ineffective strategies for keeping bananas yellow in the fridge, such as wrapping the ends, dipping them in water, or otherwise trying to prevent the release of ethylene from the stem of the banana itself. Here are some points to consider:
- While ethylene is responsible for the ripening process, there are two reasons why these strategies don’t prevent browning. Firstly, natural ethylene in commercially produced bananas is reduced – they are artificially ripened via pressurized warehouses with ethylene treatment. This inhibits the natural production of ethylene in store-bought bananas.
- Secondly, ethylene is not the major contributor to the browning process. As mentioned, polyphenolox oxidase is the main reason banana skins turn brown. With this in mind, what’s the best way to keep bananas from turning brown in the fridge?
- PPO browns banana skin when it meets the surrounding air. So, there are two main approaches to preventing browning. You can either reduce the release of PPO or prevent it from coming in contact with the air. Both of these methods should slow down the browning process.
- Temperature Extremes will damage the cell tissues, releasing more PPO and increasing browning. Bananas in the fridge or the sun will brown the fastest because of this. If you have a warmer section of the fridge that can go up as high as 50F or 10C, this temperature will be low enough to slow browning without damaging the plant tissues.
- The dry air in the fridge will also damage tissues by dehydrating them, so wrapping the bananas in a Ziplock bag or in plastic wrap may slow the process by holding in moisture while the bananas are in the fridge.
This last method works doubly well because it also prevents the released PPO from reacting with the air inside the fridge.
Finally, PPO is inhibited in an acidic environment, so much in the same way, as you would treat guacamole with lime juice to prevent it from browning, you can add an acid like citrus or vitamin C to the banana to slow the effect of the enzyme. This will affect the flavor of the banana, though, of course!
Why Does a Peeled Banana Turn Brown After Some Time?
A peeled banana turns brown in the fridge because of polyphenolox oxidase (PPO), which is responsible for enzymatic browning in bananas. It is much faster and more pronounced in the peels but has a similar effect on the flesh.
When fruit tissue is damaged in the presence of oxygen, these enzymes are released from its cells and react with the atmosphere to create the browning pigment. This is particularly common in fruits with high levels of PPO, such as avocados and apples. You may notice that these fruits go brown quickly when damaged tissues are exposed to the air.
This issue of fruit and other consumables turning brown is a huge topic of study in food preservation. Being able to reduce the effect will allow for food to be transported farther and stored for longer. Solving this issue also has powerful implications regarding food waste.
How Long Does It Take for Bananas to Go Brown in The Fridge?
Bananas go brown in the fridge within about a week. This is a slower process in green bananas but accelerated in bananas that are already yellow when put in the fridge.
It’s easy to compare the rate of change in bananas simply by setting one banana in the fridge and another on the table at room temperature. You’ll find that within a week, the room temperature banana has softened, while the banana in the fridge is still firm.
But the colder banana will darken much faster than the banana at room temperature.
So, bananas go brown at room temperature, elevated temperatures, and low temperatures. There’s no avoiding it. But they do seem to go brown much faster in the fridge. To find out how fast, we had to do some delving.
- How long does it take for bananas to go brown in the fridge? Well, this depends on how brown they were before going into the fridge. Bananas that went into the fridge before they were fully yellow browned slower than bananas at the same stage but were left out of the fridge. Bananas that had begun turn showed accelerated browning when placed in the fridge, turning black within a week.
- This rate of browning was also affected by whether the banana was enclosed in a bag or a box – both of which slowed down the browning process.
- Regardless of the speed of the browning of the banana peel, the refrigeration process significantly decreased the ripening process! While the skin may go more brown or black in the fridge, the flesh stays closer to the stage of ripeness it was at when it was put in the fridge.
What Makes Bananas Turn Brown When Exposed to Room Temperature?
Bananas turn brown when exposed to room temperature because the degraded tissues in the skin of the banana release polyphenolox oxidase (PPO), which reacts with oxygen to create a chain reaction leading to the production of pigmentation such as melanin in the banana peel.
The browning process of banana peel is an inevitable part of the degenerative process. As soon as the bananas are mature, they will enter a series of enzymatic and oxidation processes that lead to starches in the fruits being converted into sugars and tissues gradually deteriorating and oxidizing.
While some storage methods can speed up or slow down these processes, they are unstoppable, and essentially what makes bananas turn brown when left at room temperature is the same thing that makes them turn brown anywhere else:
Chilling injury accelerates this browning process at cold temperatures, but it still happens when the bananas are stored at room temperature.
This process is accelerated by extreme temperatures that damage the tissues in the skin or can be reduced by preventing the release or interaction of PPO with the oxygen in the atmosphere by wrapping the banana. Treating the banana with acid also inhibits the effect of PPO, reducing browning.
What is the Best Way to Store Bananas to Prevent Them from Getting Brown Quickly?
The best way to store bananas to prevent over-ripening is to refrigerate them. This, however, will brown them quickly, so if you want to slow browning, keep the undamaged bananas at 10 degrees, out of direct sunlight, and in areas with more humidity to slow the drying process of the peel.
To recap:
- Ethylene is not primarily responsible for the browning of bananas – in fact, in commercially produced bananas, there isn’t as much ethylene as there would be naturally. So, any method claiming to reduce browning by the reduction of ethylene is bunk.
- Further, browning agents come from every cell in the peel and flesh of a banana, so wrapping the stem is ineffective and would be even if it was related to ethylene. This myth seems to have stemmed from an article in 2012 by Lifehacker, which referenced three now-deleted papers in Wilgubeast, none of which made the claim that wrapping stems-maintained freshness in bananas.
- Before this article, there was no mention of wrapping bananas for freshness, and Food scientist Ann Reardon from How to Cook That proved in a demonstration that ripeness is unaffected by wrapping. However, wrapping the entire banana in a zip lock bag did reduce the browning of the skin.
- The same mechanisms do not cause browning and ripening processes, so if you’re looking for ways to prevent premature ripening, you’re going to be doing something differently than if you want to prevent browning.
Here is a table that highlights remedies that others used for storage of bananas so that they would not turn brown quickly:
| How to store bananas | Percentage of total results |
|---|---|
| Storing the bananas in Ziplock bags will slow down the browning. | 14% |
| If you use the bananas in smoothies, peel them, cut them up, and freeze them before they start to turn brown. (I recommend the nanaRack Frozen Banana Storage Container, click here to see the current price on Amazon) | 43% |
| Keep your bananas in a fridge so that the fruit flies will not develop (they still will turn brown, but at least there will be a lack of fruit flies). | 31% |
| Rinse off your bananas after buying. This will wash off the fruit flies’ eggs. | 12% |
Why do bananas turn brown and mushy after freezing?
Bananas turn mushy after freezing because freezing introduces a lot more destructive processes to biological tissues. While refrigeration may slow enzymatic activity and result in cell death, freezing destroys cells entirely.
If refrigeration slows down the ripening process, you’d be right to assume that freezing must stop it entirely. Freezing bananas can keep them edible for well over a month, but you might find that when you take them out of the freezer, they’re not exactly in the same condition as they were when they went in.
Freezing your bananas may be a safe way to make them last longer, but they won’t be firm and fresh when they come out. More likely, they’ll be brown, and certainly, they’ll be mushy and liquified. But why do bananas turn brown and mushy in the freezer?
When the water inside cells is frozen, it expands into jagged crystals, which rupture and destroy tissues at a cellular level. When thawed, all the ripening and browning enzymes released from the cells react with oxygen to make the banana change color.
This damage is irreversible and absolute, and it’s why, when you freeze something that has high water content, like a banana, once it thaws, you’ve got a wet, mushy banana paste.
This can be exactly what you’re looking for, however, since throwing a few frozen chunks of banana into a smoothie makes it nice and cold while providing a pre-blended banana that won’t leave chunks when it thaws. I recommend the Magic Bullet Smoothie Blender, click here to see the reviews on Amazon.
Conclusion
So, forget what you’ve heard about ethylene in the browning process. And ice crystals can’t form below zero, so that can’t be the cause of tissue damage in the fridge. Ethylene is responsible for the ripening of bananas and other fruits, but the main contributor to browning is PPO.
Reduce this by keeping bananas at around 10 degrees or treating them with a safe acid like citrus juice.