How to Keep Bananas Fresh : Keeping Bananas Green

A green banana is a living system that releases ethylene and ripens gradually over time, making changes in its color, odor, texture and taste. The problem is that the bananas at home ripen so quickly that often they become brown before you can eat them. So the natural question is how to stop bananas from getting ripen too quickly and prevent spoilage.

How should you store your bananas, specially in summer to slow down ripening and to keep banana fresh longer and prolong their self-life? Should you keep them together in the bunch or separated? Will wrapping in plastic really stop bananas from getting ripen too quickly?

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Many suggestions have been made to keep banana green for long time.
  1. Some suggest to separate the bananas from the bunch.
  2. The others suggest to wrap the top of the stems with a plastic. They say that the bananas separated from the bunch and wrapped at the top keep longer than the whole bunch (crown) wrapped.
The argument behind above suggestion is that bananas release ethylene gas through the stems (crown) and by separating and/or covering the top should contain the release of ethylene, thereby slowing down the rate of ripening. According to this, separating bananas from the bunch should restrict the gas exposure to the banana, thereby slowing the ripening.

I had some doubt about the above reasoning. According to this theory, if the banana produces ethylene only from its stem, then the stem should first become ripe, and it should rot by the time the other end of the bananas ripens. Have you ever seen a banana yellow at the stem and green at the bottom?

My point is that it is not just the stem but the whole fruit emits the ethylene gas.
Just covering at the top should not prevent bananas from ripening. So I decided to test the suggestion.
Effect of wrapping on banana ripening
Effect of wrapping on banana ripening

I bought 6 equally green bananas and made 3 batches of 2 bananas each.
1. The first batch I placed on the counter.
2. I separated each banana from the 3rd batch and placed them on the counter.
3. I wrapped the top crown of the second batch with a clear plastic and placed on the counter.
4. I separated each banana from the 4th batch and wrapped the stem of each tightly with a clear plastic and placed on the counter.
All the 4 batches of bananas were kept away from each other and from any other fruit and vegetable at room temperature.

My observation after 5 days:
  1. All the bananas in the 4 batches were ripe more or less to the same degree.
  2. The wrapping did not make any significant difference in the speed of ripening.
  3. The separated bananas ripen faster than the bunch. You can find the reason for this in the following paragraphs.
My Conclusion: Wrapping at the stem does not delay banana ripening noticeably and bananas in the bunch ripen slowly than separated.

How To Store Bananas To Keep Them Fresh and Green

Banana stand
Banana stand
The bananas releases ethylene gas which is responsible for their ripening. Ethylene gas is a plant hormone, which occurs naturally in all plant tissues (ethylene producing fruits). It governs many physiological functions of the plant such as senescence and ripening. The ripening of bananas depend on four major factors; namely, ethylene gas, temperature, relative humidity and air circulation.

Many chemical and physical changes take place in both the peel and the pulp during ripening. The color of peel changes from green to yellow to brown, and the starch in the pulp is converted to sugar by the action of enzymes and becomes soft. The acid in the raw banana is neutralized and it becomes sweet. The raw bananas contain only 1 % sugar and about 20 % starch. In a ripe banana, the sugar proportion is raised to 20 % and starch reduces to 1%.

The plant and its fruits produces more ethylene as it matures; and also in response to plant stress such as wounding, sunburn, lack of water, infection and disease [1].

Ethylene is responsible for banana ripening, but oxygen is also needed for the chemical reactions involved in ripening. The ripening of fruits gives off ethylene and carbon dioxide.

These facts on ethylene and fruit ripening can be used to make strategies for delaying  ripening of bananas. It is, therefore, logical to think that the ripening process of bananas can be prolonged if the supply of oxygen is restricted.

The banana stored on the counter will ripen and turn brown quickly as it has an unlimited supply of oxygen.

The banana ripening depends on temperature and the humidity.

Effect of Temperature on Banana Ripening

The self life of bananas increases as external temperature decreases, a 1°C reduction increases storage period by 1-2 days. However, fruits suffer chilling injury at temperatures below 11°C. The optimum storage temperature for banana is 13-14°C, at which fully mature, ripe and unripe fruits will keep for 1-2 weeks. [2]

In another research, it was found that the best quality long ripe bananas (20 cm) should be stored at 18°C and relative humidity during ripening must be 80 - 85% [3].

Effect of Humidity on Banana Ripening

The research has shown that low humidity (60 - 65%) accelerates the ripening of banana and produces low quality ripe fruit, while high humidity produces good tasting fruit. The high humidity delays the ripening of bananas, increasing their shelf life. The optimum humidity conditions for banana ripening are 90 to 95% relative humidity. [4]

Effect of Wounding on Banana Ripening

Have you noticed that the side of the banana that rests on the table has ripen more than the other parts. This is because that side has the most contact with its own ethylene.

Making bruises or punctures on the banana will speed up the ripening process, as they expose it to air, speeding up the ripening process. That is why, I always place bananas on the counter facing down (tips down), the tips supporting the weight of the bananas to avoid any bruises.

There are products marketed specifically for storing bananas in a hanging position to prevent premature ripening.

Effect of Packaging on Banana Ripening

The rate of ripening is affected by the environment. So proper packaging can help us in preventing bananas to ripen fast [4, 5]. perform the following experiment to see if wrapping does really prevent banana ripening.

Take 3 equally green bananas and store them as following:
  1. Put 1 banana in a paper bag. The paper bag is porous to allow some oxygen in and at the same time contains the ethylene released by the banana inside.
  2. Put the second banana in a plastic bag. The plastic bag does not allow air from out side, but it has some air already.
  3. Wrap the third banana with a plastic tightly. The tightly wrapped banana in plastic has no air.
Open the bags after 4 days. What do you notice?

You will find that the banana in the paper bag will ripen faster than the bananas in the plastic bag. But the banana tightly wrapped in plastic will ripen most slowly because of absence of oxygen. So this should be our strategy to prevent bananas to ripen quickly.
Conclusion: The bananas exposed to air ripen faster than the bananas that were protected from air.

You can try different materials for packaging such as paper, cardboard, unsealed plastic, sealed plastic, etc. and see the effect on rate of ripening.

It is interesting to note that the packaging of bananas in 100-150 gauge polyethylene bags keeps them attractive, fresh and good flavored ripe fruit than those packed in 200 gauge polyethylene bags [6].

Banana Chemical Treatment for extending self-life

Bananas dipped in a solution of lysophosphatidylethanolamine (LPE) for 30 minutes keep firmer and thicker than untreated bananas with an improvement in shelf life by two days [7]. This method is for commercial banana growers.

Bananas With Black Spots

Ripe bananas with black spots
Ripe bananas with black spots
A banana with black spots on its peel is due to a substance called TNF (Tumor Necrosis Factor) that is powerful in fighting against cancer cells. The number of black spots increases with the concentration of TNF.

Tips For Keeping Bananas from Ripening Too Fast

  1. Select fully ripen yellow bananas for immediate eating, smoothies, baking or using in mashed banana recipes.
  2. Do not store unripe bananas with ripe fruits.
  3. Protect your bananas from heat and direct sunlight. At higher temperatures, the bananas ripen faster. 
  4. Store the ripe banana with an unripe fruit like an avocado. It will slow down the ripening of the banana, the ethylene from the banana will be absorbed by the avocado.
  5. If the bananas are dark green then you can keep them on the counter at room  temperature until they are ripe. When they are yellowing or turning brown, then put them in a closed air-tight box or bag to keep them from ripening too quickly.
  6. Buy bananas that are under-ripe. I buy 4-5 green bananas, 4-5 slightly yellow and 3-4 fully yellow bananas. I can use them without any spoilage.
  7. Store the bananas at a place in your house which is at lower temperature, like basement.
  8. You can do the following: Put all the bananas in a paper bag and keep them in fridge. In the fridge the rate of ripening decreases. Take out 3-4 bananas when need and put on the counter to ripen them. Take out a few more bananas after a few days to ripen them. This way, you can prevent bananas from ripening quickly and avoid spoilage. OR

    When bananas are ripe, I place them in the fridge to stop them from further ripening. The peel will become black, but the banana inside will remain firm and tasty.
The best way to keep bananas fresh, green and firm is to tightly wrap them in plastic. This will protect them from air exposure, reducing the rate of ripening and will keep them green longer.


Ethylene Gas Absorbing Materials & Products

Bananas remain green or unripe if the ethylene concentration is below 0.1-0.2 ppm. At low ethylene concentration or  in the absence of ethylene, the banana ripens slower and lasts longer. It is, therefore, natural to think of  a substance which can absorb the ethylene gas.

The ethylene molecule is oxidized when it comes in contact with certain compounds such as potassium permanganate (KMnO4) or sodium permanganate (NaMnO4) and converted into harmless compounds, removing ethylene  to very low levels.

Ethylene Gas Absorbents

You may purchase ethylene gas absorbents (absorbers), which come in sachets or other type of container.
  1. Epax packet is a commercial product which absorbs ethylene gas and can be useful in protecting any fruit including banana from ethylene for up to 45 days [8].
  2. Purafil Select which contains 8% of potassium permanganate, is a commercial product that is recommended for the preservation of fruits, vegetables and flowers[9].
  3. Banana case for keeping banana
    Banana case

    Caraselle banana bag
    Caraselle banana bag

  4. Zeolite is a natural microporous aluminosilicate mineral with unique absorbing properties including ethylene. Its molecular structure is similar to a honeycomb grid making it a natural odor absorber. Commercial zeolite based ethylene zeolite adsorbent is coated with potassium permanganate.
  5. Bluapple Ethylene Gas Absorber is available in USA. It claims to prolong fresheness and storage life of fruits & vegetables for 3 months.
  6. Banana Bag: British designed Caraselle banana bag claims to keep bananas ripe and fresh for up to 2 weeks by keeping oxygen out [10].

Self-life of Bananas

Unripe bananas keep well on kitchen counter up to 6 days depending on its ripeness. Once the banana is ripe, you can put them in fridge, the skin will turn dark, but the banana will remain firm and sweet for 4-5 days.

Storing Bananas in Freezer
Peeled or mashed bananas store well when frozen; they can last for 2-3 months in the freezer. To freeze, peel the bananas, place them in a plastic bag and put in freezer. When you want to use these bananas, take out from freezer and leave them at room temperature for considerable time to allow them to thaw.
  1. The average self-life of banana bought from market is given below:
  2. Kitchen Counter: 2-6 days for unripe banana in skin.
  3. Refrigerator: 5-8 days for ripe banana in skin.
  4. Freezer: 2-3 months for ripe peeled banana.
However, when I pluck bananas from the banana tree grown in my backyard (growing banana tree at home) , these times are much longer because they have not been treated in ethylene gas.

The peeled banana or cut banana in fruit salads will turn brown, to avoid this toss it with some lemon or pineapple juice.

References 

1. Scot Nelson et al, Banana ripening: principles and practice, pdf
2. Anita Pedapati, Factors Affecting for Fruit Ripening in Banana, www.biotecharticles.com/Agriculture-Article/Factors-Affecting-for-Fruit-Ripening-in-Banana-3428.html
3. S. Ahmad, et al., Improvement of Banana Quality in Relation to Storage Humidity, Temperature and Fruit Length, International J. Agriclture & Biology, 1560–8530/2006/08–3–377–380,  www.fspublishers.org
4. C. Claidborne Ray, Science Q & A, Science Times, The New York Times, FEB. 10, 1998
4. Ripening Bananas
5. How Does Packaging Affect the Ripening of Fruit? Science Buddies, Science Buddies, 4 Oct. 2014. Web. 26 Oct. 2016 , www.sciencebuddies.org/science-fair-projects/project_ideas/MatlSci_p022.shtml
6. S. Ahmad, et al., Effect of Polyethylene Film Thickness and Exposure Time of Ethylene on the Ripening Behavior and Quality of Banana
7. New treatment extends shelf life of bananas, A. Soc. Horticulture Sc.,
www.eurekalert.org/pub_releases/2015-10/asfh-nte100715.php
8. The problem - Ethylene gas, www.sorbentsystems.com/epaxtech.html
9. Purafil Select media, pdf
10. Banana bag, http://www.caraselledirect.com/_/caraselle_banana_bag_-_keep_bananas_fresh_up_2_weeks.2991-1

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