Why Do Bananas Get Sweeter as Black Spots Appear on the Peel?
– How Ripening Works as a "Timer"
You bring home a banana with a plain yellow peel. After a few days, small black spots (sugar spots) start to appear. It looks as if the fruit is going off, but this is actually when a banana is said to be at its sweetest, with the strongest aroma. What is going on inside?
Have you ever bought a greenish or plain yellow banana from the shop and left it in the kitchen? After a few days, the peel darkens, and small black dots spread across it.
Taste a banana in this state and you will notice that the flesh is softer and clearly sweeter than when you bought it. A green banana can still taste a little chalky, like starch.
It is the same banana. Why does its taste change so much just with time?
Freshly picked bananas are said to contain a lot of starch, which tastes barely sweet. Over time, this is thought to change into sweet sugars.
A gas the banana makes itself, the plant hormone ethylene, acts as the switch for this whole series of changes.
Let's go through how this change, called "ripening", works, step by step.
Ripening: How Starch Turns into Sugar
Freshly picked bananas are said to contain a lot of starch, which has almost no sweet taste. As time passes, enzymes in the fruit are thought to break this starch down, bit by bit, into sweet sugars such as glucose and fructose. This series of changes is called "ripening".
Starch is made of many large molecules linked together, and the tongue is said to have trouble tasting it as sweet in that form. Only when enzymes cut it into small sugar molecules does it taste strongly sweet.
Ethylene: The Gas That Signals Ripening
What pulls the trigger on ripening is said to be ethylene, a gaseous plant hormone that the banana itself gives off from its fruit. As ethylene builds up in the fruit, the enzymes that turn starch into sugar are thought to become more active.
Fruits that keep ripening under the action of ethylene after harvest, like bananas, are called "climacteric" fruits. Apples, avocados and melons are also said to be types that ripen through ethylene. The saying that "one spoiled apple makes the fruit around it ripen faster" is thought to be true because the ethylene the apple gives off spreads to its surroundings.
It flips the switch for change from the inside, using a gas signal of its own making.
The Black Spots: Another Chemical Reaction
So what are the black spots on the peel? They are thought to be the trace of a chemical reaction separate from the change to sugar. As ripening goes on, the cells of the peel gradually become fragile, and substances inside the cells come into contact with one another.
When the peel cells break down, enzymes and polyphenols, which are normally kept in separate places, meet and react with oxygen in the air to make dark pigments. This is thought to be basically the same kind of reaction as the browning of a cut apple or banana left out in the air. The spots form only on the surface of the peel. They are a separate phenomenon from the sweetness of the fruit itself, but they serve as a sign of how far ripening has gone.
Bananas are often picked green so they do not spoil in transit, and then exposed to ethylene gas in special facilities just before they reach the shops to bring on ripening. Conversely, technologies have also been developed that slow the ripening and spoilage of fruit and vegetables by using storage methods and containers designed to remove ethylene.
What You Can Check Yourself
- Prepare two bananas that are still a little green
- Put one in a paper bag together with an apple at room temperature. Leave the other as it is, with nothing added, at room temperature
- Over several days, watch and compare the colour of the peel and how the spots increase each day
- Check that the banana in the paper bag with the apple ripens faster, because the ethylene cannot escape easily and its concentration also rises
Use this method when you want to speed up ripening. To make bananas last longer, it is said to help to keep them away from other fruit and put them somewhere with good airflow.
Summary
A banana's sweetness comes from ripening, a reaction in which starch inside the fruit is broken down into sugar. This reaction is thought to proceed on a signal from ethylene, a gas the banana makes itself. The black spots on the peel are the trace of a separate chemical reaction, caused by cells breaking down, and are used as a sign that ripening has advanced.
A banana with more black spots is said not to be spoiled, but to be at its most delicious moment.
Ripe fruit is not always the same story, though. Astringent persimmons become sweet in a slightly different way. For details, see our article on why astringent persimmons turn sweet when dried.
For those who want to know more – terms, numbers and links to textbooksFrom middle-school science to topics under active research, with the level of each part clearly marked
- Middle schoolCovered in middle-school science
- High schoolCovered in high-school "Basic Biology"
- High school+High-school "Biology", or advanced or sidebar material in textbooks
- UniversityUniversity-level content (plant physiology) not taught in high school
- ResearchTopics researchers are still investigating, not yet taught even at university as settled fact
Middle schoolTerms: Words around banana ripening
- Starch: A carbohydrate that plants store in their fruit. It is not very sweet.
- Enzyme: A substance in the body or in fruit that speeds up a particular chemical reaction.
- Ethylene: A gas given off by plants. It is a type of plant hormone that acts as a signal for fruit to ripen.
- Ripening: The gradual change of harvested fruit into a sweeter, softer state over time.
High schoolChecking with a formula: How does temperature change the speed of ripening?
For many reactions involving enzymes, a rule of thumb is known: every 10 °C rise in temperature roughly doubles the speed of the reaction (called the Q10 rule). Using this relationship, let's estimate the difference in ripening speed between the fridge and room temperature.
Speed ratio = 2 to the power of (temperature difference ÷ 10)
| Temperature difference | The difference between the two temperatures being compared (°C) |
| Speed ratio | A rough measure of how many times faster the reaction runs at the higher temperature than at the lower one |
| Temperature difference (°C) | 25 − 5 = 20 |
| How many 10 °C steps | 20 ÷ 10 = 2 |
| Multiply by 2, twice | 2 × 2 = 4 |
| Result | Room temperature is about 4 times faster than the fridge |
This is only a rough estimate, but it gives a general picture of why a banana in the fridge ripens much more slowly than at room temperature. Bananas are also sensitive to cold, and other effects such as the peel turning black and getting damaged come into play, so ripening is said not to follow this ratio simply.
* The Q10 rule is a rough relationship said to apply to many biological reactions. It does not apply exactly to every reaction.
High school+Climacteric fruit and the sudden rise in respiration
In fruit that ripens after harvest, like bananas, it is known that partway through ripening, the fruit's respiration (the amount of oxygen taken in and carbon dioxide given off) briefly shoots up. This phenomenon is called the "climacteric rise", and it is thought to occur in step with a sharp increase in ethylene. On the other hand, some fruits, such as citrus fruits and grapes, are called "non-climacteric" fruits, and are said to show almost no ethylene-driven ripening after harvest.
UniversityHow does ethylene pass its message inside cells?
In plant physiology, researchers study a signalling pathway in which ethylene binds to dedicated receptors (sensor-like molecules) in the cell membrane, changing how genes are switched on, so that starch-degrading enzymes and others are made one after another.
ResearchWhat is still unclear
- The full detail of the pathway from the ethylene receptor to the change in gene activity is still under study in plant physiology.
- Aiming to develop varieties that resist damage in transit and whose ideal eating time is easier to adjust, researchers are also working on genetic control of the speed of ripening itself.
- Detailed surveys also continue on how much the timing and spread of peel spots vary between individual fruits and growing conditions.
Even a single ordinary banana holds a rich subject, woven from plant hormones and enzyme reactions, that is still being researched.
Links to textbooks (by level)
| Level | Subject / unit | Where in this article |
|---|---|---|
| Middle school | Science: living things and substances | Basic terms: starch, enzyme, ethylene |
| High school | Basic Biology: enzyme reactions | Calculating temperature and reaction speed with the Q10 rule |
| High school+ | Biology: plant development and growth (advanced) | Climacteric fruit and the sudden rise in respiration |
| University | Plant physiology | Signalling through the ethylene receptor |
| Research | Plant physiology and agricultural science (under research) | Genetic control of ripening speed, variation in spot appearance |
- Explanations of enzyme reactions and reaction rates (the Q10 rule) in biology textbooks and reference materials.
- Explanations of banana ripening and the conversion of starch to sugar in food science materials.
- Research reviews on ethylene signalling in fruit ripening in the field of plant physiology.
- Explanations of the respiration rise in climacteric fruit in the field of agricultural science.
- Explanations of freshness-preserving technology based on ethylene control, in materials on food preservation technology.
* Ratios of reaction speed and estimates of the number of days can vary with variety, temperature, degree of ripeness and other conditions.
* This article is a general science explainer. When judging whether food has kept well, check the smell and texture as well as the appearance, and do not force yourself to eat it.