This is an old fruit taste mystery that was solved by botanists and scientists a long time ago.
You might have been wondering why an unripe banana, mango, passion fruit, pawpaw taste sour whereas their ripe counterparts taste sweet.
I also pondered about this for a long time until I came across it in one of my biology classes.
I must say it was as interesting as it can get and I loved it and made sure I had everything about it on my fingertips.
The problem is, the scientific jargon used to explain the phenomenon is just too much for someone with little interest in biochemistry.
The main reason for the change in fruit taste from sour to sweet is the need for plants to naturally disperse their seeds for reproduction and continuity.
Let me break it down a little here.
Why do Fruits Ripen?
You might be thinking that fruits just do us a favor by ripening so that we can devour them.
In fact, you are the one doing plants a favor.
It is a clever evolutionary adaptation tactic by plants that I touched on in one of my previous articles here.
Just to recap on this magical phenomenon, plants, unlike animals spend all their life in one fixed location.
If they want to procreate and give their offspring a better chance of survival, they would need to disperse their seeds at a distant location where the offspring will not be in direct competition with the parent for essential growth commodities such as water and sunlight.
This is where you come in.
They will produce a fruit and make it so undesirable (unattractive color, sour taste, firm texture) in the initial stages as the seeds grow in them.
When the seeds are mature and have developed a hard outer cover that is indigestible, the ripening process I have described above will kick in.
The fruits thus ripen by brightening in color, becoming sweet and aromatic, and softening as a mechanism to attract you and other animals to eat them.
You will then ingest the seeds and since they are well protected, they will pass down and out of your digestive system through poop.
This will hopefully be in a different location where the seed can then germinate without competition from the parent plant.
Very clever and interesting.
So, when you eat a ripe fruit, you are in a symbiotic relationship with the plant since you fill your body with nutrients and contribute to generational continuity.
In this article, I will attempt to simplify the biochemical processes that take place within the fruits to achieve the changes in fruit taste, color, and texture.
Let’s use a banana as our case study here because it is a universal fruit that almost everyone is familiar with.
Why are Unripe Bananas Green, Firm, and Sour?
The reason I used a banana is because it represents the changes in color, texture, and taste that are synonymous with most known fruits during ripening.
In summary, the green color of an unripe banana is due to the presence of a common pigment called chlorophyll which gives plants their signature green color.
The firm texture is due to the structural composition of their cell walls consisting of compounds known as pectin and cellulose.
The sourness, on the other hand, is brought about by the presence of starch and organic acids in the unripe fruit.
- Why the Green Color in Unripe Fruits?
As mentioned above, the green color of unripe fruits has everything to do with chlorophyll pigment.
The chlorophyll pigments are usually housed in special areas in a plant cell known as the chloroplast.
Unripe fruits contain large amounts of chlorophyll pigments which masks other color pigments in fruits.
These other coloring pigments are known as anthocyanin and are usually associated with the different ripened fruit colors we know.
In our case, the yellow coloring pigments that you see in ripe bananas are always there but are suppressed by the dominant green chlorophyll pigments before ripening.
- Why the Firm Fruit Texture?
The firm texture of unripe fruits can be attributed to the presence of substances known as polysaccharides in fruit cells.
The main polysaccharides in banana fruit cell walls are Pectin and Cellulose.
If you can recall, Pectin is a compound I talked about in another article here that makes it possible for us to make jams and jellies from fruits.
These compounds are tough and thus give most parts of a plant (including unripe fruits) their structural toughness.
- Why the Sour Taste in Unripe Fruits?
Just like most fruits, green bananas are packed with starch in their flesh.
This starch partially provides food to the developing seeds inside the core and also forms a protective layer that shields the seeds from external destruction.
Starch is an almost tasteless aroma-less compound similar to the taste you get when you eat a raw Irish potato.
Unripe fruits also contain high levels of organic acids and some phenolic compounds such as tannins that combine to give unripe fruits their characteristic sour taste.
The proportion of all these compounds in a fruit such as a banana is what really matters when it comes to the taste and aroma.
This is what brings us to the next big question.
What Makes Ripe Fruits Sweeter, Colorful, and Softer?
We will still refer to our banana case study here.
Just as we have seen earlier, fruit ripening is a well-coordinated biochemical process that involves several distinct aspects.
It involves a variety of hormonal and enzymatic activities that are way too complex to explain in a short blog post like this one.
That is why I will just touch on the basics and simplify it as much as I can to give you a glimpse of the complex activities that go on inside that banana as you wait for it to ripen.
- Fruit Ripening and Color Changes
So, what makes a banana turn its color from green to yellow after ripening?
Well, if you remember what I said about chlorophyll and anthocyanin earlier, you can already guess what happens here.
The yellow anthocyanin pigments are just sited there on the fruit peel waiting for their turn to show up to the world.
Before ripening, they are masked by the large quantities of green chlorophyll pigments that are dominant at that stage.
During ripening, enzymes disintegrate the chlorophyll on the banana skin to expose the hidden yellow anthocyanin.
The attractive color is meant to attract you and other animals to eat the fruit and its seeds so that you can disperse the seeds unknowingly.
- Why Fruits Soften on Ripening
Remember the tough pectin and cellulose associated with unripe fruits?
Yes, these also need to go during ripening.
When our banana is mature enough, it will produce two enzymes called pectinase and cellulase that will break down pectin and cellulose.
This action will soften the fruit since the tough cell walls will be no more; just like the famous biblical walls of Jericho.
- What Makes Ripe Fruits Sweet?
This is where things get a little bit interesting and technical.
In summary, the tasteless starch and sour organic acids have to be taken care of.
Enters hormone regulator Ethylene Gas.
You see, the starch and organic acids were dominating the unripe fruit because they were being shielded by some anti-ripening hormones such as Auxins, Gibberellins, Cytokinins, and Indole Acetic Acid.
The onset of ripening leads to production of more ethylene gas that suppresses the other anti-ripening hormones.
This drastically reduces the organic acids in our bananas and gives room to enzymes that break down the starch into simple sugars.
Simple sugars such as glucose, fructose, and sucrose are known to be sweet, thus giving a ripe banana fruit its characteristic sweetness.
Ethylene gas is the reason why one ripe fruit in a basket will accelerate the ripening of other unripe fruits in the same basket.
The sweet fruit taste is a result of ethylene balancing the ratio of acid and sugar in the fruit in what we call Degree Brix Concentration in food science.
Whereas an unripe banana will contain more than 40% starch and trace sugars, a ripe banana will have about 8% starch and more than 90% sugar as seen in this nutritional profile.
In conclusion, the process of fruit maturation and ripening is one of those natural phenomena that everyone needs to understand.
This is important because we are the ones with brains and in control of life on our planet.
By understanding such things, we will appreciate how important plants are to our planet earth.
I hope I have burst your bubble in a good way.