Fruits are an integral part of the human diet. There is nothing more delicious for fruit lovers than a perfectly ripe fruit. It’s the peak window of time when fruit is most enjoyable, bursting with flavor and a pleasing texture. But eating fruits when fully ripe is not only about taste but also about health!
Only ripened fruits provide the essential nutritional value we need: Fruits are far less nutritious and often contain toxic compounds when picked before the plant’s seed is mature and ready to be distributed!
The importance of fruits having reached full ripeness before consumption cannot be emphasized enough in nutrition and health! By eating underripe fruits instead of ripe fruits, you are missing out on essential nutrients while potentially getting a load of plant defense compounds into your system.
Fruits signal when they are ready – and healthy!
Fruits are the main food of specialized fruit-eating animals – frugivores. This includes humans! Taste and smell are two important adaptive instincts that indicate the nutritional value of the fruit to the animal while warning it of toxicity!
The importance of fruit ripening to human nutrition is reflected in the communication and interaction we have with our food source. Fruits unmistakable signal to our taste buds when they are ready… or not! Fruits are appealing and delicious at their nutritional peak, but off-putting with unpalatable flavors and weird textures when not ripe yet.
And those features are not a coincidence but clear communication: it is when fruits are ripe and ready that they are most nutritious, rich in antioxidants, and loaded with other beneficial metabolites. On the other hand, plants dissuade us from eating their unripe fruit with acidic flavors, toxic alkaloids, anti-nutrients, and other unfriendly compounds. To understand this complex relationship, let’s look at the evolutionary background:
Humans and fruits evolve together
The plant give us tasty fruits in exchange of seed dispersion: The whole point of the delicious fruit surrounding the seed is to employ fruit-eating creatures (i.e., birds and primates – including humans) to disperse them.
Fruits signal and communicate their ripeness with a change in color and appealing smell. Those changes attract the “right” fruit consumer and ultimately promote seed transportation. The animals gain the tasty, energy-rich fruit flesh in exchange.
But why? Plants ensure that the fruit is only eaten when the seed is ready! For this reason the symbiotic co-evolution of fruits and fruit-eater is not all just roses and sunshine:
Plants protect their fruit from being eaten before seed maturation with toxic and off-putting (warning) components that keep the fruit eater away until the seed has matured and is ready to germinate. This is vital for a plant’s reproductive success and fitness: if fruits are picked and dispersed with an immature seed, the embryo cannot sprout and grow. This scenario is a substantial reproductive disadvantage and energy loss for the “mother plant,” which has invested much of its energy into flowering and fruit building.
Thus, fruits ripen and become edible only when their seeds have matured. The plant’s tools to regulate the timing of seed dispersal are potent chemicals. As a result, fruits can be toxic to their consumer beforethey ripen but become non-toxic and edible after the maturation of the seed:
Chemical changes during fruit ripening
Why is the right timing of picking fruits such as big fuzz for our health? Ripe fruits are highly nutritious and very low in anti-nutrients – a perfect human food. However, the chemistry of unripe fruit is not as friendly!
Turning more nutritious and less toxic!
To comprehend the implication of ripeness of a fruit on nutrition and health we can look at the chemical changes that take place within a fruit during ripening: Taste, smell and color molecules, which either attract or repel animals are mostly secondary metabolites, a powerful chemical family. Changes in acidity and sweetness are regulated by sugar assimilation and acid breakdown.
The ripening process begins once the fruit is fully mature and the process of cell division has stopped. It occurs as a result of hormones which activate the genes for the production of various enzymes that break down components in the mature fruit! During the ripening process fruits are transformed into the foods we know and love. They go from unpleasant, acidic and potentially toxic to juicy, attractive, sweet, and flavorful. Those are all chemical changes!
Overview of chemical changes during the ripening process
Lets look what vitamins and minerals are increased, while unfavorable compounds such as anti-nutrients and defense toxins are decreased in the process of ripening:
| Increases during ripening | Decreases during ripening |
| Energy: | Defense Compounds (toxins): |
| Glucose | cyanogenic glycosides (cyanide) |
| Fructose | Alkaloids |
| Sucrose | Antinutrients: |
| Vitamins: | Tannins |
| Vitamin C | Taste: |
| Beta Carotene (Vitamin A precursor) | Malate (acidic) |
| Taste: | Citrate (acidic) |
| simple sugars (sweet) | Color: |
| Flavonoids (flavors) | Chlorophyll (green) |
| Color: | |
| Anthocyanin (blue, red, purple) | |
| Carotenoids (yellow, orange, red, purple) |
One study on ripening blackberries found that the mineral content (Potassium, Calcium, Zinc, Manganese and Copper) stays more or less the same. Only the magnesium contain lowers around one third because of the drop in green colored chlorophyll (which contains magnesium) and iron, which increases during ripening.
Taste and color compounds changes in the fruit ripening process
Humans have taste receptors to detect and love sweetness for a good reason! The sweet taste in fruits is attributed to simple sugars, which are mobilized from the plant to the fruit. This valuable energy reward is exchanged for seed-dispersing by the animal. Sweet tastes so good for frugivores like humans because it nourishes us.
Fruit go from acidic to sweet: Complex sugars found within the fruit are broken down by enzymes into simple sugars like glucose and fructose. This transforms these otherwise bland complex sugars into the sweet simple sugars our taste buds love. This process also makes the fruit soft as the complex sugars are no longer playing their role as structural components in the cell wall. The insides of the plant cells also become released making the fruit juicy and its nutrition more bioavailable. Concurrently, fruit acidity is decreasing, because the concentration of organic acids such as malate and citrate drops.
From green to colorful: Color is an important feature of signaling ripeness in fruit. The change in color typically occurs as chlorophyll, which is responsible for the green color, breaks down and other pigments are produced. Pigments produced in ripe fruit include carotenoids, flavonoids, and betalains. Many of these, such as the flavonoid called anthocyanin, are renowned antioxidants. Color in fruit also serves to get the fruit eaters attention, as it is easier detectable between green leaves. To be able to do the best job possible in spotting tasty, ripe fruits, frugivore species have evolved specialized color vision!
Decreased toxicity in ripe fruits
During fruit development, plants often synthesize many different types of alkaloids, anti-nutrients, acids, and other distasteful compounds within the fruit. The latter is why many immature and unripe fruits have bitter and astringent flavors. These compounds may actually be toxic and have various health consequences if consumed in excess. Plants purposefully incorporate these into their immature fruit to prevent you from eating their fruits before they are ready.
Bananas, plums, citrus, and countless other fruits contain harmful compounds before they are fully ripe. Some of these may cause negligible to minor symptoms, while some can be more serious. Due to co-evolution, humans have developed a higher tolerance to the “unfriendly” compounds (and developed more effective warning signals) of fruits they have consumed regularly during evolutionary timelines.
However, once ripe, fruits are among the foods with the least anti-nutrients and toxins!
Be aware though of fruits, which are relatively new in our diet, which can be more toxic to humans! They can have more severe effects, which we sometimes only feel when it is already too late: For example, green tomatoes (a fruit of the toxin-rich nightshade family) contain various toxic compounds. The most notorious of these are the alkaloids Tomatine and Solanine. Poisoning by these compounds can cause headaches, diarrhea, cramping, dizziness, eczema, and countless other health issues. And even worse, the Ackee fruit, which is famous in Jamaican cuisine, is deadly toxic until ripe and results in deaths every year.
More sweet carbohydrate and energy in ripe fruits
Fruit development precedes the ripening process. This begins after pollination and involves the physiological development of a fruit from a flower. Most fleshy fruits originate from tissues that originally composed the female reproductive organs of the flower. Fertilized ovules develop into seeds and in most fruit it is the ovary walls that become the fleshy parts we consume. During this process, plants sink all of their energy into the development of the fruit: they move sugars produced via photosynthesis through their vascular system and finally into the fruit.
Some plants may even mobilize sugars stored in their roots and other plant tissues. These sugars will later on provide the energetic nutrition for its consumer (read here, why simple sugars in fruits are good for us). Once they reach the developing fruit, the sugars are incorporated into the rapidly dividing cells. Here simple sugars (which taste sweet) like sucrose and glucose are transformed into complex sugars like cellulose and hemicellulose (which do not taste sweet). These compose various cellular components such as the cell walls.
The complex sugars in unripe fruit are technically full of energy, but we are incapable of breaking them down. This means if we eat them, they simply act as a dietary fiber and don’t contribute any calories to our diet. Our taste buds know this, and despite them technically being sugars, they are bland and without a hint of sweetness. Only simple sugars taste sweet.
Unripe fruits will not nourish you with sweet, simple carbs as ripe fruits do because the carbohydrates in unripe fruits are still in their form as cellulose and hemicellulose – complex carbs that can hardly be broken down and used for energy.
Eating sweet and ripe fruits is imperative for health!
Conclusion
The chemical changes during ripening are not only important for taste: only truly ripe fruits offer full nutritional benefits. Also, not fully ripe fruits can contain harmful substances, which the plants produces to protect its immature seeds. So, eat your fruits ripe! And, as it is becoming more and more difficult to access ripe fruits straight from nature, at least know which ones ripen after being picked (see list of fruits that ripen or don’t ripen after harvest). I hope you live in a tropical place with loads of colorful, divine-smelling, and sweet tasty fruits, though…
This article was written in conjunction with a biologist and botany expert.
References
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