Although it hides many wonders, that of maturation is notable because we witness it frequently: a hard, green, bitter and unappetizing product becomes in an astonishing way into an appetizing delicacy, which offers us the richness of its flavor, its sweetness, its juiciness and aromas. Citrus fruits that make you think of Andalusian patios filled with orange trees, smooth skins that inspire poets, and striking and varied colors.
But before unraveling ripening, what is a fruit?
From a botanical point of view, a fruit is the matured ovary of a flowering plant that houses the seed or seeds with which the plant will reproduce. The ovary can be hard and dry, as in the case of walnuts, peanuts and other products we call dried fruits, or it can be fleshy and succulent, as is the case with apples, tomatoes, bananas or avocados. These last examples also tell us that fruits can be sweet or not.
Once fertilized, the ovary of a plant undergoes a series of transformations to protect the ovules it contains, which when mature are the seeds that will give rise to new plants. Thus, a small yellow flower the size of a thumb can develop into a huge, juicy watermelon. It is reasonable to ask then what are the reasons why plants have evolved in such a way that they dedicate such an enormous amount of resources (water, photosynthesis, nutrients, chemical processes) to create that fruit.
The answer is twofold. The fruit is a seed protection and dispersal strategy aimed at ensuring that as many of them as possible germinate and flower, perpetuating the species.
It must be clarified that when talking about ‘strategy’ we are using a human metaphor to explain a natural evolutionary process that, unlike the strategies of our species, is neither voluntary nor conscious. Over millions of years, plants that, through natural variation or mutations, have better fruits (that is, ovaries that better protect and disperse seeds when they mature) have reproduced with a little more success… and those accumulated successes go leading to the results we see today, as if they were a conscious goal.
Having clarified that, and speaking only of non-dried fruits, this strategy consists of having certain nutrients or being tasty or sweet, attracting animals with varied colors that indicate their maturity and with seductive aromas that tell different species, such as Alice in Wonderland cake: “Eat me.” The ‘business deal’ that is involuntarily made between the plant and the animal could be summarized like this (only as a metaphor): I give you appetizing and nutritious food and you, in return, take my seeds away from here and throw them away or, better yet, , you deposit it with your feces as food so that they can germinate and develop.
Animals, be they birds or mammals, are thus the vehicles for the seeds to carry them further and further, with a ticket paid for by the flesh of the fruit. This result, evolutionarily useful for the survival of the species, makes the energy cost of fruit production a ‘good deal’ for the plant that produces it.
Fruits as such were the logical result of the evolution of a type of plant, angiosperms, which appeared on our planet in the late Cretaceous period, between 100 and 125 million years ago, and fruits probably began to evolve at the same time. : the best protected seeds were less likely to be digested by the animals that ate them, so dry fruits with hard protection appeared first and, later, fruits that surrounded seeds that were difficult to digest, to disperse the seeds without were part of the food.
Every fruit, then, is originally a flower or, more precisely, the ovary in the center of the flower. The different layers of the ovary (which is a structure formed by modified leaves called carpels) give rise to the tissues of the fruit: the epicarp, which is the peel or skin, the mesocarp, which is the flesh of the fruit itself, and the endocarp, that protects the seeds.
Since it is not good for the survival of the seeds or embryos of the plant to be devoured in their early stages of development, the developing mesocarp that forms the fruit is not initially attractive to predators, as anyone who has sunk their teeth into it knows. in an unripe fruit. It is only when the seeds are ready to germinate that the fruit takes on its desirable characteristics for those of us who eat it. When it matures.
And the ripening process is a complex series of coordinated events that profoundly transform the fruit in all its sensory and nutritional aspects.
Let’s start with the softness of the fruit. An unmistakable sign of an unripe fruit is precisely that it is hard, a characteristic that we frequently use to tactilely assess whether the avocado we are buying is ready to be part of our meal. The hardness of the fruit is given by the thickness of the walls of its cells, walls formed mainly of layers of polysaccharides, especially pectin. For ripening, enzymes come into action that allow pectin to dissolve in water.
The nutrients in fruits accumulate mainly in the form of starch, another polysaccharide that, when affected by enzymes during ripening, breaks down into shorter, water-soluble molecules such as fructose, glucose and sucrose, giving the fruit its sweetness or its characteristic flavor. At the same time, its acid content decreases.
The enzyme-mediated process also causes essential oils to be produced in the fruit in well-defined mixtures that give each fruit its characteristic flavor. At the same time, its generally green color due to chlorophyll changes as it is replaced by pigments called carotenoids, which range from yellow to red and are also precursors of vitamin A.
A complex succession of phenomena developed over more than 100 million years that result in our enjoyment… a word that means, precisely, ‘to bring out the fruit’ or ‘to obtain the benefit’ of something.
Your plastic and your fruit
The enzyme that triggers ripening in fruits is ethylene, a colorless, odorless gas with which polyethylene, a common plastic, is made. As the seeds mature, the plants produce ethylene, but it can also be applied externally, as is done with many fruits that are picked green and ripened ‘in chamber’ with ethylene. We can do the same at home by putting unripe fruits next to other ripe ones such as bananas, which therefore emit ethylene.