There are no words to describe the moment when eating a rich, juicy fruit. The scent, the taste, the whole experience when biting into ripe fruit is pure satisfaction.

But to ensure that satisfaction, the food industry needs to know how fruits ripen. And this knowledge has helped by finding strategies to control and manipulate fruit ripening to extend their shelf-life.

Fruit ripening involves various physiological, biochemical, and developmental changes that lead to a fruit moving from a green-unripe stage to a colored (usually) ripe stage.

This article will teach how fruits ripen, types of fruits, how industry controls the fruit ripening artificially, and what happens when animals eat overripe fruits.

In this article:

How fruit develops

Physical and chemical changes during fruit ripening

Physical changes

Chemical changes

Types of fruits

Climacteric fruits

Non-climacteric fruits

Ways to manipulate the fruit ripening

Regulation of organic fruit ripening

Effects of eating overripe fruits


How fruit develops

Before discussing how fruits ripen, let me briefly explain fruit formation.

A fruit is regarded as a plant organ containing seeds. After fertilization, when male sex cells called sperm cells fuse with the female sex cell (egg) inside an ovule in the plant, the ovary of a flower develops a fruit

how a fruit develops when a plant sperm cell fertilizes a plant egg cell in the plant ovary. the plant ovary enlarges and a fruit develops

The ovary in a plant is the female reproductive part of the flower. Here, the pollen grains are deposited by bees or other animals, even with wind. And then, pollen releases sperm cells that later fertilize the flower's egg cell. The fusion of both male and female cells forms the zygote.

The zygote leads to embryo formation. An embryo is like a baby plant that grows inside a seed. After fertilization, the ovary grows bigger, forming fruit, and the ovules develop into seeds within the ovary. The combination of these parts forms the fruit.

Physical and chemical changes during fruit ripening

When fruits ripen, they become more attractive to fruit-loving species, such as birds, bats, and people. Therefore, ripening facilitates seed dispersion and further successful propagation of the plants in other environments and the generation of new plants.

Once the fruit is formed, different physical and chemical changes occur where fruit becomes ripe. In this section, we will explain these physical and chemical changes that occur during ripening.

Physical changes

Three main physical changes are detected when fruit ripens. The first change is associated with sweetness. Although this is an invisible change, it is recognized once a consumer eats the fruit. Commonly, when fruits ripen, they taste sweet because of the accumulation of sugars.

The second physical change is related to coloration. Usually, unripe fruits are green, while ripened fruits are colored (depending on fruit type). Orange, reddish, purple, and yellow fruits, for example, indicate ripened fruits that are ready to eat.

Color change occurs because pigments inside fruits are produced while others are degraded.

Finally, when fruits ripen, they become soft. The softness is related to changes in the cell wall of plants cells (explained in the chemical changes section below).

physical changes that occur as fruit develops

Chemical changes

The above three physical changes are explained by the chemical changes occurring inside fruits. Let me explain each of them.

1. Sweetness:

Sugars accumulate as fruit ripens. This accumulation is due to a chemical modification that leads to the sweet taste of fruit.

But these sugars are commonly monosaccharides (sucrose, glucose, and fructose) generated by the breakdown of starch. Starch is a large, complex molecule formed by the union of many small sugar molecules (monosaccharides). Starch is converted into simple sugars by enzymes such as α-amylase, β-amylase, and α-glucosidase.

These simple sugars make fruit more attractive and prepare the seeds for future germination once the fruit is harvested.

starch breakdown leads to ripened fruits becoming sweeter. Illustration of starch breakdown into simple sugars.

2. Coloration change:

Coloration is the most evident change when fruit ripens. Unripe fruits are usually green, which is due to the chlorophyll content.

Once fruit matures, chlorophyll is degraded, and other pigments such as anthocyanins and carotenoids are revealed.

The type of coloration depends on the plant species. For instance, in some fruits like tomatoes, carotenoids are responsible for the reddish and orange colors. In other fruits like grapes and berries, the coloration is given by the anthocyanin pigments.

Different enzymes are involved in producing each of these pigments, like phytoene synthase and anthocyanidin synthase for carotenoids and anthocyanins, respectively.

3. Softness:

Softness is critical when picking fruits. If fruit looks too soft, it is an indication of bruising.

Chemically, soft fruit is associated with the status of the cell walls. The plant cell wall is an external layer covering the cell membranes, protecting and giving form to the plant cells. Polysaccharides make the cell wall. These are molecules composed of many small sugars that form branched or linear chains.

how fruit softens as it becomes ripe - the cell wall illustration

Cellulose, hemicellulose, and pectin are the main polysaccharides making the plant cell wall.

Different enzymes induce cell wall breakdown, making the fruit softer.

Among these enzymes are cellulases, hemicellulases, and pectinases. Furthermore, fruits start to lose water by transpiration affecting the turgor pressure. Turgor pressure is the outward pressure caused by the water inside the cells. It is similar to having a ball filled with air, and then is slowly deflated.

illustration of turgor pressor in plants

chemical changes occuring when fruits ripen - fruits get softer, colorful and sweeter.

Types of fruits

According to their ripening behavior, fruits are traditionally classified into climacteric and non-climacteric.

Climacteric fruits

Climacteric fruits are fruits that continue ripening after harvesting. These fruits emit ethylene during the ripening process and present an increased respiration rate.

Ethylene is a phytohormone (plant hormone) that can trigger the initiation of ripening and senescence (the aging process in plants). These fruits are usually harvested green, and they are allowed to mature while in transit or close to commercialization areas. We find apples, avocados, bananas, tomatoes, peaches, and pears among climacteric fruits.

Non-climacteric fruits

Non-climacteric fruits stop ripening once they are harvested. These fruits likely produce low amounts of ethylene or are considered ethylene-independent.

They do not produce ethylene during the ripening process and therefore do not have an increased respiration rate. These fruits are usually harvested when ripe, and their consumption is suggested to be soon.

Non-climacteric fruits are delicate fruits for commercialization purposes. Among these fruits, we find cherries, cucumbers, grapes, watermelons, and citrus fruit like oranges and lemons.

illustration of the 2 types of plants: climacteric and nonclimacteric

Although this fruit classification is commonly accepted, new studies are having trouble classifying certain fruits.

For instance, melons are classified as non-climacteric, however, some melons and reticulatus produce elevated levels of ethylene, ripen rapidly, and have a short shelf-life (Paul et al 2012).

Another contradictory example is guava. Although guava is classified as climacteric, some authors reported guava's climacteric or non-climacteric behavior depends on the variety. For example, the Pedro Sato guava shows the maximal rate of respiration once the fruit is ripened; therefore, it contradicts the climacteric behavior for guava (Azzolini et al. 2005).

Ways to manipulate the fruit ripening

Sales losses from overripe fruits are a significant concern in the food industry. The knowledge about how fruits ripen is now applied at an industrial scale to overcome this issue.

Main efforts have been focused on controlling banana ripening because it is a highly consumed fruit in Europe and the USA.

Thanks to the understanding that the ethylene hormone acts as an enhancer of fruit ripening, ethylene is sold commercially as ethylene gas in pressurized cans to promote fruit ripening.

However, the concentration of ethylene required for different fruits to enhance maturation is different. For instance, some fruits require between 24-48 hours (about 2 days) to ripen. Using ethylene gas is common in developed countries because the generation of gas by catalytic generators is expensive.

Acetylene is comparable to ethylene. It is a product of the hydrolyzation of calcium carbide. Commercial calcium carbide is hazardous because it contains arsenic and phosphorous hydride traces. Therefore, acetylene is also toxic (not used in the USA).

Another product used in other developing countries to induce ripening artificially is Ethephon (2-chloroethylphosphonic acid). Ethephon is a compound that penetrates fruit and breaks down into ethylene.

However, it has been reported to have hepatotoxic potential, because ethephon is an organophosphorus compound, which is rapidly absorbed in the gut and can damage the liver. Furthermore, ethephon has been also used as an antibiotic against Streptomyces.

Ethylene is the only product widely accepted to induce fruit ripening safely from all these compounds.

Regulation of organic fruit ripening

According to the Code of Federal Regulations and the United States’ National Organic Standards Board (NOSB), the use of ethylene is allowed for post-harvest ripening of tropical fruit and degreening of citrus.

The issue with the rule is the term “tropical fruit” is not well defined, and some producers can interpret their organic fruits as “tropical.” However, it is advisable for any artificial ripening of organic fruits to make a request to a certifier to review the use for compliance, for instance the USDA could help in the process.

At this time, currently in the USA ethylene is allowed for use on organic bananas, mangoes, avocados (Catalytic Generators, 2021).

Effects of eating overripe fruits

An interesting effect of letting fruits get overripe is they ferment. Thanks to the rich concentration of simple sugars like sucrose, glucose, and fructose, microorganisms eat them and produce ethanol as a byproduct.

Some wild animals love to eat the marula (Sclerocarya birrea) fruit. It is a fruit native to Southern Africa, the Sudano-Sahelian range of West Africa, the savanna woodlands of East Africa, and Madagascar.

What animals do not know is that when they eat fermented fruits, they can get a little drunk. A funny video about it can be found here.

In the United States, people report flocks of cedar waxwing birds that appear dead on the side of the road. However, they are not dead. They have just passed out from eating so many fermented berries.

Cedar waxwings eating berries (not necessarily fermented berries).

Image: Cedar waxwings eating berries (not necessarily fermented berries).


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