How Ethylene Induces Ripening in Fruits

Ethylene is a simple two-carbon gas (C₂H₄), but its impact on fruit ripening is enormous. Often called the “ripening hormone,” ethylene is the primary signal that tells climacteric fruits – bananas, tomatoes, apples, mangoes, avocados – to begin the transformation from hard, green, and starchy to soft, colourful, and sweet. Understanding how ethylene works as a ripening inducer is essential for anyone involved in postharvest management, food processing, or agricultural science.

What is ethylene and why does it matter? 🔗

Ethylene is a naturally occurring plant hormone produced by virtually all parts of higher plants. It is a colourless, flammable gas present in trace quantities, yet it triggers powerful physiological responses even at concentrations as low as 0.1 parts per million (ppm). As a fruit matures, it produces ethylene as a chemical signal to initiate the ripening cascade. After harvest, ethylene production continues to rise, which progressively reduces shelf life and storage potential.

The discovery of ethylene’s role stretches back centuries. Ancient Egyptians would slash figs to hasten ripening – a practice that, unknowingly, stimulated ethylene release through wounding. By the late 19th century, the Russian scientist Dimitry Neljubow identified ethylene as the active component in street lamp gas that caused unusual growth responses in pea seedlings. In 1910, Cousins hypothesised that gases released by oranges were responsible for ripening bananas during mixed commercial shipments, though those oranges were likely infected with fungi that produced the ethylene.

How ethylene is produced inside the fruit 🔗

Ethylene biosynthesis follows a well-characterised metabolic pathway. It begins with the amino acid methionine, which is converted to S-adenosyl-L-methionine (SAM) by SAM synthetase. SAM is then converted to 1-aminocyclopropane-1-carboxylic acid (ACC) by the enzyme ACC synthase (ACS). This step is considered the rate-limiting step – meaning it controls the overall speed of ethylene production. Finally, ACC is oxidised by ACC oxidase (ACO) to release ethylene gas, along with carbon dioxide and hydrogen cyanide as by-products.

Two key enzyme families – ACS and ACO – are encoded by multigene families, and their expression is regulated by developmental signals, environmental stress, and other hormones. Higher expression and activity of ACS and ACO genes lead directly to increased ethylene output, which in turn accelerates ripening.

System 1 and system 2 ethylene production 🔗

Ethylene production in fruits operates through two distinct systems. System 1 is active during normal vegetative growth and early fruit development. It produces low levels of ethylene and is auto-inhibitory – meaning ethylene suppresses its own further production at this stage. System 2 kicks in at the onset of ripening in climacteric fruits. It is characterised by massive autocatalytic ethylene production, where ethylene stimulates its own synthesis in a positive feedback loop. This burst of autocatalytic ethylene is what drives the rapid and coordinated changes associated with fruit ripening.

Climacteric vs non-climacteric fruits 🔗

Fruits are broadly divided into two categories based on their ripening behaviour: climacteric and non-climacteric.

Climacteric fruits – such as bananas, tomatoes, mangoes, apples, avocados, papayas, and peaches – show a dramatic spike in both ethylene production and respiration rate at the onset of ripening. This is called the respiratory climacteric. Because they produce autocatalytic ethylene, these fruits can continue to ripen after harvest. That is why bananas can be picked green and ripened later in controlled rooms.

Non-climacteric fruits – such as grapes, strawberries, cherries, citrus, and blueberries – do not exhibit a burst of ethylene or respiratory peak during ripening. They must be harvested at or near full ripeness because they will not ripen significantly after being detached from the plant. Their ripening is governed more by hormones like abscisic acid (ABA) rather than ethylene.

This classification has practical significance. Postharvest strategies differ sharply between the two groups. Climacteric fruits benefit from controlled ethylene exposure, while non-climacteric fruits require careful handling to preserve the quality they had at harvest.

Physiological changes triggered by ethylene 🔗

When ethylene initiates the ripening process, it sets off a series of interconnected physiological and biochemical changes that transform the fruit.

Increased respiration 🔗

The peak of ethylene production coincides with an increase in the respiration rate in climacteric fruits. This respiratory surge provides the metabolic energy needed to drive the various biochemical transformations of ripening, including starch-to-sugar conversion, acid degradation, and aroma compound synthesis. The rise in respiration also generates heat, which is an important consideration in ripening room management.

Chlorophyll breakdown and colour change 🔗

One of the most visible effects of ethylene is colour change. Ethylene promotes the degradation of chlorophyll – the green pigment in unripe fruit. As chlorophyll breaks down, other pigments that were previously masked become visible. In tomatoes, this reveals red lycopene; in bananas, yellow carotenoids and xanthophylls emerge. In tomato, although ethylene cannot force immature fruit to ripen immediately, exposure will shorten the “green life” and hasten the onset of colour development once the fruit reaches physiological maturity.

Softening and texture changes 🔗

Ethylene activates genes encoding cell wall-modifying enzymes such as polygalacturonase (PG), pectin methylesterase (PME), cellulase, and expansins. These enzymes break down pectin and other structural polysaccharides in cell walls, causing the fruit flesh to soften. In persimmon, ethylene-responsive transcription factors directly activate genes involved in cell wall modification, confirming that softening is tightly controlled by ethylene signalling.

Sugar accumulation and flavour development 🔗

As ripening progresses, complex carbohydrates (particularly starch) are broken down into simple sugars like glucose and fructose, making the fruit taste sweeter. Organic acid levels typically decrease, and volatile aroma compounds are synthesised, giving the fruit its characteristic flavour and scent. These flavour-related changes are a key reason why ethylene-treated fruits are commercially valuable – consumers prefer fruits that taste ripe.

Commercial use of ethylene for fruit ripening 🔗

In the global fruit trade, ethylene treatment is standard practice for several major commodities. Since climacteric fruits are typically harvested at a mature-green stage to withstand long-distance transport, they must be ripened artificially before reaching retail shelves.

Banana ripening rooms 🔗

Modern banana ripening rooms are designed to control temperature, humidity, and ethylene gas concentration, with proper ventilation and exhaust systems. Bananas arrive at these facilities still green – often after spending up to 20 days in refrigerated shipping containers at about 13°C, a temperature that suppresses natural ripening during transit.

Once in the ripening room, ethylene is applied at a concentration of 100-150 ppm for a minimum of 24 hours during the initial phase. The ripening process then takes roughly 4 to 8 days depending on the desired ripeness stage. Catalytic generators are the most common method for releasing ethylene in these rooms. The gas mixture used commercially is typically about 4% ethylene in nitrogen – a ratio that ensures effectiveness while keeping the mixture non-flammable.

Tomato and mango ripening 🔗

Similar principles apply to tomatoes and mangoes. Tomatoes picked at the mature-green stage are exposed to ethylene to achieve uniform red colour and consistent quality. Mangoes, which ripen unevenly on the tree, benefit from ethylene treatment that synchronises ripening and reduces the need for extensive sorting. The concentration of ethylene used is generally about 100 ppm for a 12 to 24 hour period for both bananas and mangoes.

Ethephon as an ethylene-releasing agent 🔗

Ethephon is an ethylene-releasing chemical that can be applied as a preharvest growth regulator to promote ripening. It decomposes within plant tissues to release ethylene directly. However, ethephon also accelerates fruit drop (abscission) and can negatively affect storability, so its use must be carefully timed.

Managing ethylene treatment: concentration, temperature, and humidity 🔗

Effective ethylene-induced ripening is not as simple as flooding a room with gas. Three critical factors must be carefully controlled.

Ethylene concentration 🔗

When ethylene concentration reaches 0.1-1.0 ppm, the ripening process in climacteric fruits becomes essentially irreversible. Commercial ripening rooms typically use much higher concentrations (100-150 ppm) to ensure rapid and uniform initiation. However, excessive ethylene can lead to over-ripening, off-flavours, and premature senescence. Precise dosing is essential.

Temperature control 🔗

Temperature directly influences both ripening speed and fruit quality. For bananas, the ideal pulp temperature during ripening is typically between 14°C and 18°C depending on the desired ripening schedule. If temperatures drop below about 13.3°C (56°F), chilling injury can occur, causing a dull, grey appearance on the peel. Conversely, excessively high temperatures can “cook” the fruit, resulting in a brown or orange peel and shortened shelf life. Bananas generate significant heat during ripening, so the refrigeration system must have enough capacity to maintain stable temperatures throughout the load.

Humidity and ventilation 🔗

For optimal ripening, humidity should be maintained between 85% and 95%. Low humidity causes moisture loss from the fruit, leading to shrivelling and weight loss. Adequate ventilation is equally critical because ripening fruits produce carbon dioxide alongside ethylene. Carbon dioxide levels above 1% (10,000 ppm) can actually retard the ripening process and impair quality, so rooms should be vented periodically – typically by opening doors for 20 minutes every 12 hours after the initial 24-hour ethylene exposure.

Controlling ethylene to extend shelf life 🔗

While ethylene is useful for inducing ripening, it becomes a problem when the goal is to preserve fruit during storage and transport. Several strategies exist to manage unwanted ethylene effects.

1-Methylcyclopropene (1-MCP) 🔗

1-MCP is a synthetic compound with a structure very similar to ethylene that blocks ethylene perception by binding to the fruit’s ethylene receptors. This delays postharvest ripening, maintains firmness, and extends storage life. It is widely used in apple storage, though it can sometimes enhance physiological disorders during prolonged storage.

Controlled atmosphere storage 🔗

Reducing oxygen levels and increasing carbon dioxide concentrations in storage facilities slows down ethylene biosynthesis and the fruit’s response to ethylene. This is because oxygen is a required substrate in the reaction catalysed by ACC oxidase, and carbon dioxide can compete with ethylene at the receptor site. Controlled atmosphere (CA) storage is a standard technology for apples, pears, and kiwifruit, among others.

Genetic approaches 🔗

Researchers have also explored genetic modification to delay ethylene-induced ripening. These approaches include suppressing ACC synthase gene expression using antisense technology, inserting an ACC deaminase gene from soil bacteria to divert ACC away from ethylene production, and suppressing ACC oxidase gene expression. Each method reduces the fruit’s ability to produce ethylene, thereby extending shelf life while maintaining desirable quality traits.

Risks of improper ethylene use 🔗

When ethylene is not managed correctly, the consequences can be significant. Over-exposure leads to premature senescence – the fruit ages too quickly, becoming mealy, off-flavoured, or prone to decay. Mixed storage of ethylene-producing fruits (like apples) with ethylene-sensitive produce (like lettuce or broccoli) can cause unintended spoilage. In some developing countries, the use of calcium carbide as a cheap ethylene substitute remains a concern. Calcium carbide releases acetylene when exposed to moisture, which mimics ethylene’s effects, but commercial calcium carbide contains traces of arsenic and phosphorus hydride, both of which are toxic. Its use is banned in most countries for fruit ripening.

Ethylene’s role beyond ripening 🔗

While ripening is the most commercially important function of ethylene, this hormone also plays broader roles in plant biology. It stimulates leaf and fruit abscission (shedding), promotes root hair growth, induces adventitious root formation during flooding, and contributes to senescence in flowers. In agriculture, ethylene is also used to induce flowering in pineapple and to promote latex flow in rubber trees.

What do you think? Given how central ethylene is to the modern fruit supply chain, how might advances in ethylene management change the way fresh produce reaches your table in the future? And should there be stricter global regulations on artificial ripening agents to protect consumer health?

References

1. https://extension.umd.edu/resource/ethylene-and-regulation-fruit-ripening
2. https://www.mdpi.com/2311-7524/10/8/840
3. https://www.frontiersin.org/journals/plant-science/articles/10.3389/fpls.2024.1475496/full
4. https://pmc.ncbi.nlm.nih.gov/articles/PMC11579711/
5. https://pmc.ncbi.nlm.nih.gov/articles/PMC3550874/
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7. https://www.sciencedirect.com/science/article/abs/pii/S0925521420301691
8. https://pmc.ncbi.nlm.nih.gov/articles/PMC6521425/
9. https://www.catalyticgenerators.com/banana-ripening
10. https://www.isaaa.org/resources/publications/pocketk/12/default.asp