What is Ethylene Gas? - How it Affects Fruits and Vegetables
We’ve all at one point or another heard the idiom “One bad apple, ruins the bunch”, but not everyone knows that this particular saying is directly related to the effect that one overripe apple can have on the other apples, as it continues to emit ethylene into the surrounding atmosphere, accelerating the ripening and senescence of each other apple in the bunch as a result. So, what exactly is ethylene and how is it being utilised to maximise the outgoings of fruits & vegetables across global supply chains?
What is Ethylene?
Ethylene is the most commercially produced organic compound in the world and is used in many industrial applications. Some of these industrial applications for ethylene include; ethylene oxide for the sterilising of medical devices, polyethylene as a more durable plastic over polyvinyl chloride and polypropylene, ethylene glycol for creating polyester fibres in fabric, and many other applications but in this particular case, we’ll look at its use within the fresh produce industry.
A colourless and flammable gas, ethylene (C2H4) is a small hydrocarbon natural gas and a naturally occurring plant hormone produced by plant cells that aid in the ripening and ageing process of fresh products. Fruits such as apples, bananas, and pears emit a greater amount of ethylene gas, which as a result affects their ripening process. Other fruits, like cherries or blueberries, have very little ethylene production, therefore, it does not greatly impact the ripening process.Ethylene is a hydrocarbon gas, with four hydrogen atoms bound to a pair of carbon atoms that are connected by a double bond.
While ethylene in plants is the way that fruits and vegetables tend to ripen in nature, due to ethylene biosynthesis being so important for the ripening of some fruits and vegetables within the food supply chain, it is sometimes used by commercial agribusinesses to speed up the ripening process of crops.
What are its effects on Fruits and Vegetables?
While ethylene is a valuable agent for initiating the ripening process in various fruits and vegetables, it can also have detrimental effects on many plant varieties, flowers, and other produce by accelerating aging and diminishing product quality and shelf life. The extent of damage is influenced by factors such as ethylene concentration, exposure duration, and product temperature.
Fruits and vegetables play a crucial role in human nutrition, and their economic value is often impacted by the relatively short ripening period and limited postharvest life. Over-ripening can lead to excessive softening, causing spoilage and damage during shipping and handling. Slowing the ripening process and preserving quality extend the storage and shelf life of fresh produce, benefiting both growers, who reduce postharvest losses, and consumers, who enjoy prolonged freshness.
Understanding the intricate relationship between ethylene, ethylene producers, and respiration rates during fruit ripening is essential for effectively managing harvesting, storage, and distribution processes. When exposed to fruits and vegetables, ethylene binds to their receptors, stimulating the maturing stages and influencing flavor, vitamins, firmness, aroma, texture, and color. However, excessive ethylene exposure before reaching destinations can lead to overripeness, reduced quality, and increased Volatile Organic Compounds (VOCs) throughout the stock.
A significant contributor to global food waste is the overexposure of fruits and vegetables to ethylene during ripening and storage, accounting for 45-50% of all harvested fresh products lost or wasted in the food supply chain annually (approximately 1.3 billion tonnes, equivalent to $680 billion USD). Strategies to mitigate ethylene effects include low-temperature storage, ventilation, ethylene removal using potassium permanganate or ozone, storage in high CO2 atmospheres, and air scrubber systems for storage cleansing. Adverse reactions to ethylene include chlorophyll loss, abortion of plant foliage and stems, stem shortening, and stem bending (epinasty), underscoring the importance of careful ethylene management in preserving the quality and longevity of fresh produce.
Conventional Practices are Harmful to Fruits and Vegetables
Any closed environment, such as truck trailers, shipping containers, warehouses, and cold rooms, will have a similar effect resulting in an increased concentration of ethylene.At the latter part of post-harvest, artificial fruit ripening by using ethylene gas is a general practice as it ensures that the product reaches the retail outlets with a degree of ripeness, which brings out its best in terms of taste, colour, texture, and nutritional value. One of the most common examples is the ‘forced’ ripening of bananas during high-demand periods.However, these conventional methods adopted by most warehouses and cold stores of lowering the temperature and humidity and using gases like nitrogen and sulphur increase the shelf life but have several drawbacks.
The use of toxic gases
Dangerous for human health
These gases change the natural properties of fresh items as well
Refrigeration and humidity control slow decay but are not enough to halt the production of ethylene gas in cold stores and warehouses. Even a small amount of ethylene gas during storage is enough to hasten the process of decay of most fresh products. This makes ethylene control absolutely necessary in the cold chain.Ethylene gas is an odourless, colourless gas that exists in nature and is triggered at maturity in climacteric fruits.
Ethylene, also known as either the “death or ripening hormone”, plays a regulatory role in many processes of fruit & vegetable growth, development, and eventually decay.Fruits are mostly either high ethylene producers or absorbers. Apples, bananas, melons, pears, and peaches are ethylene-producing fruits, while tomatoes are moderate ethylene-producing foods. Broccoli, cabbage, and cauliflower are just a few examples of ethylene-sensitive foods.
So, by the rule of thumb, it is preferable to avoid storing ethylene-producing fruits with ethylene absorbing ones. Ventilating the storage area also serves this purpose but just to an extent.
Commercial use of ethylene gas to ripen fruit
The overall impact of ethylene gas is to expedite ripening, aging, and eventual spoilage, manifesting in detrimental effects such as scald, brown spots, decay, sprouting, mold, odor, yellowing, bitterness, and loss of color in common fruits and vegetables. Ethylene gas, present in concentrations ranging from a few parts per billion (ppb) to a few parts per million (ppm), can diminish the vitality of fresh produce, curtail product lifespan, and compromise overall stock quality.
Climacteric fruits, including bananas, mangoes, tomatoes, and avocados, are often harvested at a physiological stage termed 'commercial maturity,' typically in a hard green but mature state just before natural ripening begins. This allows for extended storage, transport, and distant marketing, with subsequent ripening occurring within controlled cold storage environments. Unripe fruit undergoes the ripening process in specially designed rooms with controlled temperature, humidity, and ethylene exposure.
The sensitivity of fruits to ethylene varies, with ethylene-sensitive fruits requiring specific concentrations for optimal ripening. For instance, bananas need a precise ethylene exposure to de-green and mature to a market-ready state. Ethylene-sensitive fruits, ripening at an accelerated pace, may necessitate controlled atmosphere storage instead of ripening rooms to extend their life cycle, ensuring they reach the grocery store at the most opportune time.
Regardless of ethylene sensitivity, the process involves introducing a prescribed amount of ethylene production into the cold storage environment as a ripening agent, often through a "catalytic generator" that converts liquid ethanol or commercially available gas supplies into ethylene gas. Forced-air cooling systems guarantee uniform exposure to room ethylene concentration, initiating the respiratory climacteric pattern in fruit and promoting coordinated ripening.
Treatment times vary based on conditioning and ripening stages, with a 24-hour treatment at a room ethylene concentration of 10 microliters per liter (10 µL/L) being sufficient for many climacteric fruits. As fruit respiration rates increase in response to ethylene, proper ventilation is crucial to prevent the accumulation of carbon dioxide, a by-product of fruit respiration, ensuring optimal ripening conditions.
‘Ripe ‘n’ ready’‘
Ripe ‘n’ Ready’ is a retail strategy used for marketing climacteric fruits to consumers in a ready-to-eat state. Consumers are increasingly purchasing smaller quantities per shop, yet they are shopping more often.Traditionally consumers would need to plan ahead when purchasing climacteric fruit in the early stages of ripening and wait for the fruit to ripen before consuming, otherwise resorting to using consumer-level techniques to introduce more ethylene gas to their leafy greens, citrus fruits, and other ethylene sensitive produce.An example of consumer-level ripening may be to store fruits such as bananas, apples, or other ethylene-producing products, with an unripe avocado inside a paper bag to help produce ethylene levels that will ripen the avocado faster.The Ripe ‘n’ Ready strategy is to pre-condition/ripen climacteric fruit to be able to present consumers with a ready-to-eat product and save them the hassle. Achieving this requires a synchronised supply chain that can ensure the timely delivery of common fruits after the ripening treatment has been applied.
‘Vine-Ripe’
Some new varieties of tomatoes are marketed as ‘vine-ripened’. Certain truss tomatoes develop their colour and flavour while still attached to the plant. They have been developed through crossbreeding with non-ripening tomato varieties and are usually grown hydroponically in greenhouses.These tomatoes develop colour without softening like conventional tomatoes and remain firm for harvest, packing and distribution. These tomato varieties do not exhibit climacteric qualities like conventional tomato varieties but remain susceptible to other fruits that act as external ethylene producers.
Final Thoughts
Monitoring and controlling the ripening of fresh produce is becoming a very important issue in the fruit and vegetable industry since the state of ripeness during pre-harvest, harvest, storage, and market distribution determines the quality of the final product measured in terms of customer satisfaction.Many methods to monitor ethylene and the maturity of fruits and vegetables have already been proposed. The main disadvantage of the majority of these common examples of techniques is that they are not practical in a sustainable sense for cultivars or storage stations. Moreover, most methods are invasive and require the destruction of say bananas or apples as test samples used for analysis.This is why, nowadays, optimal harvest dates and predictions of storage life are mainly based on practical experience. Leaving these critical decisions to subjective interpretation implies that large quantities of fruit are harvested too soon or too late and reach consumer markets in poor condition.
How PostHarvest helps optimise Ethylene levels across supply chains PostHarvest has developed a form of sensor technology, which allows users to accurately measure and forecast optimal delivery times, maximise outgoings and minimise food wastage. PostHarvest’s amplification technology allows for atmospheric reads in the form of parts per billion (PPB), this measurement is often used to describe amounts of ethylene and contaminants found within an atmosphere in its most precise and finite form (currently an unmatched industry standard).
PostHarvest’s Environmental Sensor is a wall-mounted device that sits within storage facilities as it continuously captures ethylene samples, along with other chemical compounds from within the controlled atmosphere of a cold chain operation, providing the most accurate product health readings in the industry.
This data provides conditioning & health reports to help cool store operators forecast their processes, supplies, and optimal delivery times, maximising outgoings and minimising food wastage as a result.If you would like to know more about ethylene and its relationship to fruits and vegetables, go to PostHarvest’s free online learning platform.