Ethylene, also known as ethene, is one of the simplest organic compounds, possessing the formula C2H4. This hydrocarbon gas features a double bond between its two carbon atoms, making it highly reactive. Ethylene serves as a powerful signaling molecule in nature and is also a foundational material for the modern petrochemical industry.
The Sensory Profile of Ethylene
Pure ethylene is a colorless gas at normal temperatures and pressures. While often described as odorless, some sources indicate that high-purity ethylene can possess a faint, sweet, or musky odor. This subtle scent is generally not strong enough for humans to reliably detect the gas at low concentrations. This lack of a strong, distinctive smell is why specialized electronic sensors are required to detect its presence in industrial settings.
The concentration at which the average person can smell ethylene, known as the odor threshold, is estimated to be around 270 parts per million (ppm). This threshold is significantly higher than the concentrations that affect plants, meaning humans cannot naturally sense its presence in agricultural environments. Industrial-grade ethylene can sometimes contain trace impurities from the manufacturing process, which may contribute a more noticeable smell. Relying on smell alone is therefore not a reliable warning sign for either safety or agricultural management.
The Dual Origin of Ethylene
Ethylene originates from two distinct sources: the biological world and industrial manufacturing. Natural production occurs in virtually all parts of higher plants, including the roots, stems, leaves, flowers, and fruits. In plant cells, ethylene is synthesized through a biochemical pathway that begins with the amino acid methionine. The rate of this natural production can vary widely, increasing dramatically during specific life stages like the ripening of certain fruits or the aging of leaves.
This biological production is most noticeable in climacteric fruits, such as bananas, apples, avocados, and tomatoes. These fruits exhibit a pronounced surge in ethylene output as they mature. This self-amplifying signal acts as a hormonal trigger, coordinating the final stages of ripening even after the fruit has been harvested. Other environmental factors, such as mechanical wounding, drought, or disease, can also stimulate plants to produce a burst of ethylene as a stress response.
The majority of the world’s ethylene is manufactured for industrial use, making it the most produced organic compound globally. Over 95% of this production relies on a high-temperature process called steam cracking. This method involves heating hydrocarbon feedstocks, such as ethane derived from natural gas or naphtha from petroleum, to temperatures between 750°C and 900°C. This thermal breakdown “cracks” the larger molecules into smaller ones, yielding a mix of products from which pure ethylene is then separated.
Other environmental sources contribute smaller amounts of ethylene to the atmosphere. Incomplete combustion of organic materials, such as in car exhaust, gas flares, and burning biomass, releases ethylene as a byproduct. Historically, trace amounts have also been associated with volcanic emissions and natural gas leakage.
Ethylene’s Role in Plant Biology and Commerce
Ethylene functions as a gaseous phytohormone, regulating various aspects of plant development and senescence. Its primary role in mature plants is to initiate and coordinate the ripening process in climacteric fruits. It triggers a sequence of biochemical changes, including the breakdown of chlorophyll to reveal new colors, the activation of enzymes that convert starches into simple sugars, and the softening of the fruit’s cell walls.
Beyond ripening, ethylene also controls the aging process in other plant parts. This includes the shedding of leaves in autumn, known as abscission. Furthermore, it accelerates the wilting and fading of cut flowers.
This precise biological function is heavily manipulated in the food supply chain for commercial benefit. To extend the shelf life of produce, fruits like apples and pears are often stored in specialized facilities that actively remove or inhibit ethylene to delay ripening. Techniques like controlled atmosphere storage use ethylene scrubbers, and chemical inhibitors like 1-Methylcyclopropene (1-MCP) are applied to block the fruit’s ability to perceive the hormone. Conversely, distributors apply external ethylene gas to batches of fruit, such as bananas and tomatoes, to force-ripen them just before they are sent to grocery stores.
In the industrial sector, ethylene serves as the fundamental building block for countless manufactured goods. The gas is polymerized to create polyethylene, which is the world’s most common plastic used in packaging, films, and containers. It is also converted into other chemical intermediates, such as ethylene oxide for antifreeze and polyester production.