Ethylene gas is a colorless gas produced naturally by plants and is recognized as a powerful plant hormone, initiating significant biological changes in fruits, flowers, and leaves. Ethylene’s behavior is governed by its physical properties as a gas and its chemical function as a signaling molecule. Understanding the basic physics of this compound is necessary to manage its profound effects on produce quality and longevity.
Understanding Ethylene’s Physical Movement
The fundamental question of whether ethylene gas rises or falls is answered by comparing its molecular weight to that of air. Ethylene has a molecular weight of approximately 28.05 grams per mole, while the average molecular weight of air is about 28.97 grams per mole. Because it is slightly lighter than air, ethylene gas does have a natural tendency to rise in a still environment.
This difference in density is minimal, however, which means the gravitational effect causing it to rise is extremely slow and weak. In virtually any real-world setting, factors like temperature gradients, air currents, and molecular diffusion are significantly more impactful on gas movement. For instance, in a cold storage facility or a kitchen, the movement of the gas is dominated by dispersion and convection rather than buoyancy.
This rapid diffusion ensures that ethylene mixes throughout the air space rather than pooling at the ceiling. While ethylene is technically lighter than air, it is more accurate to describe its behavior as rapidly dispersing and mixing within the environment. This behavior is a significant consideration for post-harvest handlers who must ensure effective ventilation to prevent the gas from accumulating to damaging concentrations.
Ethylene’s Biological Function in Plants
Ethylene’s primary relevance comes from its function as a chemical signal produced by plants to regulate growth and development. It is best known for its ability to regulate the ripening process in climacteric fruits, such as bananas, apples, and tomatoes. The plant’s response is triggered by extremely low concentrations.
In these fruits, the presence of ethylene triggers a dramatic spike in the fruit’s respiration rate and an autocatalytic increase in its own ethylene production. This positive feedback loop accelerates the physiological changes associated with ripening, including softening, color change, and the conversion of starches to sugars. Ethylene achieves this by binding to specific receptor proteins within the plant cell, initiating a cascade that changes the expression of genes responsible for ripening-related traits.
Beyond ripening, ethylene also plays a role in senescence, the aging and eventual death of plant tissues. High ethylene concentrations can hasten the yellowing of leaves and the wilting of flowers, reducing their shelf life. The hormone also regulates abscission, the process by which plants actively shed organs like leaves, flowers, or mature fruit.
Managing Ethylene in Commercial and Home Environments
In large commercial storage facilities, maintaining quality requires strict control over the gas concentration to slow the ripening and senescence processes. One method involves using Controlled Atmosphere Storage, which uses low oxygen and high carbon dioxide levels to suppress the plant’s natural ethylene production.
To actively remove the gas, commercial operations employ ethylene scrubbers or catalytic converters. These devices utilize various technologies, such as activated carbon filters, ozone generation, or photocatalytic oxidation (PCO) systems, to chemically convert ethylene into harmless byproducts like carbon dioxide and water. Some techniques also involve the application of 1-methylcyclopropene (1-MCP), a compound that physically blocks the ethylene receptors on the produce, inhibiting the hormone’s action.
At home, managing ethylene relies on simple separation and ventilation, leveraging the gas’s dispersive nature. Ethylene producers, like apples and bananas, can accelerate the ripening of sensitive produce, such as broccoli and leafy greens, so they should be stored separately. Increasing ventilation in storage areas allows the gas to disperse and prevents it from reaching levels needed to trigger unwanted ripening.
For home ripening, the same principle can be used intentionally by placing a high-ethylene producer, such as a ripe banana, in a paper bag with an unripe fruit. The bag traps the gas, allowing it to build up the concentration needed to initiate the autocatalytic ripening response.