Ethylene is a simple, naturally occurring plant hormone that exists as a gas at ordinary temperatures. This colorless gas functions as a signaling molecule, coordinating various aspects of a plant’s lifecycle, including the maturity and eventual decline of leaves and flowers. In the context of produce, it is the primary initiator for the physiological changes that define ripening. Ethylene production acts like a master switch, setting off a cascade of internal biochemical reactions that transform hard, green, and often tart fruit into soft, sweet, and aromatic produce.
Fruits That Produce Ethylene
The ability of a fruit to ripen is categorized by its response to and production of this gaseous hormone. Fruits are broadly classified as either climacteric or non-climacteric based on their post-harvest behavior.
Climacteric Fruits
Climacteric fruits are those that exhibit a dramatic increase in respiration rate, often called a “climacteric rise,” and a corresponding spike in ethylene production after being harvested from the plant. This autocatalytic process means that the initial release of ethylene stimulates the fruit to produce even more of the gas, fueling the ripening process. Well-known examples of climacteric fruits include apples, bananas, avocados, mangoes, and pears. Because these fruits continue to ripen off the vine, they are often picked when mature but still firm and green, allowing them to withstand the rigors of long-distance transport. This physiological trait makes them highly responsive to external ethylene applications, which is a common practice in commercial agriculture to synchronize ripening before sale.
Non-Climacteric Fruits
Non-climacteric fruits, by contrast, produce very low levels of ethylene and do not experience the climacteric rise in respiration. These fruits, such as grapes, strawberries, blueberries, and citrus varieties, must be fully mature and ripened before they are harvested. Once picked, a non-climacteric fruit will not become sweeter or softer, though exposure to high levels of ethylene can still cause quality deterioration or senescence. This distinction means that if a non-climacteric fruit is picked too early, its eating quality will not improve, regardless of how long it sits on the counter.
How Ethylene Triggers Ripening
Ethylene initiates ripening by binding to specific receptor proteins within the fruit’s cells, effectively signaling the cell to begin the transformation process.
Texture and Sweetness
One of the most noticeable changes is the softening of the texture, which occurs because the gas activates enzymes like pectinases and cellulases. These enzymes systematically break down the structural components of the fruit’s cell walls, turning a firm texture into a yielding one. Simultaneously, ethylene drives the conversion of stored carbohydrates into simple sugars, dramatically increasing the fruit’s sweetness. In many climacteric fruits, starches are broken down into simpler molecules like glucose and fructose by enzymes such as amylase. This chemical transformation not only increases the sugar content but also often reduces the concentration of organic acids, minimizing the tartness of the unripe fruit.
Color and Aroma
The hormone also directs changes in the fruit’s appearance and aroma. Ethylene promotes the degradation of chlorophyll, the pigment responsible for the green color, which unmasks or allows the synthesis of other colorful pigments like carotenoids and anthocyanins. Furthermore, the final stages of ripening involve the production of various volatile organic compounds, which are responsible for the complex and distinct aromas associated with fully ripened produce.
Using Ethylene Knowledge for Better Storage
Understanding which fruits are high ethylene producers allows consumers to control the ripening process at home and minimize premature spoilage.
Accelerating Ripening
To accelerate the ripening of an unripe climacteric fruit, such as an avocado or peach, placing it in a paper bag with a high-ethylene producer like a ripe banana or apple will concentrate the gas. The trapped ethylene gas will then speed up the ripening of the unripe item.
Preventing Spoilage
Conversely, preventing spoilage requires separating high-ethylene-producing fruits from produce that is sensitive to the gas. Items like leafy greens, broccoli, carrots, and cut flowers are highly sensitive to ethylene and can rapidly yellow, wilt, or decay when stored near apples or bananas. Storing these sensitive vegetables away from ethylene-releasing fruits helps to maintain their freshness and extend their shelf life. In commercial cold storage, this principle is managed with sophisticated ventilation systems and ethylene scrubbers, which actively remove the gas to slow the ripening and decay of large quantities of produce.