Ripening is a physiological process in fruits, during which they undergo biochemical and structural changes that lead to enhanced sweetness, reduced acidity, softer texture, and development of characteristic flavors and aromas. Fruits can be divided into those that ripen after picking (climacteric) and those that do not (non-climacteric); see lists of climacteric fruits and non-climacteric fruits. Compare bletting, which is when fruit continues to change after ripening.
Biochemical processes of ripening
The ripening process is primarily driven by enzymes and plant hormones, with ethylene (C₂H₄), a simple hydrocarbon gas, playing a key regulatory role in climacteric fruits. During ripening, complex carbohydrates, primarily starch, are broken down into simpler sugars, such as glucose, fructose, and sucrose, which contribute to the sweetness of the fruit. Concurrently, organic acids (such as malic and citric acids) are metabolized, resulting in a reduction in acidity.
Cell wall-degrading enzymes, such as pectinases and cellulases, break down pectin and cellulose in the plant cell walls, leading to fruit softening. This is why ripe fruits tend to be less firm than unripe ones. Pigments such as chlorophyll degrade, revealing other pigments like carotenoids and anthocyanins, which cause the change in color typically associated with ripening (e.g., green bananas turning yellow or tomatoes turning red).
Ripening fruits release volatile organic compounds that contribute to their aroma. These are produced through enzymatic pathways that modify fatty acids, amino acids, and terpenoids, giving rise to the range of flavors characteristics of different fruits.
The role of ethylene
Ethylene is a plant hormone that plays a critical role in the ripening of many fruits, particularly those classified as climacteric. Climacteric fruits, such as bananas, tomatoes, and apples, exhibit a rise in ethylene production as they ripen, triggering and accelerating ripening processes. Non-climacteric fruits, such as citrus, grapes, and strawberries, do not rely on ethylene for ripening, and once harvested, their ripening processes slow or stop.
Ethylene affects several ripening-related genes, enhancing the activity of enzymes involved in starch breakdown, pigment change, and cell wall degradation. In a home setting, ethylene production can be manipulated to control ripening, particularly by storing fruits that produce ethylene (like bananas) with those that are sensitive to it (such as avocados or kiwis).
Post-Harvest Ripening in a Home Context
In a home environment, managing the ripening process involves temperature control, atmospheric conditions, and understanding the ethylene response of fruits. Climacteric fruits can be ripened at room temperature, often accelerated by placing them near other ethylene-producing fruits (e.g., storing bananas with apples). Wrapping in a paper bag is a common method to increase ethylene concentration around the fruit, speeding up the process.
Refrigeration slows ripening by inhibiting enzymatic activity and ethylene production. Bananas, pears, avocados, apples, and stone fruit will all ripen more slowly when chilled. Note that some fruits, like tomatoes, suffer from cold damage, which will affect their texture and flavor.