Tomato ripening involves shifts in color, texture, and sugar content, and is a continuous chemical process occurring both day and night. While the plant needs daylight to produce sugars, the final stage of maturation is an autonomous internal function. Tomatoes do ripen at night, but the speed of this transformation depends heavily on external conditions, particularly temperature, rather than sunlight.
The Mechanism of Ripening
Tomatoes are classified as climacteric fruit, meaning they undergo a distinct phase of increased respiration and a surge in the production of the gaseous plant hormone, ethylene. This internal surge triggers a cascade of chemical reactions within the fruit’s cells. Once a tomato reaches the mature green stage, ethylene production initiates the breakdown of chlorophyll, the green pigment. The hormone also activates enzymes, such as polygalacturonase, which degrade the fruit’s cell walls, leading to softening. Simultaneously, starches convert into simple sugars, enhancing flavor, and new pigments like lycopene and carotene are synthesized.
Temperature’s Impact on Ripening Speed
Temperature is the most significant external factor that regulates the pace of the chemical reactions involved in ripening. The ideal temperature range for efficient tomato ripening is between 68°F and 77°F (20°C–25°C). Within this “Goldilocks” zone, the enzymes responsible for color change and softening operate most effectively, resulting in the fastest and most uniform ripening. Temperatures that fall outside this optimal range can dramatically slow the process.
Cool night temperatures dropping below 50°F (10°C) significantly inhibit the activity of the ripening enzymes. Conversely, high temperatures above 85°F (29°C) inhibit the synthesis of lycopene, the pigment responsible for the deep red color. This heat stress can lead to fruit that turns a pale orange or yellow, or exhibits a condition known as “yellow shoulder.”
The Role of Light
Light is not required for a mature green tomato to complete its final stage of ripening and turn red. Sunlight’s primary role occurs earlier, powering the photosynthesis that creates the sugars transported to the fruit, which contributes to flavor. Once the fruit reaches full size and internal ethylene production begins, the color change occurs just as readily in the dark.
Direct, intense sunlight can be detrimental during the final ripening stage. Exposure to excessive sun causes the fruit surface to heat up beyond the ideal range, inhibiting the production of red lycopene pigment. This effect can result in sunscald or uneven coloring on the exposed side of the fruit.
Practical Steps for Ripening Off the Vine
When the growing season ends or frost threatens, mature green tomatoes can be successfully ripened indoors by replicating optimal conditions. Maintain a consistent room temperature, ideally between 60°F and 75°F, and avoid the refrigerator, as cold temperatures halt the necessary chemical processes. Placing the fruit in a confined space, like a paper bag or a cardboard box, helps concentrate the naturally produced ethylene gas. To expedite the process, add a ripe apple or banana to the container. These fruits are prolific ethylene producers, and the concentrated gas will stimulate the tomatoes to ripen more quickly and uniformly.