The end of the growing season often leaves gardeners with a heavy harvest of green tomatoes that have not had time to fully mature on the vine. When the threat of frost approaches, harvesting these firm, green fruits is necessary to save the crop. Fortunately, tomatoes are climacteric fruits, meaning they continue ripening even after being picked. By understanding the science and utilizing simple indoor methods, you can successfully turn green tomatoes into a steady supply of flavorful, ripe ones.
When to Harvest and Why Tomatoes Ripen
The successful ripening of a tomato off the vine depends entirely on its maturity level at the time of harvest. A tomato must reach the “mature green” stage before it is picked, or it will likely rot before achieving full color and flavor. You can distinguish a mature green tomato by its physical characteristics, including reaching nearly its full size and developing a slight waxy sheen on its skin.
The most reliable indicator of maturity is the internal development of the seeds, which are fully formed and surrounded by a jelly-like substance. This is the point at which the fruit has stored enough sugars and starches to begin the final phase of ripening. The key biological agent for this transformation is the naturally occurring plant hormone called ethylene gas.
Ethylene is a colorless, odorless gas that initiates changes within the fruit after it has reached maturity. This hormone stimulates the breakdown of chlorophyll and triggers the production of carotenoids. It also causes the fruit to soften and flavor compounds to develop, transforming the hard, tart green fruit into a juicy, sweet tomato. Because tomatoes are sensitive to cold, pick all green fruit before temperatures drop below 50°F, as chilling injury can slow or halt the ripening process.
Ethylene-Trapping Methods for Accelerated Ripening
Once the mature green tomatoes are harvested, clean them gently and separate them from any attached stems to prevent puncture wounds during storage. The goal of indoor ripening is to create an enclosed environment that traps the naturally released ethylene gas, concentrating it around the fruit. This concentrated gas accelerates the color and flavor development of the entire batch.
One effective technique is using a simple cardboard box, which allows for a small amount of air exchange while still containing the gas. Place tomatoes in the box in a single layer or stacked no more than two layers deep, using newspaper or tissue paper to separate them and absorb excess moisture. Check the fruit weekly and remove any showing discoloration or softening to prevent the spread of mold or decay.
For smaller batches, a paper bag is an excellent method to accelerate the process. The paper material is permeable enough for breathability, yet it effectively concentrates the ripening gas. To boost the ethylene concentration, place a high ethylene-producing fruit, such as a ripe apple or banana, inside the bag with the tomatoes. This additional source of the hormone can significantly reduce the time required for the tomatoes to ripen.
Monitoring Conditions and Preventing Spoilage
Maintaining the correct environmental conditions is necessary for successful indoor ripening. The ideal temperature range for ripening mature green tomatoes is between 64°F and 70°F, which encourages the optimal production of pigments and flavor compounds. Temperatures outside of this range, particularly above 86°F or below 55°F, can slow the process or result in poor color development.
Contrary to a common misconception, light is not required for tomatoes to ripen once they have been picked. Storing the fruit in darkness or indirect light is preferable, as direct sunlight can cause the tomatoes to heat up too quickly and turn soft before the flavor fully develops. A dark room, closet, or pantry that maintains the recommended temperature range works best for storage and ripening.
Regular inspection of the stored tomatoes is important to prevent a single spoiled fruit from ruining the entire batch. Check the tomatoes every few days, looking for any signs of mold, rot, or excessive softness. Immediately removing any compromised tomatoes and the ripe ones ensures that the remaining fruit continues to ripen in a healthy, controlled environment.