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There are a few methods that can be used for ripening tomatoes that have been removed from the vine too early. One of the most well known ways is to place the tomatoes in a paper bag to accelerate the ripening process. A method for large batches, which also can help to preserve the tomatoes, involves wrapping each individual fruit in newspaper and then leaving it in a dark area. A tomato also can be placed on a windowsill in the warm sun to help the ripening process, although sunlight is not necessary for the fruit to ripen. Ripening tomatoes also can be accomplished with the help of other fruits and vegetables that are already ripe.
One factor that is necessary for ripening tomatoes is exposure to the gas ethylene. Ethylene is emitted by many fruits and vegetables as they ripen. Tomatoes emit ethylene gas as they slowly turn red and process the water and sugar contained inside. In commercial settings, entire crates of tomatoes are fast-ripened by placing them in a room flooded with a high concentration of ethylene gas and, sometimes, carbon dioxide. Using extremely high volumes of ethylene to ripen tomatoes, however, usually affects only the surface of the fruit and leaves the inside hard, which is why a gradual method is better for truly ripening tomatoes.
When a tomato is placed in a paper bag, the ethylene that it naturally produces is held close to the tomato and encourages the fruit to ripen. This method can be accelerated with the addition of other ripe produce, such as bananas or apples, although the exact type of produce that is added might affect the flavor of the tomato, so vegetables such as onions should be avoided. Wrapping the tomatoes individually in newspaper can have a similar effect, although the ripening tomatoes will develop more slowly, allowing them to be stored for a longer period of time. A second effect of paper bags or newspapers is to wick away excess moisture from the ripening tomatoes so they do not rot or start to develop surface mold.
No matter what method is used for ripening tomatoes, it is important to check on the tomatoes regularly to ensure that they are still viable. Cracked, soft or moldy tomatoes should be separated from any other tomatoes and discarded, because they will quickly cause the other tomatoes to rot, as well. Tomatoes also should not be packed too tightly in a bag or container, partly because they could crush fruits on the bottom, and partly because it will create a favorable environment for mold and insects.