This age-old method for ripening tomatoes quickly has been tried and true for generations. That’s because it takes advantage of an odorless, colorless chemical called ethylene gas.
Ethylene is a major plant hormone that occurs naturally in tomatoes and other climacteric fruits. When these fruits start to ripen, they release ethylene, which speeds up the ripening process of other nearby fruits.
To ripen your tomatoes this way, simply place a few unripe, unblemished tomatoes inside a paper bag. Don’t smush them in there; you want a little airflow so the fruits don’t mold.
Add a ripe banana, apple, or pepper to the bag and loosely seal it. (I just fold the top over a few times so it stays closed.) Place the bag in a warm spot and the tomatoes should ripen in one to two weeks, depending on how much color they had in the beginning.
If you’re wondering why you couldn’t just use a plastic bag or zip-top bag, well, technically you can. You’ll trap more ethylene this way, but you’ll also trap moisture, increasing the chances of mold.