Tomatoes, like peaches, are one of the many fruits and vegetables that will continue to ripen after they've been picked. They won't ripen quickly and they won't usually ripen perfectly, but you can coax an underripe tomato to ripen at home. While tomatoes won't ripen as well as peaches do, underripe tomatoes can definitely be improved upon. And best of all: it's beyond easy and requires barely any equipment so if you find yourself with not-quite-ripe tomatoes, there's no reason not to give it a shot.
As they ripen, tomatoes and other botanical fruits let off a gas called ethylene. The more ethylene they're around, the more they'll ripen. So putting the not ripe tomatoes in a situation where they're around more ethylene is the key to ripening them further.
Ripening Tomatoes Is Easy
Put all of your under-ripe tomatoes together in a breathable container. This could be a cardboard box, a paper bag, or a plastic bag with holes cut in it. The goal is to help them ripen by capturing the ethylene the tomatoes naturally release. Note that using sealed plastic containers will capture too much of what they let off; intact plastic bags and containers will trap in humidity along with ethylene. The humidity will make the tomatoes rot, often before they even have a chance to further ripen. In short, you want to capture the ethylene, but you want a breathable container because you don't want them to get moldy in there!
If They're Not Ripening Fast Enough
If you want to speed things up, add a ripening banana to the breathable container. Bananas let off more ethylene than other fruits, so they help their brethren along the ripening path. Other fruits work, too; avocados and apples are good choices to bump up the ethylene and move things along.
Keep an Eye on Them
Check the tomatoes regularly and remove them as they ripen. They are unlikely to ripen at exactly the same rate, so examine each tomato by taking it out of the bag, feeling if it feels heavy for its size, looking at its color, and smelling to see if it smells like a ripe tomato. Tomatoes ripened this way are unlikely to rival a tomato fully ripened on the vine, and tomatoes of poor quality can't really be helped much. This method will help a quality tomato that was picked just a tad before its time.
How to Store Ripened Tomatoes
Once your tomatoes ripen, store them on the counter and use them as quickly as possible. Tomatoes do not last long once they're ripe. Keeping them out at room temperature and in-view is the key to maintaining their quality and ensuring that you remember to eat them sooner rather than later. Never refrigerate tomatoes; temperatures under 50 F turn them mushy and/or mealy. If you need to keep them longer, it's better to pop them in the freezer and use them when you are ready.