Put unripe tomatoes stem-side up in a paper bag with a few holes: The setup may help them ripen by keeping ethylene close to the fruit

A carton of firm, pale tomatoes from the farmers market need not wait weeks on the counter. Stick the unripe fruit, stem end up, in a paper bag with a few holes punched in it, then fold the top over and let it sit. According to the University of Illinois Extension, the punctured bag traps the tomato's own ethylene gas close to the fruit and, depending on how green it was to begin with, can lead to full ripeness in just one to five days.

The real reason tomatoes keep ripening after harvest

Tomatoes are climacteric fruit, which means they don't stop ripening after harvest; they keep going. Ethylene, a plant hormone, stimulates this process, and it occurs naturally in plants. According to the United States Department of Agriculture, after a tomato reaches physiological maturity, ethylene forms in the fruit tissue and triggers ripening, bringing changes in color, texture, and sugar content. Other fruits that ripen the same way include bananas, pears, and avocados.

Setting up the paper bag the right way

A couple of factors determine whether your tomatoes ripen evenly or rot before they get there. Instead of placing them in a closed container, use a paper bag and arrange the tomatoes stem-side up. The University of Nebraska-Lincoln Extension advises that the shoulder area near the stem bruises more easily than the rest of the tomato, so keeping that end up keeps pressure off the fruit's soft part. Before folding the top of the bag closed, poke several holes in it. The University of Illinois Extension notes that punching holes and folding the top over, rather than sealing the bag shut, keeps some of the tomato's natural ethylene close to the fruit. Place the bag somewhere it won't be disturbed or in direct sunlight, and check it daily. Depending on how underripe the tomatoes are, Illinois Extension notes they can take anywhere from one to five days to ripen. Adding a ripe banana or apple to the bag can help too, since those fruits also give off ethylene as they mature.

Signs a tomato has finally ripened

The first sign is color. The University of Nebraska-Lincoln Extension suggests looking for tomatoes with even coloring, which signals they're ready to eat. The color should be red or reddish orange. The skin should also give slightly under light thumb pressure. If the tomato is still firm and has white streaks or spots, it needs more time. Once a tomato is fully ripe, it needs care to keep it from spoiling too quickly. According to Illinois Extension, only extra-ripe tomatoes should go in the refrigerator, since the cold slows down any further ripening.

Mistakes that slow the process down

Two practices that can hurt the ripening process without seeming harmful are refrigerating green tomatoes and exposing them to direct sun. According to Illinois Extension, tomato flavor and texture start to deteriorate once the temperature drops below 54 degrees Fahrenheit, so a tomato placed in the fridge before it's ripe has already lost some of its ability to improve. Sunlight causes a different problem: Illinois Extension notes it can ripen the fruit unevenly, leaving one side redder than the other. Therefore, it's better to keep tomatoes in the shade or on a countertop rather than on a sunny windowsill.

The takeaway for your next batch of tomatoes

Picking unripe tomatoes isn't a task to avoid, but one to prepare for. A simple paper bag with a few holes poked in it, kept away from the fridge and bright sunlight, and you'll be well on your way to a plate of ripe, tasty tomatoes. Next time green tomatoes pile up on your counter, skip the windowsill and reach for that paper bag.