How do I ripen unripe oranges and grapefruits?

You don't.

Citrus fruits, unlike most other fruits, do not ripen after being picked from the tree.

The only solution is to be proactive and not buy unripe citrus.

I agree with the suggestion that it is best to buy ripe citrus fruits. I respectfully disagree with the assertion that they don't ripen after picking.

I stumbled across this ancient question today and looked at it because I have a lot of very immature oranges that I thinned off my orange tree a few weeks ago, and I wondered what gems of wisdom might have been suggested in the past for hastening their ripening, and was surprised that the answer was quite definitely wrong, at least, for untreated citrus. It is possible that a wax coating might affect this.

I'd like to clarify what happens to the citrus I have picked unripe and left to ripen (generally when pruning the tree). My experience is mostly with lemons and oranges, but it should carry over to other citrus. My experience is mostly with fruit from my own trees but also fruit from local farmers markets that have no wax coatings.

They do continue to ripen.

• The skin will continue to develop color from green through mottled green-yellow on to the final color.

• The scent and flavor of the skin and oils will change from a very "green", slightly bitter, citrusy smell and flavor (very generic, the lemons and the oranges at this stage smell and taste nearly the same) to a fruitier smell/flavor. The lemons will start to smell more lemony, the oranges will start to smell and taste like oranges.

• They will eventually lose a lot of moisture, unless they have been waxed. Citrus fruits have a porous, moist, spongy skin and they are full of water. Unripe citrus tends to be dryer to begin with in my experience -- they continue to fill up with water as they ripen and grow on the tree.

• They can ripen faster if kept with ripe apples and pears, but they also can become moldy if there isn't good air circulation. Check them periodically, remove any that are becoming squishier -- those are spoiling, not ripening.

In summary, as the unripe citrus ripens, the flavors and colors of the flesh will develop, but it will also dry out. It's a bit of a race, will they ripen enough to be pleasing before they dry out too much to be used?

Here's a picture of some unripe oranges before I picked them, tiny and green:

And here they are now, several weeks later, in a box next to where I'm sitting:

Unfortunately I don't have any really green ones left on the tree (and the green ones I picked ripened) so I can't show that comparison. But for oranges, they start off green outside and in, then go through yellow and on to orange or even red. I just cut some open and the ones that have been ripening in the house are mostly a little less juicy than the green and yellow ones I just picked, but they look the same.

Front-right are picked a few weeks ago and ripened in the house (I chose the most ripe and the least ripe I could find in my box). Back-left are picked today from the tree (I chose ones that were splitting and needed to be picked anyway, but cut and photographed the most and least ripe again).

When I picked them, I tasted the skin and found it to be too bitter on the ones that were still fully green, although the ones that were a bit yellow were edible. Tasting now, the same is true. For the fruit picked today vs the fruit picked a few weeks ago, it really just follows the color scale. You can see the difference here between the paler (not quite as ripe) skin and the one that was pretty much ripe that I picked today. Both taste nearly identical, with the riper one tasting just that shade riper, but essentially both taste like the skin of a reasonably ripe, fresh orange.