The folks America's Test Kitchen wanted to see how to keep lemons juicy and fresh as long as possible. The answer is simple: put them in a Ziploc bag and toss them in the fridge.
If you buy lemons in bulk, colloquial wisdom is to just let them stay on the countertop or store them in a cool, dry place until you get around to slicing them open and using them. However, those lemons slowly harden and lose those tasty juices over time. To figure out how to keep them fresh as long as possible, the folks at ATK tested three different methods of lemon storage, both at room temperature and in the fridge:
First we put them in an uncovered container, then in a sealed zipper-lock bag, and then in a sealed zipper-lock bag with 1/4 cup water added. We recorded the fruits' weights at the start and end of the experiment and then every few days to measure moisture loss.
All the lemons stored at room temperature hardened after a week. But the refrigerated fruits fared much better: The uncovered lemons (which we kept in the crisper drawer) began to lose a small amount of moisture after the first week and 5 per cent of their weight in the following weeks; the lemons stored in zipper-lock bags, both with and without water, didn't begin to dehydrate until four weeks had passed. As it turned out, the water wasn't offering any preservation benefits, but the zipper-lock bag did seal in some moisture.
While there's nothing wrong with keeping your lemons out on the counter (especially if you don't buy more than you can use before they dry out), they will definitely stay fresh longer if you put them in the fridge in a sealed plastic bag.
We Prove It: The Best Way to Store Lemons [America's Test Kitchen]