Resting your lemon juice for a few hours may actually improve its flavor.
When more than one bartender friend told us that lemon and lime juices that are several hours old taste better than the just-squeezed stuff, we decided to see for ourselves. We pressed several fresh lemons and limes and refrigerated their juices, covered, for four hours. We then used the samples in batches of lemonade and limeade, comparing them with versions made with just-squeezed juice. Sure enough, tasters preferred the aged-juice drinks, noting that they had a more mellow yet complex flavor.
It turns out that terpenes, aromatic compounds in lime and lemon oil, start to oxidize when the juice rests. In this case, oxidation is a good thing: A chemist in the juice industry explained that oxidized terpenes consistently present a truer, more preferred citrus flavor than unoxidized terpenes.
So if you’ve got the time, letting juice rest for a few hours before mixing your drinks can make for better-tasting citrus ades and cocktails. Any longer than six hours, though, and the juice may start to lose its potency and develop off-flavors. (Note: This aged-is-better phenomenon is not true of oranges, since they contain a compound called LARL that can make their juice turn bitter when exposed to air.