It’s pretty easy to spot a good lemon from a rotten one: white and black patches on a cut lemon or green and white mold on the outside mean it’s time to toss it.
The intermediate stages, between fresh and spoiled, are a bit more difficult to figure out. At that time, oxidation—when oxygen molecules begin to react and break down the molecules in lemons—is beginning to affect the fruit, but hasn’t yet turned the flesh moldy or brown.
Buy the freshest lemons you can to ensure that they last long once you get them home. Look for skins that are firm to the touch and don’t have any soft spots, which are a sign of age. Lemons with thin, smooth skins generally have more juice than those with thicker, more bumpy skin.
Begin to check more closely for signs of spoilage toward the end of their typical storage times. Whole lemons stay good for up to one week at room temperature and two to three weeks in the refrigerator. Cut lemons stay good for three to four days in the fridge. Keep both cut and whole lemons in the refrigerator wrapped in plastic to extend their shelf life.
Whole or cut, lemons stay good indefinitely in the freezer, but their quality suffers since the fruit’s flesh can turn mushy. You can still use the juice, though.
Lemons that have gone bad may not yet have visible mold growing on them, but they may have other signs of spoilage:
- Brown or soft spots on the skin.
- Brown or squishy areas of the lemon flesh.
- White or dark spots or splotches on either the skin or the flesh.
- Slimy surfaces on the skin or flesh.
- Oozing and runny juices.
In some foods, such as cheese or firm fruits and vegetables like bell peppers or cabbage, mold is contained within a specific area, and you can just cut it off.
But in soft or high-moisture fruits like tomatoes, peaches, or lemons, there’s a good chance that any mold you see has also spread invisibly to other parts. The U.S. Department of Agriculture recommends that rather than trying to cut out any mold, you should play it safe and discard the questionable lemons.