When you see white and black patches on a cut lemon or green and white mold on the outside of a whole lemon, it’s obvious that the lemon is rotten. Throw those lemons away.
The intermediate stages, between fresh and spoiled, are a bit more difficult to determine. At that time, oxidation — when oxygen molecules begin to react and break down the molecules in lemons — is beginning to affect the lemons, but has not yet turned the flesh moldy or brown.
Buy the freshest lemons you can to help ensure that they last the longest time once you get them home. Look for lemons whose skins 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 in lemons toward the end of their typical storage times. Whole lemons stay good for up to 1 week at room temperature and two to three weeks in the refrigerator. Cut lemons stay good for three to four days in the refrigerator. Keep both cut and whole lemons in the refrigerator wrapped in plastic to extend their shelf life.
Use the juice from lemons that you’ve placed in the freezer. Whole or cut, lemons stay good indefinitely in the freezer, but their quality suffers, with the fruit’s flesh turning mushy.
Lemons that have gone bad may not yet have mold growing on them that you can see, but they may have other signs of spoilage. Signs that indicate lemons have gone bad include:
- 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 such as bell peppers or cabbage, mold is contained within a specific area, and you can just cut away the moldy sections.
But in soft or high-moisture fruits, such as tomatoes, peaches or lemons, there’s a good chance that any mold you see has also spread invisibly to other parts of the fruit. The U.S. Department of Agriculture recommends that rather than trying to cut out any mold, that you play it safe and discard the produce.