What you see: Dark spots on your grapefruit’s peel.
What it is: A lot of issues could cause dark spots, but in this case a physical injury is likely.
Eat or toss: As long as the fruit looks normal under the peel, it’s an “eat.”
Why does this grapefruit have little spots all over it?
Citrus fruits are particularly vulnerable to funny spots on their peels, whether from an injury or disease, explained Mark Ritenour, a professor of postharvest technology at the University of Florida. But, he said, as long as the fruit underneath the peel looks and tastes good, it’s still good for eating.
“This all looks completely isolated to the peel,” he told me after I shared the image above. “In subtropical, tropical environments, we get more peel disorders and blemishes. We have a tolerance for a lot of those disorders because they’re not decay related.”
Peel blemishes are hard to identify, but the fruit below is usually fine
While peel blemishes that are only skin deep rarely affect eating quality, they do impact marketability. Growers obviously want to avoid such blemishes, but it can be hard to figure out what exactly is causing them. In this case, Ritenour suspected that this grapefruit suffered a physical injury and then repaired itself somewhat. However, the scars, aka those black spots, remain.
To me the spots looked pretty similar, but Ritenour pointed out that their shapes and sizes varied, and that each spot didn’t have a “center” from which diseased peel radiated out. Another hint that this is a physical injury rather than a disease: the areas around each spot are slightly green. That suggests that they didn’t turn yellow along with the rest of the peel. Physically injured tissue tends to resist ditching its green because injuries break open oil glands. The oil messes with the surrounding cells in such a way that their chlorophyll doesn’t readily degrade. If a disease were present, rather than staying green those areas around the spots would be more likely to turn brown as the tissue died and the infection spread.
Physical injuries and microorganisms can cause spots on grapefruit peel, but the fruit is often still edible
So what could have caused an injury with this pattern? Ritenour said a number of scenarios were possible, but here’s one explanation: it got knocked around with some other grapefruits, including some whose stem nub might have been particularly sharp. Most grapefruits are harvested by hand with a twist and snap motion. However, sometimes pickers snip them from the tree, leaving a sharp bit of stem behind. (They do this if a batch of grapefruits have particularly clingy peels that break during the twist and snap.)
Dirt and any rough edges in the packing line could also cause injuries. If a mold spore is present during the injury you’ll probably get growth, but in this case it looks like nothing was able to take root and the peel simply healed over.
Of course, from looking at a photo alone, Ritenour couldn’t entirely rule out the possibility of some kind of microorganism at work. Fungal diseases and certain insects can also leave spots on the peel, even as the fruit beneath it is still fine (here’s a collection of images of various issues). Storage conditions, particularly temperature and humidity fluctuations, before and after harvest, can lead to pitting.
