You cut into a potato that looked normal on the outside and find a gray, brown, or nearly black patch. Dark internal tissue is not one single diagnosis. Potatoes can discolor because of bruising, pressure, growth disorders, or poor storage conditions.
A hidden bruise is one common explanation
University of Nebraska Extension describes internal blackspot as an impact bruise that forms beneath the skin after the tuber strikes a hard surface. The dark area may take one or two days to develop, so the outside can still look normal.
The spot is usually dry and localized. If the rest of the potato is firm, normal-smelling, and sound, the damaged portion can generally be cut away generously and the remaining potato used.
Some dark areas are physiological disorders
UC Davis describes internal brown spot as dry, corky reddish-brown to black areas associated with calcium uptake problems during growth. Blackheart is different again: under restricted airflow and high respiration, the center of a tuber can become brown and eventually black because too little oxygen reaches the interior.
Wet and foul-smelling changes are more concerning
A watery, slimy, very soft, moldy, or foul-smelling area suggests active decay rather than a simple bruise. If deterioration is extensive, discard the potato rather than trimming a small visible piece.
Black is not the same issue as green
Greening is related to light exposure and can accompany increased glycoalkaloids. A very green, bitter, extensively sprouted potato should not be judged by the same rules as a small dry black bruise.
Use texture and odor, not color alone
A firm potato with one dry defect is a quality problem. A potato that is collapsing, leaking, slimy, moldy, or smells wrong is a spoilage problem.
The condition of the surrounding tissue tells you more than the color of one spot.
What to do after you cut the potato open
First look at the character of the dark area. A small, dry, firm patch surrounded by normal-smelling, normal-textured potato is more consistent with bruising or a physiological defect than with active rot. You can cut away the discolored tissue generously and use the unaffected portion if the rest of the tuber is firm. That is different from a potato with spreading wet decay, softness, leakage, or a foul odor.
Do not confuse internal black bruising with green skin. Greening is associated with light exposure and can accompany increased glycoalkaloids, so a heavily green or very bitter potato deserves more caution. Sprouting, severe shriveling, and extensive damage are additional reasons to choose a better tuber. A black spot in the center and a green surface are visually different problems with different causes.
Storage can reduce future defects. Keep potatoes in a dark, cool, well-ventilated place and avoid heavy pressure or dropping bags, because impact damage may not become visible until the potato is cut later. Refrigeration is not usually the preferred household storage method for raw potatoes because very cold temperatures can alter sugars and cooking behavior.
If several potatoes from the same bag show extensive internal damage, review how they were stored and consider using the remaining good ones sooner. When the cause is uncertain and the tissue looks wet, moldy, hollow with decay, or smells wrong, discard the potato rather than trying to salvage around a large affected area.
After trimming a small dry bruise, cook the potato normally and pay attention to the remaining flesh. If the interior is uniformly firm and smells neutral, the defect was likely local. If cutting reveals tunnels, extensive hollow areas, spreading discoloration, or additional soft pockets, the problem is broader than the first spot suggested and the whole potato may be a poor choice.
Do not use taste as a test for a suspicious raw potato. Bitterness can be a warning sign of elevated glycoalkaloids, but the absence of bitterness does not diagnose every defect. Visual extent, firmness, odor, greening, and storage history provide a safer set of clues. When in doubt, discarding one potato is a small loss compared with trying to rescue severely damaged tissue.
Cooking does not make rotten tissue acceptable. Heat can destroy many microbes but it does not reverse advanced decay, remove every heat-stable compound, or restore a potato with extensive structural damage. Trimming is appropriate only when the defect is small and the surrounding tissue is clearly sound. For a broadly affected tuber, disposal is the simpler and safer decision.
Sources
- University of Nebraska-Lincoln Extension — Bruises of Potato Tubers — Explains internal blackspot caused by impact and pressure bruising.
- UC Davis Postharvest Research and Extension Center — Potato — Describes blackheart, black spot, internal brown spot and other disorders.
- Cornell Vegetables — Detection of Potato Tuber Diseases and Defects — Distinguishes black spot from other potato defects and diseases.