Potatoes turning green is a common sight. This visual change indicates the potato tuber has been exposed to light, triggering a natural defense mechanism. While the green color itself is harmless, its presence signals a chemical shift that can lead to the accumulation of potentially toxic compounds. Understanding this process helps determine when a potato is safe to eat and when it should be discarded.
The Mechanism Behind Green Color
The green hue on a potato’s skin is caused by the synthesis of chlorophyll, the same pigment that gives leaves their color. A potato tuber is essentially a modified stem, and light exposure triggers a process similar to photosynthesis, upregulating genes involved in chlorophyll biosynthesis.
Greening can occur at any point, from insufficient soil coverage during growth to prolonged exposure on a store shelf or in a home kitchen. As the light intensity and duration increase, the amount of chlorophyll produced under the skin also rises. This serves as a straightforward warning sign for consumers.
Understanding Glycoalkaloid Toxicity
The greater concern associated with green potatoes is the simultaneous production of toxic compounds called glycoalkaloids. The two primary glycoalkaloids are alpha-solanine and alpha-chaconine, which accumulate just beneath the skin alongside the chlorophyll. Glycoalkaloids are naturally produced by the potato plant as a defense mechanism to deter pests and animals.
The concentration of these compounds defines “too green” for consumption. A total glycoalkaloid (TGA) concentration above 20 milligrams per 100 grams of fresh weight (20 mg/100g) is the accepted safety limit. Most properly stored potatoes contain significantly less than this, typically ranging from 1 to 15 mg/100g. However, once the potato greens, the TGA level can rise rapidly toward or exceed the safety threshold.
The bitterness often associated with green potatoes is the most immediate warning sign of high glycoalkaloid levels. Adverse effects have been observed in humans at doses around 1 milligram of TGA per kilogram of body weight. At this level, the compounds can disrupt cell membranes and inhibit the enzyme acetylcholinesterase, leading to gastrointestinal distress.
Symptoms of mild poisoning include nausea, vomiting, diarrhea, and abdominal pain, usually resolving within 24 hours. In rare, severe cases where higher doses are consumed, neurological symptoms such as drowsiness, confusion, headache, and weakness can appear. If a potato tastes noticeably bitter, discard it entirely, as the flavor indicates a concentration that may cause discomfort.
Proper Storage and Safe Preparation
Preventing greening starts with correct storage, as light exposure is the main trigger. Potatoes should be kept in a cool, dark, and dry area with good ventilation, such as a pantry or root cellar. The ideal temperature range for long-term storage is between 40 and 55 degrees Fahrenheit.
Avoid storing potatoes near heat sources or next to fruits like apples, which release ethylene gas that accelerates sprouting. Potatoes should not be washed before storage, as moisture can hasten spoilage. If a potato has only minor greening, you can salvage it by peeling the skin much thicker than usual, since the toxins are concentrated in the outer layer.
Peeling can remove 25 to 75% of the glycoalkaloids, and cooking methods like boiling or frying can further reduce the content. Any sprouts or “eyes” that have developed should be completely cut out before cooking. If a potato is extensively green, has numerous sprouts, or tastes distinctly bitter after a small sample, it should be thrown away.