why does potatoes turn green when stored?

Q: why does potatoes turn green when stored?

Potatoes turn green when exposed to light because they produce chlorophyll, like any plant. More importantly, this light exposure triggers the synthesis of solanine, a bitter-tasting and potentially toxic natural defense chemical in the tuber.

The Deep Dive

A potato is a modified stem, a tuber, evolved for underground storage. Its skin is naturally protected from sunlight. When exposed to light, specialized organelles called chloroplasts develop in the potato's skin cells. These chloroplasts produce chlorophyll, the green pigment essential for photosynthesis. This is the plant's signal that it's above ground and should start photosynthesizing. Concurrently, the potato ramps up production of glycoalkaloids, primarily solanine and chaconine, which are its natural chemical defenses against pests and pathogens. Light is a key environmental trigger for this toxic compound's biosynthesis. Solanine is bitter and disrupts cell membranes in animals, acting as a deterrent. The concentration of these toxins increases with the intensity and duration of light exposure, and they are most concentrated in the green skin and just beneath it. This is a sophisticated, inducible defense system that protects the plant's energy-storing organ but renders the tuber unsafe for human consumption if the green areas are ingested in significant quantities.

Why It Matters

Understanding this process is critical for food safety. Consuming green potatoes can cause solanine poisoning, with symptoms ranging from nausea and vomiting to neurological issues like confusion and paralysis in severe cases. For consumers, it guides proper storage: potatoes should be kept in a cool, dark place, ideally in a paper bag or basket. For the food industry, it impacts harvesting, packaging (using opaque bags), and retail display to minimize light exposure, reducing economic losses from spoiled produce and liability from consumer illness. It also highlights the fine line between a plant's natural survival mechanisms and human dietary needs.

Common Misconceptions

A common myth is that the green color itself is the harmful part. The green is merely a visual indicator of chlorophyll; the real danger is the co-produced solanine toxin, which is present even without significant greening if storage conditions were poor. Another misconception is that cooking or frying destroys solanine. Solanine is heat-stable and not broken down by typical cooking temperatures, so simply cooking a green potato does not make it safe; the green and any bitter-tasting areas must be thoroughly peeled and discarded.

Fun Facts

Solanine is so effective as a pesticide that it has been investigated as a natural, biodegradable alternative to synthetic chemical pesticides for crop protection.
The green skin of a potato can contain up to 100 times more solanine than the normal, non-green tuber, and the toxin can diffuse slightly into the white flesh just below the skin.