Exposure of potato tubers to light either in the field, in storage, on the store shelf, or at home, will induce the formation of a green pigmentation on the surface of the potato. This is called greening and indicates the formation of chlorophyll. This pigment is completely safe and is found in all plants, lettuce, spinach etc.
But, in potato tubers, the green indicates an increase in the presence of glycoalkaloids, especially the substance solanine. When the potato greens, solanine increases to potentially dangerous levels and it is not advised to eat them.
Commercial varieties of potatoes are screened for solanine levels, and most have a solanine content of less than 0.2mg/g. However potatoes that have been exposed to light and started to green can show concentrations of 1mg/g or more. In these situations a single unpeeled potato can result in a dangerous dose.
Solanine biosynthesis occurs parallel but independent of chlorophyll biosynthesis; each can occur without the other. Unlike chlorophyll, light is not needed for solanine formation but is substantially promoted by it. The formation of solanine in potato is localized to the skin, usually no deeper than 3 mm. In processed potatoes such as chips and fries, there is little hazard since peels are removed.
Potatoes show a green area on the surface. Green areas may be surrounded by purple markings like 'bruising'.
Choose potato varieties that set tubers deeper in the ground to reduce the risk of exposure to light. Avoid excessive or late application of nitrogen. Avoid harvesting early as young tubers have higher levels of solanine. Store potatoes in the dark for a short period. Wash before cooking so that green areas can be identified.
- Pavlista, Alexander D. (2009). Green Potatoes: The Problems And The Solution. University of Nebraska, Lincoln.