Sometimes, when I boil potatoes, after I drain them I find gray or black spots on the surface. What is this, and are they still safe to eat?-- Marianna
Those spots are caused by a chemical reaction involving iron, the phenolic compound chlorogenic acid and oxygen. Because it occurs more often at the end of the potato closest to the end where the stem was attached, it is called stem end blackening. While somewhat unsightly, the color isn't as noticeable if the potatoes are mashed, and is not harmful, at least in the small quantities present.
The chemical reaction is more likely to happen in water that is more alkaline, so blackening may be reduced by adding an acidic ingredient such as lemon juice, vinegar or cream of tartar to the cooking liquid. In his book On Food and Cooking: The Science and Lore of the Kitchen, Harold McGee suggests adding the acidic ingredient after the potatoes are half-cooked. The amount that you will need to add depends on the hardness of your water and the amount of water used to boil the potatoes.
Although some cooks like the added taste from using vinegar, I would suggest using cream of tartar, as it is the most neutral in terms of flavor, and trying perhaps 1/4 teaspoon in a pot of potatoes, to start. If the blackening continues, try adding another 1/4 teaspoon. If you can taste an unpleasant acidity in the final product, reduce the amount of acid added.