How do we prevent potatoes that we planted from turning black when frying or cooking (not when peeling)?A:
This sounds like a job for Dr. Potato to send in his nurse to take vital signs.
Dr. Potato: “Yes, nurse what do you see?”
Nurse Florence Nightenbaker: “It’s the potato’s temperature, doctor! It’s so cold…reading (gasp!) 36°F, and the sugar reading is high. Oh my!”
This sounds like an acute case of potatoes that were stored too cold. Wherever it occurred (at the packing shed, in transit, or in your storage area), potatoes stored for too long below 40°F will trigger the starches within to convert to sugar, changing the cooking chemistry. This imbalance will cause fries to darken on the outside too quickly, while the interior is not fully cooked. In other dishes, darkening can result, such as you describe.
An easy pre-test is to use diabetic test strips (found in most pharmacies) for glucose analysis. Cut the raw potato in half, place strip on the interior cut surface and take a reading. Compare to the color chart on the side of the test container. If the color is dark, the potato sugar content is high. Fear not, as the good doctor has a ready remedy: To recondition chilled, high-sugar potatoes, place in a dark, well-ventilated room at 60-70°F for one to two weeks. This will burn off much of the sugar and you should have better preparation results. For immediate relief, you can try blanching the cut potatoes in hot water (170°F) for several minutes to leach out the sugars. In the meantime, remember to store potatoes cool (but not cold - do not store in the refrigerator) – rather, store in a dark, 45 - 55°F area, and leave unwashed until ready to prepare.