The way to keep prepped potatoes from turning brown is to submerge them in cold water. This is a very handy trick that will serve you best when used as follows:
– The longest potatoes should sit in water is overnight.
– If keeping potatoes in water for more than an hour, refrigerate.
– Small cuts, like diced potatoes, get waterlogged faster, so are best kept in cold water for a short time, like while you prep other ingredients.
– Bigger cuts can hang out happily in water for 8-12 hours. One of the best ways to use this trick is when making mashed potatoes—you don’t have to cut them small and you’ll be adding liquid anyway. In the morning, put peeled, quartered potatoes in a pot of cold water and refrigerate; when it’s time for dinner, just put the pot on the stovetop and fire it up!
– Don’t keep shredded potatoes in water. Such a fine cut takes on water quickly and is typically cooked until golden (think latkes), so none of that discoloration will show anyway.
– If your dish depends on the most crisp exterior (like pommes Anna) or requires your potatoes to give off very little moisture (like a gratin with other water-heavy veggies) your best bet is to cut those tubers right before use.