As with pasta water, there's a reason to liberally salt the water in which the potatoes will cook: As the starches in potatoes warm up, they open up and absorb water (and salt if you season the water). When they're finished cooking, the cells close off.
But the most essential suggestion, per Bon Appétit, is always to salt the water you are boiling your mashed potatoes in. It acts as pre-seasoning, and Bon Appétit recommends adding a full cup of kosher salt to the boiling water.
Add enough cold water to cover the tops of the potatoes. Add ½ to 1 teaspoon salt to the water. Turn the burner on high and bring water to boiling. Reduce the heat to medium-low or low.
Too much whipping (or mashing) will give you sticky — not fluffy — mashed potatoes. Make the best of it and turn them into cheesy mashed potato cups: Mix the potatoes with grated cheddar and an egg, then scoop the mixture into lightly greased muffin cups.
Tips for the Best Mashed Potatoes
Start cooking the potatoes in cold water: This ensures that the potatoes cook evenly. Otherwise, if you start with hot or boiling water, the outsides of the potatoes cook and soften while the middles are still hard and crunchy.
For most potato dishes it's important to add the potatoes to cold water and allow the water to come to a boil with the potatoes in the water. The potato starch can react as soon as it comes in contact with hot water, which will promote uneven cooking and mealy potatoes.
Should I soak my potatoes before making mashed potatoes? You can, but I don't. If you do – just soak them for 5 minutes and rinse. Doing so is a science thing – soaking cut potatoes removes excess potato starch.
Here's how to do it: For every pound of potatoes in your mash, drizzle 1 tablespoon of melted butter over the dish and fold it gently into the potatoes. If the mash is still too gluey for your liking, repeat the process with another tablespoon of butter. It's that easy!
To thicken runny mashed potatoes with a thickening agent, add 1 tablespoon of cornstarch, flour, or powdered milk to the potatoes. Stir and continue to add more thickener, 1 tablespoon at a time, until you have the desired consistency.
Ideally, you should wait until your water is at a rolling boil. The boiling water will agitate and dissolve the salt quickly. You can add salt to your cold water if your prefer, though. You don't want to forget it after all!
If the salt concentration in the cup is higher than inside the potato cells, water moves out of the potato into the cup. This leads to shrinkage of the potato cells, which explains why the potato strips get smaller in length and diameter.
The potato in the salt water shrinks because water moves from the potato into the more concentrated salt water. In contrast, water moves from the less concentrated distilled water into the potato causing it to expand.
Dense potatoes don't absorb seasonings easily, so you'll need to salt the water liberally so that the water the potatoes do drink up also carries in seasoning. And because potatoes are so timid in flavor, they need that salt to bring them out of their shells. Pour some salt in, then keep going.
It Gives a Firmer Texture
Since soaking potatoes in salt water draws out excess moisture and reduces their ability to absorb oil, it helps them have a firmer texture.
Salt is the key here. Water will move from an area of less salt to more salt (more water to less water), and so when the potato is placed in the saltwater, all the water that is inside the potato (yes, plants have a lot of water inside of them, that's what gives a plant it's structure) moves out by osmosis.
Soupy, watery mashed potatoes usually happen because the cooked potatoes were cut into pieces that were too small, weren't drained well enough, or were overcooked. Fix them: Scoop the watery mashed potatoes into a pan and stir constantly over low heat to evaporate the excess liquid.
Whisk a little cornstarch into the warm potatoes, only a teaspoon at a time, until the potatoes thicken to your liking. The potatoes must be warm. You can also use tapioca starch or potato starch, although most cooks are less likely to have those products on hand.
Tip #3: Don't Overcook or Undercook the Potatoes
If they're undercooked, you'll have pockets of crispy potato chunks-—a big no-no for classic fluffy mashed potatoes. If you overcook them they disintegrate and your potatoes will be soupy.
The trick to the most fluffy mashed potatoes is to add a leavening agent like baking powder or baking soda. Just a pinch of baking powder added to the drained, cooked potatoes can help make them so fluffy.
Soaking potatoes in water helps remove excess starch. Excess starch can inhibit the potatoes from cooking evenly as well as creating a gummy or sticky texture on the outside of your potatoes. Cold water is used because hot water would react with the starch activating it, making it harder to separate from the potatoes.
A word about butter: Don't melt butter before stirring it into the potatoes because the milk solids and fat will separate. You can add cold butter to your hot potatoes since the butter will melt as a whole and distribute the fat and milk solids evenly.
Potatoes can be peeled prepped and cut into water up to 2 days before boiling for mash.
You can cover the potatoes while they are cooking but you don't have to. Do you put potatoes in cold water to boil? Yes. If you place potatoes in already boiling water the outsides will cook first resulting in unevenly cooked potatoes that are more likely to break down.