The Mistake: Overworking the Potatoes with a Food Processor, Blender or Mixer. Too much — or too vigorous — mashing will produce gluey potatoes. Your best tool is an old-fashioned masher, fork, ricer or food mill.
Some cells will inevitably burst, releasing tacky gel that helps give mashed potatoes their cohesiveness. But if too much gelled starch seeps out of the cells, either from overcooking or too-vigorous mashing (or both), gumminess ensues.
Instead of using an electric hand mixer, food processor, or blender — all of which will overwork the potatoes — use a ricer, food mill, or hand masher to gently break down the spuds, per The Spruce Eats.
Add a Thickening Agent
This is the most common, and perhaps the simplest way, to thicken mashed potatoes. You can use what you have on hand: Flour, cornstarch, or powdered milk are all solid options that are probably already in your pantry.
When potatoes are mashed, starch is released. The more you work the potatoes, the more starch gets released. When too much starch gets released, the potatoes become gummy, gluey, and unappetizing.
As already discussed, pastiness is caused by too much potato starch being worked into the mashed potatoes. The main way we combat this problem is by rinsing away starch, both before and after cooking potatoes for fluffy mashed potatoes. Rinsing off starch means that there is less of it that can end up in the dish.
To thicken mashed potatoes, you can add a thickening agent like cornstarch, flour, powdered milk, or tapioca. Add one tablespoon at a time until you reach your desired consistency. You can also add heat, more potatoes, or even strain the mashed potatoes in order to reach your desired consistency.
However, if you have already mixed them together, only to realized there's too much milk, the easiest thing to do is to add more potatoes to the mixture. Or if you have any dry potato flakes, that will work as well. I don't recommend adding flour or corn starch to it, unless you are plan to warm it again after adding.
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.
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.
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.
Mashed potatoes, like other starchy dishes that are made with dairy (think: creamy casseroles, macaroni and cheese, scalloped potatoes), can develop a dry, gluey texture when reheated for too long, or at a temperature that's too high.
If you find your potatoes are too salty, adding more liquid, such as milk or buttermilk, or more potatoes can help tone it down. No more potatoes on hand? Try adding some mashed cauliflower or root vegetables to the mix.
No-Mashed potato flavors are enhanced by many different ingredients-too much butter makes them taste greasy.
Make sure the milk or cream you add to your potatoes is HOT. This helps it absorb better so you don't feel the need to overmix.
To reheat mashed potatoes in the oven:
Your final product should be smooth but not runny. Cover the dish with a lid or foil and place in an oven preheated to 350 degrees.
They absorb liquids brilliantly, which is why they mash so well. But when you boil them in water, the liquid they are absorbing is just that, water, which can make for a less flavorful mash. By boiling the potatoes in salted milk, they are absorbing creaminess and seasoning, which makes them inherently more flavorful.
The flavor and texture of instant mashed potatoes doesn't suit everyone, but they can save the day and can work wonders in this case. Whisk a little cornstarch into the warm potatoes, only a teaspoon at a time, until the potatoes thicken to your liking.
Too much — or too vigorous — mashing will produce gluey potatoes. Your best tool is an old-fashioned masher, fork, ricer or food mill. If you've already done the damage, turn pasty potatoes into a casserole: Spread them in a baking dish, drizzle with melted butter and sprinkle with grated cheese and breadcrumbs.
The perfect mash is smooth and creamy with a fluffy finish and no lumps.
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.
Use a Thickening Agent – Thickening agents such as corn starch or flour are super effective in, you guessed it, thickening recipes. I recommend corn starch when thickening mashed potatoes as it does very little to affect the taste of your side dish.