Add more cream and butter: More moisture and fat are the two secret ingredients to reheating any leftovers. For potatoes, reach for half-and-half or broth and butter. Cover and heat low and slow: Cover the baking dish and set it in a 350°F oven alongside any turkey leftovers you might be reheating too.
You can use milk, cream, half-and-half, tangy buttermilk, or chicken or vegetable stock to rehydrate your mashed potatoes. Start with a ½ cup of liquid for 4 cups of mashed potatoes, and add it gradually so you don't end up with potato soup. Adding extra butter is a good idea, too.
Oven method:
If you refrigerated your mashed potatoes in an oven-safe casserole dish, set them out on the counter to take the chill off for at least 20 minutes before you pop them into the oven. When you're ready, bake, covered, at 350 degrees for 15 to 20 minutes, or until the potatoes are hot throughout.
You can use what you have on hand: Flour, cornstarch, or powdered milk are all solid options that are probably already in your pantry. Potato flour and potato starch would work as well. Stir in the thickening agent gradually, about a tablespoon at a time, until the potatoes have reached your desired consistency.
According to Serious Eats, when your mashed potatoes end up with a pasty or a gluey consistency, it's due to an excess of starch. As a result, limiting the amount of starch in your dish is the key to nailing perfect mashed potatoes. Luckily, there's an easy way to do this: Don't overwork your potatoes.
Fix them: Scoop the watery mashed potatoes into a pan and stir constantly over low heat to evaporate the excess liquid. If the potatoes are really watery, you can add either cornstarch or instant mashed potatoes a half teaspoon at a time to help the spuds thicken to your preferred consistency.
Texture: Texture is another sign of whether mashed potatoes have gone bad. If the mashed potatoes are slimy or have a watery consistency, they should be discarded. Additionally, if the mashed potatoes are hard and dry, they may be stale and not suitable for consumption.
Use heat to evaporate the excess milk
If you don't have any thickening agent, the next best thing to to remove the milk using heat. You can use a large shallow baking pan and spread out the mash potatoes. Put it in the oven under low heat. You can stir it every 30 mins.
When you're pressed for time, a microwave can be used to thicken runny mashed potatoes. Place the potatoes into a microwave-safe container and heat them on high for 1 minute. Check the potatoes after 1 minute and gently stir. If the potatoes need to thicken up some more, heat them for another minute and check again.
Usually, restaurants reheat mashed potatoes inside the crockpot for several hours. If you also want to try using this method at home, be mindful of signs of spoilage such as changes in texture or color when keeping it for long hours. Also, check the temperature setting when you reheat the dish to avoid overheating.
Take your mashed potatoes out of the fridge and set them on the counter for about 20 minutes to bring them to room temperature. Meanwhile, preheat the oven to 350°F. Then, pop them in the oven, covered, for about 15-20 minutes, or until warmed through.
If this occurs, the outlet recommends correcting course with more fat: Adding more liquid in the form of melted butter, extra-virgin olive oil, cream, or full-fat milk.
Adding cold dairy to your mashed potatoes also cools the mash down — and very few people like cold mashed potatoes. Ready to put this tip into action? Bring the milk and butter to at least room temperature or, better yet, gently heat them on the stovetop before adding them to your potatoes.
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.
To reheat mashed potatoes in the oven:
Stir in melted butter, melted vegan butter, or oil and milk/cream, vegan milk, or broth. Start with about 1/2c liquid for every 4 cups of potatoes. Your final product should be smooth but not runny.
Substituting Milk for Heavy Cream
Whole milk alone can work in some instances as a substitute. At 3.5%, its fat content is low, but it's a good substitute in mashed potatoes or potatoes au gratin (you'll just need to add extra flour to prevent the sauce from separating).
When too much starch gets released, the potatoes become gummy, gluey, and unappetizing. Overworking the potatoes can happen in a couple ways: either by simply handling them too much, or by using a food processor, blender, or similar tool, which mixes the potatoes too aggressively.
Salt the water generously.
Potatoes love salt. Don't be shy with the salt. If you don't add enough salt, the potatoes will be bland. It's surprising that such a simple ingredient would make a big difference, but it will.
Mashed potatoes stored in an airtight container last 3-5 days in the refrigerator. But, keep in mind, that recipes with added dairy like butter, milk, or sour cream may go bad more quickly than plain potatoes. And be sure to store them towards the back of the refrigerator for best results.
Add a Leavening Ingredient
Baking soda needs to be combined with an acidic ingredient of some kind (buttermilk, lemon juice, vinegar, honey, yogurt) to create carbon dioxide, a gas that emits small bubbles and makes the goods fluffy.
Heavy cream can make instant mashed potatoes taste indulgent. Half and half or even regular milk (whole is better than skim, but 2% will do) adds extra creaminess. To give your mashed potatoes a light tanginess, go with buttermilk or sour cream. If you don't want to go all in, try half water and half dairy.