One of the best parts of a big holiday dinner is leftovers and when it comes to leftovers, mashed potatoes is at the top of the list. You can eat them cold out of the fridge, but what if you want to reheat them? Is there a right way and a wrong way to reheat mashed potatoes?
If you've ever made mashed potatoes, you probably know that one of your main objectives is that you want them to come out smooth and creamy, not stiff and gluey. And as it turns out, that's the same issue you face when reheating mashed potatoes.
But exactly what causes gummy mashed potatoes? Why does that happen in the first place?
The Gelatinization of Starches
Potatoes contain starch and there are two kinds: amylose and amylopectin. In their natural state, these starch molecules have a crystalline structure. When heated using dry heat, like when you put bread in a toaster, these starches undergo dextrinization: the starches turn brown and convert to sugar.
But when heated using moist heat, the starches absorb water, causing the starch granules to swell up. It's mostly the amylose absorbing water, but it happens with amylopectin to a lesser degree. While warm, the cooked starches have high viscosity, which means it will be smooth and creamy.
But when the starches start to cool, they undergo gelatinization. They thicken, and tend to form a solid mass. Other factors can encourage this, such as overheating or excessive stirring or mashing, which can rupture the expanded starch granules, causing the mixture to thicken.
That's why it's best to simmer your potatoes for mashed potatoes, rather than boil them, and avoid over-mashing them. Certainly using an electric mixer on them is a no-no.
(Note that the presence of sugar, salt, fat and/or acid all help to prevent the rupturing of the starch granules by blocking their ability to absorb water, thus ensuring creaminess rather than gumminess. So salt, butter, and cream in your mashed potatoes are definitely helpful as well as tasty.)
But with leftovers, the starches cool fully and are stored for a period of time. During this time, the starches undergo what is called retrogradation, meaning the starch granules give back some of their water, and the starches partially revert to their crystalline structure. The challenge when reheating them is to achieve creaminess without gelatinization.
And it turns out, the best way to do that is by managing the temperature. In other words, don't let your mashed potatoes get too hot when you're reheating them. And how hot is too hot? The answer is, the gelatinization starts to occur at 131 F and up to around 185 F, depending on the type of starch. Which is not to say that you need to use a thermometer to measure the temperature, but simply that your goal when reheating mashed potatoes is to try to keep them from getting too hot.
Note that different potatoes contain different starches. Starchy or mealy potatoes like russet potatoes have more amylose than amylopectin, so they are more prone to gelatinization. Waxy potatoes like Yukon golds contain more amylopectin. They can get stiff, but it takes more heat. Obviously if you're reheating leftovers, you can't go back and change what kind of potatoes you use, but it's still good to know for next time.
The Best Tool for the Job? It's Your Microwave
Some people will discourage using a microwave for reheating mashed potatoes, likely because microwaved mashed potatoes sometimes turn out gummy. But that's not the microwave's fault. It's actually due to user error.
It's very easy to get something extremely hot in a microwave. Like, depending on how much food you're reheating, you can achieve 131 to 185 F within a matter of seconds. But that doesn't mean a microwave is no good for reheating mashed potatoes. It's actually perfect for the job. No other technique works as well.
Reheating on the stovetop can cause mashed potatoes to stick to the bottom of the pot. A slow cooker can take an hour or longer. Reheating in the oven can work for smaller amounts, but it still takes too long. And using a double boiler is just way too much trouble.
The microwave, it turns out, is actually perfect for reheating mashed potatoes. Simply add your mashed potatoes to a microwave-safe bowl, along with a tablespoon or so of milk or cream. Cover the bowl tightly with plastic wrap and microwave on 50 percent power for 60 seconds. Stir, taste for heat, and repeat as necessary, heating for 60 seconds at a time, stirring in between, until you achieve the right temperature.
These 60 second bursts at 50 percent power ensure that the potatoes never get too hot, thus ensuring that they turn out creamy, not gummy.