Thanksgiving is the one time a year when home cooks get to think like line cooks. Just like in a professional kitchen, orchestrating a turkey day meal requires scrupulous planning, detailed timelines, and lots and lots of prep. While pies, gravy, and most casseroles can be made a day ahead, some dishes and ingredients don’t fare as well to the test of time. One example is the potato.
Once peeled and cut, the tuber can turn brownish-black in the time it takes to boil a pot of water. But when you have a boatload of mashed potatoes to make—not to mention an entire bird to babysit in the oven—the temptation to prep potatoes in advance can be enticing. The good news is you can get the peeling and cutting out of the way, but to do so without causing potatoes to brown requires a few extra steps. Before we get into the how, let’s talk about why potatoes turn brown in the first place.
Why do potatoes turn brown?
The fancy science term for browning is enzymatic discoloration. And potatoes aren’t the only ones susceptible to this chameleonic phenomenon—as my colleague Zoe Denenberg reported, apples go through the same process when cut. Both foods contain an enzyme called polyphenol oxidase (PPO) as well as polyphenolic compounds (or a collection of phenols), which protect plants from UV radiation, pathogens, and parasites.
When fruits and vegetables are intact and encased within their protective peels, these two elements are contained in separate parts of the cell. But once you put a knife and a peeler into action, the enzymes and polyphenolic compounds start to mingle, and oxygen from the air joins the party, setting off the timer until your apple slices or potato chunks take on a sepia tone (also known as oxidation.)
The rate and speed of oxidation varies greatly between potato cultivars. A 2009 study published in the journal of Postharvest Biology and Technology tested the stability and browning of five different cultivars of freshly cut potatoes. Among the examined varietals, the Marabel variety (a type of waxy potato) was least susceptible to enzymatic browning because of its “low phenol content and PPO activity” and “high antioxidant activity.”
While that might lead some to conclude that potatoes with low PPO are less prone to browning, it’s a little more complicated than that. Much like apples, different potato varieties have varying amounts of phenols and PPO. And the study explains that “not all the phenols have the same affinity with PPO enzymes.” For example, some phenols, like caffeic acid, do a better job at fighting enzymatic browning than others, like tyrosine. Another variable in the browning equation is sugar content. The study explains that “the high sugar content of this [the Marabel] variety may be an additional factor” in keeping browning at bay.
Scientists are more than privy to the finicky nature of enzymatic discoloration, which doesn’t just affect home cooks; it also results in tons of commercial waste. 20 years of research and careful crossbreeding of Honeycrisp and Enterprise apple varieties led Washington State University to discover Cosmic Crisp—an apple cultivar that’s naturally slow to oxidize. Potato experts at Idaho-based Simplot Plant Sciences developed the Innate potato by reducing PPO activity. But these new-and-improved foods are still susceptible to browning, just at a much slower pace. The good news is that oxidation, regardless of varietals, is easily avoidable, and below, we dig into the how.
So how do I stop potatoes from turning brown?
Of course, if you're throwing that peeled potato straight into a pot of boiling water, that creamy color will stay frozen in time because heat deactivates the browning enzymes. But for most of us, prepping for an enormous family feast means that the potatoes will realistically need to sit around for a little while before we're ready to cook them. Luckily, you can ensure they remain unblemished by controlling external factors like ambient temperature, access to oxygen, and the pH level of your potatoes.
1. Keep potatoes submerged in water: I spent the first few weeks of culinary school dicing, slicing, and tourneing pounds and pounds of potatoes. Of course, the grades for our first practical exam depended on the level of meticulous right angles we achieved on our dices, but if we forgot to put them in a bowl of ice-cold water, we received an automatic zero. That’s because submerging potatoes in cold water is the best way to prevent potatoes from browning. As my instructor so fervently drilled into our heads, “oxygen is a raw potato’s worst enemy,” and water works as a shield to keep the air at bay.
Gustavo Teixeira, a potato postharvest physiologist and assistant professor at the University of Idaho, explained to me over email that water helps potatoes in two ways. First, as my instructor had suggested, “it reduces the oxygen access to the active site [i.e., the potatoes] and controls the oxidation of phenolic compounds.” Secondly, “it reduces the enzyme [polyphenol oxidase] activity, as at low temperatures the rate of chemical reaction will be significantly slower.” That’s why if you’re prepping potatoes in advance, it’s crucial to soak them and store them in the refrigerator to lower the temperature.
But water, once a friend, can turn into a foe within a matter of a day. You can keep peeled and/or cut potatoes soaked in the refrigerator for no longer than 24 hours. Like humans, tubers are made mostly of H2O, and keeping them submerged any longer can lead to waterlogging, which disintegrates the starches. Meaning, you’ll end up with sad, soggy taters, which will make your mashed potatoes grainy and coarse.
2. Lower the pH level with some acid: A meta-analysis of studies published in 2020 explains that enzymatic discoloration is most active between pH 5 and pH 7 and least active below pH 3. This means that in order to reduce the chances of your potatoes browning, you’ll need to lower their pH. Luckily, this doesn’t require an elaborate science experiment—adding some acid, like lemon juice, white distilled vinegar, or citric acid, to your bowl of submerged taters will suffice. I’ve found that adding one tablespoon of lemon juice (or the entire half of a lemon) per half gallon of water does the trick.
Are brown potatoes edible?
If you peeled 15 pounds of potatoes, only to get distracted by pie drippings setting off the fire alarm, it’s not the end of the world. Eating oxidized potatoes will do you no harm. But blemishes are less than ideal if you’re going for snow-like mashed potatoes or perfect pommes Anna for Thanksgiving. So, if your goal is style and substance, I recommend taking those extra few minutes to fill a big mixing bowl full of water before you get peeling. But if your potatoes do fall victim to oxidation, don’t throw them out! Just as a rose by any other name would smell as sweet, potatoes with any amount of discoloration taste just as delicious. So fry ’em or roast ’em for a while-you-cook snack.
