Round as around as my apple. An apple is out of season.
Gertrude Stein wrote those two lines in 1920 as part of her poem In Circles, A Circular Play. Today, with apples coming back into season, I've been re-writing them in my head.
Brown as a-brown as my apple.
That's because apples, along with some other fruits like pears and bananas, have a tendency to brown quickly once the inner flesh is damaged and exposed to air. So, why do they brown and what's the best way to stop it?
Why Apples and Pears Turn Brown Once Cut
Apples are one of many fruits that contain a large amount of an enzyme called polyphenol oxidase. As its name suggests, it is capable of oxidizing polyphenols, molecules that play a variety of roles in plants, from protecting against infections to giving them their pigments. Polyphenol oxydase and the polyphenols themselves are stored in separate areas of the plant's cells, but when the cells are damaged—say, by slicing open an apple or dropping and bruising it—the cells are ruptured and the enzyme comes into contact with its prey...er...I mean substrate. With the help of oxygen, which is in the air around the damaged cells, the polyphenol oxidase initiates a series of chemical reactions, transforming the polyphenols and eventually producing melanins: brown pigments.
The general name for this process is "enzymatic browning," and the problem is that it doesn't just change the appearance of produce: it also alters flavor, scent, and nutritional value...and usually not in a good way. (Although it's worth mentioning that it does help produce desirable effects in some of our foods, like teas, cocoa, and dried fruits like raisins.)
Preventing browning, then, becomes a question of how to put a halt to enzymatic browning. There are scores and scores of scholarly articles out there that have examined this in depth—after all, there's a pretty big financial incentive for the produce industry to find a solution, since as much as 50% of some produce is lost annually due to the process.
Most solutions involve blocking the oxygen in one way or another, reversing the oxidation reaction, changing the pH of the environment, or halting the reaction through exposure to either high or low temperatures.
Often, on the industrial scale, these methods are used in concert with each other to maximize their effect, say by combining the powers of ascorbic and citric acids. But that's not practical for the home cook who just wants to stave off browning for a short period—you know, so the apple slices don't turn a spongy mud color while they sit on your cocktail party's cheese tray.
I've been playing with more practical methods to test out which one works best at home. In my tests, I used Red Delicious apples, which I selected for their tendency to brown relatively quickly. I repeated all my tests with Bartlett pears, which aren't pictured since my results were the same as the apples.
Here's the short version: The best way to prevent browning is to soak the cut fruit in a saltwater solution (1/2 teaspoon kosher salt per cup of water) for 10 minutes; then drain and store until ready to use. The mild salt flavor can be rinsed off with tap water before serving—the best part is that even after rinsing, the browning is still just as effectively reduced.
Water, Lemon Juice, and Citric Acid
One of the easiest things you can do to prevent browning is to submerge the cut fruit in plain water, which reduces the amount of air, and therefore oxygen, that can get to it. Since a lot of fruits float, it helps to either lay a clean paper towel on top, which, once wet, will push them under, or to put them in zipper-lock bags with the air pressed out. I did the latter, since it's easier to see the apple slices in the photos that way.
A lot of people will tell you to squeeze some lemon juice in the water first, which acidulates it; lemon contains ascorbic acid, which not only lowers pH (along with the citric acid also found in lemons) but can also reverse the oxidation reaction through a process chemists call reduction. In my tests, I did 3 tablespoons fresh lemon juice (more or less equivalent to the juice of an average lemon) per quart of water.
Then, just for the heck of it, I also played around with citric acid in two different (very strong) solutions and sprinkled dry crystals of it directly onto the cut apple surfaces. The concentrations of the acid in these samples was too strong for them to actually be edible, but it's interesting to see what higher levels of the acid can do.
In this first photo, which I've defined as zero in the timescale (though technically it took me a few minutes to bag it all up and arrange it on the table for the photograph to be taken), you can already see that the citric acid is keeping the cut surfaces ever so slightly more white. It's worth pointing out, though, that the light source in the photos was at right, illuminating the samples closer to it more than the ones on the left. To the naked eye under the room lights, I wasn't able to see the difference between the plain water and lemon water samples at all. For comparison's sake, note the white table surface underneath, which is a uniform shade of white but in the photos appears darker at left as well.
Fifteen minutes later, and already differences are becoming more apparent—the light source at the right alone isn't responsible for the yellowing of the untreated apples on the top row at left.
At 30 minutes, we have more significant browning of the plain apple slices. The plain water sample (in the bag at far left) is maintaining its color better, but not as well as the citric acid samples in the two rightmost bags. The lemon water (second from left) is not very different from the plain water and definitely more yellow than the citric acid-treated samples. Note also the slight pink tint in the citric acid solutions, from pigment being leeched out of the apple skins.
Jumping ahead to one hour after the first photo, and the differences become more stark. The plain water and lemon water are holding up better than the air-exposed pieces of plain apple, but they don't look great. Eat a slice of any of the water-soaked samples at this point, and you'll notice that the apples have become waterlogged and slightly mushy.
Take a look at the above photo. That's an image of the same samples after three hours and 15 minutes. At this point, the lemon-water sample (second from left in bottom row) has browned more than the plain water sample; this lines up with scientific studies I've read, which have reported increased browning when the apple is exposed to lower concentrations of ascorbic acid. Meanwhile, the citric acid samples look practically bleached, while the skins have taken on a neon quality as pigments have been drawn out even more.
At this point, all the samples were near inedible. The citric acid ones, of course, were inedible from the start (unless you like the idea of Sour Patch Kid-style apple slices). The plain and lemon water apples, meanwhile, had reached an undesirable level of browning, and both were waterlogged and unpleasant to eat. The lemon water also subtly alters the apple flavor with a distinct citrus quality.
I later did a series of tests with much lower concentrations of citric acid (1 teaspoon per quart of water), which I found more palatable, but they didn't work nearly as well at preventing browning. I wasn't able to find a concentration of citric acid that prevented browning well enough while also not tasting overly acidic.
I also did a test of simply rubbing a cut lemon all over the cut surface of an apple. You can see a photo of that below in the salt section, but I'll sum the results up now: Skip it, since it adds a heavy lemon flavor to the apple and only marginally reduces browning.
My take on this: Soaking apples and pears is a method that works fine if you use plain water, but only for a very short time—I'd say less than 30 minutes, and ideally less than 15. Any longer, and your fruit will brown and the texture will suffer. Lemon water, meanwhile, actually speeds up browning while changing the apple flavor, so avoid it.
Check out this time-lapse GIF below to see the apples brown over time (like Gertrude's poem, it goes round as around as an apple...).
Sodium chloride, common table salt, is another chemical that can interfere with oxidation. For my test, I soaked cut apple and pear slices for 10 minutes in a salt solution made from 1/2 teaspoon Diamond Crystal kosher salt dissolved in 1 cup of cold water. I then drained them and let them stand for two hours alongside samples of untreated cut apple.
In the photo above, the saltwater-soaked apple is on the bottom right, with two untreated apple samples to the left left (the top row is my lemon-rubbed apple, at right, and its untreated control at left).
As you can see, the saltwater apple resisted browning the best: even after two hours, when this photo was taken, it was still a respectable white color. The salt flavor is very mild on the surface of the apple—one taster didn't even notice it—but it's there. The good news is a quick rinse under cold running water completely washes away any traces of salt, leaving you with a fresh tasting and looking apple, well after it was originally cut.
But here's even better news: Even after rinsing off the salt, the apple and pear slices continue to maintain their fresh white color. This means you can soak your fruit in the saltwater for 10 minutes, then rinse it off, pat it dry, and put it out on a cheese platter for at least a couple hours without a major loss in quality. Packing a lunch for your kids? They'll have fruit that looks freshly sliced for lunch, and they won't come home complaining that it tasted salty!
Eventually, the salted apple will slowly start to brown after several hours. For most home cooks, that's not a big issue.
Bottom line: Using salt is the most effective method I tested, with the least damage to the apple's texture and flavor.
And in case you're curious, extreme temperature can also be used to prevent browning, as you can see in the photo above. The diced apple at right was blanched for two minutes in boiling water, then drained and shocked with cold water. High heat completely shuts down the browning reaction. Unfortunately, it also softens the apple considerably and gives it a cooked flavor...because you cooked it. If you need the apple for baking, that might be okay, but otherwise it's not a useful method.
Stick with the salt and you won't need to worry about the browning any more. Which leaves you plenty of time to repetitively ponder the shape, and not the color, of your apple.