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 "A Circular Play." Today, with apples coming back into season, I've been rewriting 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 oxidase and the polyphenols themselves are stored in separate areas of the plant's cells, but when the cells are damaged—say, when an apple is sliced open, or dropped and bruised—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 stop 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 types of 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 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 the color of a muddy sponge 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 with the apples.
Here's the short version: The best way to prevent browning is to soak the cut fruit in a saltwater solution (half a teaspoon of 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 into the water first, which acidulates it; lemon contains ascorbic acid, which not only lowers pH (as does the citric acid also found in lemons) but can also reverse the oxidation reaction through a process chemists call reduction. In my tests, I used three 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, sprinkling dry crystals of it directly onto the cut apple surfaces. The concentrations of the acid in these samples were too strong for the apples 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 whiter. 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 differences are already 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 sample (second from left) is not very different from the plain-water one, and definitely more yellow than the citric acid–treated samples. Note also the slight pink tint in the citric acid solutions, from pigment being leached out of the apple skins.
Jumping ahead to one hour after the first photo, the differences become more stark. The plain-water and lemon-water slices are holding up better than the air-exposed pieces of plain apple, but they don't look great. If you ate a slice of any of the water-soaked samples at this point, you'd notice that the apples had 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 the 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 even more pigments have been drawn out.
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 altered the apple flavor with a distinct citrusy quality.
I later did a series of tests with much lower concentrations of citric acid (one 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 while the texture suffers. Lemon water, meanwhile, actually speeds up browning while changing the apple's flavor, so avoid it.
Check out the 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, or common table salt, is another chemical that can interfere with oxidation. For my test, I soaked apple and pear slices for 10 minutes in a salt solution made from half a teaspoon of Diamond Crystal kosher salt dissolved in one 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. (The top row contains 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 that 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's 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 salt water 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, slowly, after several hours, the salted apple will start to brown. 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.
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've 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 anymore. Which leaves you plenty of time to repetitively ponder the shape, and not the color, of your apple.