It’s the curse of the sluggish apple eater—brown apples. We’ve all been there; you leave a few apple slices out too long, or take too long to eat your way around an apple, and you’re confronted with an unpleasant sight. Your once crispy, juicy white apple has turned a dismal shade of brown. Not very appetizing. The good news is that a brown apple is perfectly safe to eat. The bad news is that it’s ugly. The cause is enzymatic browning, a molecular reaction that occurs when certain enzymes and a class of chemicals known as phenolic compounds, or phenols, naturally found in some plants combine and react in the presence of oxygen. One of the products of their chemical reaction is a brown pigments called melanin. And it’s not only apples that are susceptible to this phenomenon. Pears, bananas, avocados, eggplants and potatoes can also undergo enzymatic browning, because they, like apples, contain phenols. Fun fact: Bruises in fruit are caused by the enzymatic browning too!
Slicing (or biting into) an apple allows the phenols and enzymes that are normally kept separate by the plant’s cell structure to commingle. As soon as this phenol-enzyme mixture is exposed to oxygen (usually the second you cut the apple open…unless you’re slicing an apple in the vacuum of space…if so, kudos!), the browning process begins. The longer it sits, the more brown pigments are formed, until all the available phenols have been used up and the fruit is as brown as it’s going to get.
Enzymes are simply proteins, but they have very special functions in nature. Enzymes allow chemical reactions to take place that otherwise would never occur (at least not in any significant amount) because they require too much energy (most chemicals are pretty lazy; they usually prefer the path of least resistance). Enzymes are the sort of lever or pulley of the biological world–they make jobs (reactions) that should be impossible much easier to accomplish, and they never get tired! They will repeat the action over and over until they run out of supplies! Sometimes the results of these reactions are undesirable, like enzymatic browning in cut fruits, but enzymes are key in many foods that are important to our American culture too! For example, the enzyme rennet is needed to change liquid milk into the curds that make cheese; the enzyme lactase breaks down lactose for us in our bodies (unless you’re lactose intolerant); and the food industry uses measurements of certain enzyme levels in food to determine the freshness of seafood, the safety of pasteurized milk, and other helpful info. Not to mention this very same enzymatic browning reaction contributes to the desirable color of tea, coffee, and raisins.
Because enzymes are proteins, they, like all proteins, denature in the presence of heat or acidity. When a protein denatures, it loses its unique shape and therefore its function. Enzymes interact with other molecules by acting much like a puzzle piece, so shape means everything; once they’ve lost it, poof! They’re toast. Once denatured, an enzyme cannot revert to its original shape, so the effect is permanent.
You may know that adding lemon juice to sliced apples is a common way to prevent browning. Lemon juice works because its acidity denatures the enzyme. Lemon juice is usually the type of acid used because its flavor is complementary to apples. However, there are lots of different ways to stop or slow enzymatic browning. I’ve worked up a little guide (below) for some at-home ideas to stop or slow this reaction from ruining your fruit salad or guacamole.
Your guide to perfectly un-brown food:
Works for apples, bananas, pears, peaches, grapes, potatoes, eggplant, and avocados.
Method 1. Stop the enzyme. These methods denature the enzyme and prevent it from producing brown pigments.
Acidity: Add some vinegar, cream of tartar (which is tartaric acid), lemon or lime juice to water to bring down the pH on the surface of your food. Choose an acid based on what flavors will go best with your food. I would guess that cream of tartar would have the mildest flavor, but you’d have to judge for yourself. If you find lemon juice too tart, you could add some sugar to balance the flavor. The citric acid in the lemon juice will still do its work regardless of whether you choose to sweeten it up or not.
Heat: Blanch, boil, steam, bake, broil, fry, poach, microwave, or otherwise heat the item in question. This is great for potatoes or eggplant, but not so great if you want to serve your product as fresh. Heat will destroy the enzymes, but it will also cook your food, so even an quick blanch is probably not a good idea if you are using a delicate food (avocado or banana) or want to retain the crispiness of the fresh item.
Method 2. Stop the oxygen. Oxygen is required for the browning reaction to move forward. Here are some ways to limit oxygen’s access to your sliced fruits and veggies.
Water: Dunking your cut fruits in plain water is often very helpful in slowing browning. The layer of water helps limit oxygen’s access to the cell surfaces. A dunking in simple syrup (a 1:1 mixture of sugar:water) will achieve the same effect with a little added sweetness and perhaps a bit more staying power if the surfaces are left out long enough to dry (the syrup should leave a bit of a film). Some people recommend dunking in water, then dredging in dry powdered or granulated sugar. For me that would be too sweet, but that should work at least as well as plain water.
Antioxidants: This is the method most commonly used in the food industry to prevent browning on pre-cut apple slices. If you’ve ever eaten the apple slices from McDonald’s, Burger King, or Subway, you’ve tried apples prepared using this method. The antioxidant of choice is Vitamin C, also known as ascorbate or ascorbic acid; it’s not acidic enough to denature the enzyme, but it will steal oxygen right out of the enzyme’s proverbial mouth. It’s preferred because the protection it offers can last a long time (though it is not permanent) and the flavor is relatively mild and citrusy. You can replicate this method at home by dissolving a Vitamin C tablet in water and dunking your fresh cut fruits and veggies. Industrially, you will almost always see this paired with calcium. Sometimes you’ll see it listed as calcium ascorbate, which is a combination of both in one molecule. Calcium is commonly added to canned foods and pickles because it keeps food firm, even when it’s sitting in liquid. In this application, it works to firm the cell walls in the apple, thereby reducing the leaching of phenols out of the sliced cell structure. Sort of like a band-aid. It’s not very effective by itself, but together, Vitamin C and calcium are more effective than either one would be alone. So throw a pinch of calcium supplement into that waterbath while you’re at it.
OR, if you’re not interested in emptying your medicine cabinet of its (probably expensive) contents, here is what I think is the smartest combination of the above principles. Use apple (or orange juice, if you don’t mind the flavor) that is fortified with Vitamin C (and calcium, if you can get it). Apple juice has a really mild flavor and will give you the benefits of the plain water dunk, plus the added boost of Vitamin C. If you use a juice that is also fortified with calcium (pretty common in orange juice) you’ll get a powerhouse of antioxidants that will keep your food un-brown until serving time! For the full whammy, add some lemon juice (or other acid) to that apple juice to lower the pH and you’ll have those browning enzymes crying for their mamas! Goodness help those enzymes if you should decide to COOK your sliced fruit in such a liquid! Whew! There’s no hope in sight for those lonely brown pigments now!! Of course, that’s really going overboard…really just one attack method should suffice. Honestly, the Gala apples I used in this experiment took many hours to get as brown as they did, so that might be a good variety to choose if making apple slices. I’ve eaten Red Delicious apples before that were brown on the other side before I even finished eating them! NOT a great choice for sliced apples.
Keep your sliced fruit away from copper and iron. These metals help the browning enzyme do its job. Fast. So don’t mix or serve your fruit salad or guac in a copper bowl, and don’t (for so many reasons) cut your food with a rusty knife or other ferrous metal.
Refrigerate cut fruits and veggies. The enzymes can still do their thing in the fridge, but they are much slower at low temperatures. By the same token, don’t set your sliced bananas by the heater; warm cozy temps (up to about 104F) make the enzymes work even faster.
To see the wonders of the apple juice + Vitamin C technique, check out these photos from my experiment with it. I used this technique in my post “buffalo blue pulled turkey sliders with a crisp celery apple salad“. I made the salad, including the sliced apples, well in advance of dinner time. The apples looked great, and you can compare them to the untreated apple slices, which turned brown. Enjoy!