Apple, whole, fresh, and brown

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!

Banana, whole, fresh, and brown

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.

Avocado, whole, (diced) fresh, brown

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.

Potato, whole, fresh, and brown

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.

Additional Tips: 

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!

Tagged with →  

20 Responses to Why Apples Turn Brown

  1. mei says:

    thanks for the insightful article. I have been doing quite a bit of research into whether stainless steel knives cause fruits to oxidise more than other knives such as ceramic knives. There are many claims of such on the website but no one has explained why. Your article comes closest to explaining as you mention iron speeds up oxidation! Do you have more information on why iron speeds up the oxidation process? thanks very much in advance!

  2. Terri says:

    I read that using Sprite or the other soft drinks like it would also work. I have tried in the past and it did… but not any better than using Fruit Fresh or juices. The only benefit was it was not too lemony.

  3. heidi says:

    So fascinating

  4. Steve Erasmus says:

    Excellent article, have subscribed immediately ! Thank you so much ! :-)

  5. Keke says:

    so if you put them in water will the still turn brown

    • Amanda says:

      Apple slices in plain cold water will still brown, but at a much slower rate. I use plain cold water all the time when I only need to keep them fresh for a 10 or 15 minutes (like while I’m prepping to make pie or applesauce). However, if you need them to stay fresh for hours in a packed lunch or overnight, bathing them in something acidic, like lemon water, will stop the browning once and for all.

  6. paul says:

    I found this article after searching – I also found this http://hortsci.ashspublications.org/content/45/8/1150.full

    The reason I looked was I had bought apples here in the UK that were grown in Poland and I had cut one in half – and left the cut half unwrapped and out on my desk over night a full 14 hours. It showed only a few and very light brown marks very light.

    My question would be this – Is there any dietary benefits or nutritional benefits to apples that brown quickly versus these that do not. If the browning is caused by enzymes then do we want apples with more enzymes or not.

    • Amanda says:

      Paul,
      Thanks for your insightful comment and interesting link! As that study showed, the amount of browning you’ll see in a given apple will vary based on the type of apple, growing conditions, age of the apple, and (possibly) the age of the tree on which the apple grew. Fascinating! Typically older (i.e. more ripe) apples will brown less than their younger counterparts (though this study only examined age up to harvest time; not any time spent ripening on your counter at home).

      Nutritionally speaking, less browning is often correlated with less phenolic compounds in the apple. Phenolic compounds act as antioxidants in our bodies, which may help prevent disease and keep us healthy. However, there is some argument that the other nutrients and fiber in the apple do as much or more to keep us healthy then the phenolic compounds. All nutrients, phenolic and otherwise, in apples will vary based on agricultural conditions, maturity and apple variety. So, realistically, the ease with which your apple browns has no significant impact on its overall nutritional value.

      The major reason for scientific interest in low-browning apples, as in the study you linked to, is not for fresh apples. It’s for apples that are processed into packaged goods (e.g. applesauce, fruit cups, apple juice, dried apples). Apples that brown too much before they can be processed often have to be thrown out. A slow or low browning apple would prevent this waste and improve processing efficiency and product quality. But, again, that’s just for apple processors. For the rest of us, fresh apples are good for us just the way they are!

  7. Abirami Nadarajan says:

    Hai, good day. I’m a student from Malaysia, currently working on food science educational videos. I came across your images and was wondering whether I could use it in my videos. These videos are purely for educational purposes and will be uploaded in YOUTUBE for global learners! Thank you in advance. =)

  8. Prissy says:

    The browning increases with age of fruit and the soil in which parsnips are grown influences their susceptibility to browning with parsnips grown in sandy soils being more susceptible to browning.

  9. Pam Greene says:

    I eat a Gala apple every day – some get brown really quickly, and some do not – so there must be other factors involved other than variety – like where the apple’s been before I got the knife at it.

    Interesting post!!

  10. Adeline says:

    i think about this all the time, and some apples brown so quickly, I can’t keep up! super interesting stuff

  11. Ori says:

    Interesting!

  12. David Lemus says:

    Are certain apples more
    Susceptible to browning than others? Recommendations for making a fruit salad with apples and bananas. Just add lemon juice? But I don’t want my salad lemony…

    • Amanda says:

      Interesting question. In fact, different apples do have different browning rates, since exact levels of enzymes and phenols can vary based on variety, maturity, seasonal changes, and storage conditions. I wasn’t able to find much info on which apples were the best and worst about it, but all apples will brown eventually, unless given a surface treatment. Some sources mentioned that Red Delicious, McIntosh, and Fuji were more prone to browning. I saw a mention of Cortland apples as being low-browning (due to a naturally low enzyme content), but they are difficult to find. I will update this if I find better info. I used Gala apples for my experiment, which I would say browned at a medium-slow rate (certainly not too fast to eat!)

      If you want to avoid a “lemony” flavor in your fruit salad, a few solutions come to mind. The simplest is to use plain water instead of lemon-water. It won’t inhibit the enzyme, but even a dunk or soak in water will inhibit the ability of oxygen to come in contact with the cut apple. Your fruit will still turn brown, but more slowly than it would have dry. Keeping cut fruits in the fridge will also slow down browning, since enzymes slow down in the cold. If you still want to use lemon-water to permanently deactivate the enzyme, but don’t like the tart flavor, add some sugar to balance the sourness; that should make the lemon less noticeable. And here’s my last idea–Vitamin C (aka ascorbate) is an anti-oxidant, which means it will essentially steal any oxygen it can out of the hands of the enzyme. Without oxygen, the brown pigments can’t form, so you get a pretty apple even though those enzymes are still kickin. For the cleanest flavor and lowest cost, I would recommend dunking your sliced fruit in apple (or other mild-tasting fruit) juice fortified with Vitamin C (and Calcium if you can get it, which, when used alongside Vitamin C, synergistically helps prevent browning by firming cell walls and slowing the leaching of phenols from the cell structure). Alternately, dissolving a Vitamin C tablet in water should work too, if you don’t mind the citrusy flavor, but that could get pricey. I hope this helps!

  13. MJ says:

    Great post! Check out how USDA research helped get this concept ‘up to speed’ to add cut fruit to your local fast food menu: http://www.ars.usda.gov/is/pr/2005/050428.htm

    • Amanda says:

      Thanks for the link! I’d bet good money that the “invisible, vitamin-and-mineral based coating” their referring to is a Vitamin C + calcium combination like I mentioned in my post! I’m sure the scientists put a lot of time and effort into finding the perfect ratios. 28 days without browning is a long time! Amazing the chemical impact of some of the most common food substances like vitamins and minerals!

Leave a Reply

Your email address will not be published. Required fields are marked *

%d bloggers like this: