Why does lemon juice keep apple slices from turning brown?

Have you ever packed a school lunch with fresh apple slices and instinctively reached for the lemon juice to squirt on them? If you are like me, you did this without questioning why lemon juice keeps the apples from browning. I like to think of it as my “mommy intuition”.  But one day I wondered why this happens and what alternative to lemon juice I could use in a pinch.

So yesterday I told Allie we were going to do a little experiment. She said, “Mommy, I want to do science I can eat.”  That’s my girl!

I told her we were going to experiment with different juices on apples to see which one would keep the apple slices from browning.

I did a little research and selected the following juices in our kitchen:  lemon juice, apple juice, orange juice, soda (carbonic acid), and apple cider vinegar.  All of these juices are acidic.  Then I decided to test milk and water to see if something other than acid would work.

    

Every scientist needs to test the quality of the experiment, so Allie enjoyed the apple juice and some apple slices while we worked.

Then she painted each apple slice with one of the liquid samples. We had a ‘control’ apple slice with nothing on it for comparison.

Her guess: I let Allie taste all the liquids and guess which one would keep the apples slices from turning brown. She guessed lemon juice because it was the most sour.

Here are the slices at the start of the experiment.  I also looked up the pH of each liquid. Remember the lower the pH the stronger the acid.  Lemon juice happens to be the most acidic of our juice samples with a pH of 2.

An hour later all the slices were turning brown except the slice with lemon juice.  The ‘control’ slice and water slice looked the worst.

Four hours later the lemon juice slice won hands down and it was surprisingly still delicious!

What happened:  I found an amazing explanation here.  Basically, when apples are sliced they release an enzyme called polyphenol oxidase.  When this enzyme combines with air it oxidizes and starts the browning process.  The enzyme is “deactivated” when an acid is applied but the acid has to have a pH less than 3.   So all of the acidic juices we chose had a pH greater than 3 except lemon juice.  The article says lime juice and cranberry juice are just as effective as lemon juice!  Cranberry and lime apple slices… sounds yummy to me!

Follow up: Did you know there is a genetically modified apple that doesn’t brown? They are called ‘Artic’ apples and their enzyme is modified to dramatically slow the browning process.  You can read the full article here.  For now  I think I’ll keep squirting lemon juice on my apple slices for Allie’s lunch.

Have fun packing those lunches and I hope this post will inspire you to try your own apple slice experiment!