BJ's Science Fair Project

For this year's Science Fair, BJ decided to investigate how best to keep apple slices from turning brown. Apples are a favorite fruit, and since he's losing teeth now, the apple slices make for easier eating. But brown slices? Not so appetizing.

I knew a little about the basic chemistry behind the question he was asking -- acids inhibit the enzyme reaction that leads to brown fruit. I'd used lemon juice myself to keep apples from browning. We decided we'd test a variety of acids: lemon juice, lemonade Kool-Aid, apple cider vinegar, and white vinegar. (We diluted 1/2 teaspoon of powder in 6 tablespoons water.) Each apple slice was dipped in the liquid for two minutes and then set out on waxed paper to see what would happen.

He included water and a control slice of apple for comparison.
After three hours, the control and vinegar slices were decidedly brown. The lemon juice and lemonade Kool-Aid did the best job of keeping the apples from browning. The one soaked in water was a little bit brown. Surprisingly, the vinegar-soaked slices were as brown or even more brown than the control. (The results don't show up as well as we would have liked in the picture.)
He then did a taste test -- since the idea behind the experiment was to keep the apple slices from browning so that they would be more appetizing. The vinegar ones were "OK" but not all that yummy, he told me. The control and water slices were "normal." The lemon juice one was "Yum" and the lemonade Kool-aid was deemed to be yummiest of all.

So the citric acid and ascorbic acid in the Kool-aid was the winner!

Unfortunately, I didn't have any pH strips to test acidity directly. (Then again, it's a first-grader's project. The lack of pH strips isn't that big a deal in this context.)

After the experiment was over, we did what BJ usually does these days when faced with a question: GOOGLE IT!

Here's what we discovered on FAQ KIDS:
Q: Why does lemon juice prevent cut apples from turning brown and vinegar doesn’t?

A: Cut apples turn brown because of a reaction called oxidation. Oxygen in the air reacts with a natural substance in apples called phenolase and turns the cut surface brown. Mild acids like lemon juice can stop this reaction; tossing cut apple chunks with a little lemon juice will keep the apples from turning brown. Vinegar which literally means “sour wine” goes through a fermentation process, which makes it stronger than lemon juice. The key here seems to be the mildness of lemon juice.

If I had it to do over again, I'd include those pH strips to quantify the acidity of each liquid. But BJ enjoyed the experiment, learned a little bit about chemistry, and discovered a practical cooking tip in the process. I call that success.