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To prevent cut apples from turning brown, a result of oxidation, toss them with citrus juice. The juice of oranges, lemons, or limes will work equally as well. If you want to include chopped apples in a fruit salad, allow them to marinate a few minutes in the citrus juice before adding them to the salad bowl.
I have also heard that crushing some Vitamin C tablets in water and washing the cut apples in it works - but I haven't tried it myself.
You could also buy ascorbic acid in the canning section (close to the Certo or Sure-Jell) of your supermarket to wash them in.