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.
Answer provided by Dr. Universe