If brown apple slices are getting you down, think twice before reaching for a box of baking soda to stop the discoloration. Keeping sliced, cubed or mashed apples looking fresh requires a strong acid to stop the chemical reaction that happens when the sweet, crisp inner flesh is exposed to the air. Baking soda, being alkaline, simply isn't the right tool for the task.
Soaking apple slices in lemon juice is better at stopping them from browning because it is higher in acid than baking soda is.
Apple browning is caused by a chemical reaction known as enzymatic oxidation. When a cut apple comes in contact with the oxygen in air, polyphenol enzymes begin to break down. It is this enzyme breakdown that results in browning. This reaction may be a natural defense mechanism used by plants to guard against pest invasion when the fruit becomes blemished. Although this chemical reaction cannot be reversed, you can slow it down.
The chemical reaction that occurs when cut apples meet oxygen can be slowed by the introduction of an acid. Ascorbic acid and lemon juice are traditional additives to prevent apples from browning. You can also use vinegar, though it may give your apples an unpleasant flavor. What all of these ingredients have in common is their low pH. The lower a substance's pH, the higher it is in acid. It is acid which slows down the enzymatic reaction and keeps your apples looking and tasting fresh.
Unlike acidic items, baking soda has a high pH, which means that it is more alkaline. Substances with a high pH contain higher amounts of oxygen. The more oxygen there is to react with the polyphenol enzymes, the more browning you will see on the apples. So, exposing an apple slice to baking soda can actually make it turn brown even faster.
The simplest way to stop apple slices from browning is to cover them completely with cool water. This will protect them from the oxygen in the air and slow their browning a bit. The most effective way to stop browning is to dip your apple slices in a bowl of lemon, orange or apple juice. Or spritz your apple slices with a small amount of ascorbic acid. You can also buy commercial anti-browning products at the supermarket. After thoroughly dousing your apple slices with a fruit acid, ascorbic acid or commercial anti-browning product, place them in an airtight container and store them in the refrigerator.