Most plants contain polyphenol oxidase, an enzyme stored in their cells that reacts to the oxygen in the air, a process called oxidation that causes apple flesh to turn brown. The acid in lemon juice interferes with this by stopping the reaction between the two substances. The process is believed to serve as a defense mechanism by discouraging animals and insects from further damaging the plants.
Once added to sliced apples, the ascorbic acid, or vitamin C, in lemon juice creates a barrier between the oxygen and the polyphenol oxidase. Until the lemon juice has evaporated or been completely absorbed, the oxygen reacts with the ascorbic acid, preventing the apple from turning brown. The more acidic a substance is, the more capable it is of preventing apples from turning brown during preparation. Among other characteristics, fruits are categorized by their pH, or acidity, content. The pH of lemon juice is about 2.0, which makes it acidic enough to completely deactivate the polyphenol enzyme.
To keep just a few apple slices from turning brown until you are ready to use them, brush their cut surfaces with pure lemon juice. If you're preparing a larger number of apples for a pie or for applesauce, soak the slices in a solution made from 1/4 cup of lemon juice and 1 quart of cold water. Increase proportions based on the amount of apples, because you want to keep them completely submerged in the solution until you are ready to use them. Soak them for about five minutes, and then drain completely and pat them dry.
Using the acid in lemon juice to prevent apples and other fruit from darkening is a useful aid when preparing desserts of all kinds, from pies and breads to cakes and cobblers. While the browning of fruit is not in itself harmful, it detracts from the apples' visual quality and affects their appearance. Untreated fruit may also cause a change in the consistency or flavor of a baked good or dessert, as the brown color indicates that the tissues and sugars in the apple have started breaking down, which can lead quickly to spoilage.
Several other ways to keep apples fresh and crisp during preparation involve blocking the chemical reaction that causes them to turn brown. You can soak the sliced apples in apple juice, which has the added benefit of matching their flavor, or another juice such as lime or orange. A solution of 1/2 teaspoon salt dissolved in 4 cups of water is another choice, as is just plain cold water, as long as the apples are completely submerged. While lemon juice has one of the highest concentrations of ascorbic acid and a very low pH, the University of Illinois Extension states that it is not as effective as using the acid in its pure form. It's available wherever canning supplies are sold.
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