Apples will keep fresh for several weeks inside of a refrigerator, but once you cut one open, it browns in a few minutes. A process called oxidation causes this. Once the flesh of the fruit hits the air it reacts to the oxygen which causes the flesh to turn an unappetizing color. Not only does the apple look bad, oxidation also can cause a loss of vitamin C, according to the University of Illinois Extension Service. In order to stop oxidation from occurring you can treat apples with citrus fruit juice.
Wash the citrus fruit of your choice and the uncut apples in warm soapy water. Rinse them well until all the soap is gone.
Roll the citrus fruit back and forth across a hard surface, applying pressure. This will allow you to get the most juice from the citrus fruit.
Cut the citrus fruit in half. Squeeze as much juice from each half of the citrus fruit as possible. Use a citrus fruit juicer if you have one available. If not, use your hands.
Cut the apples into pieces, removing the seeds at the center.
Place the apples in a bowl and pour 3 to 4 tablespoons of the citrus juice on top of them.
Mix the apples with the citrus juice until they are well coated. This will prevent the apples from turning brown.