Article in English: Apples turn brown due to an enzyme in the fruit that reacts with oxygen in the air. This process is known as "oxidation." Lemon juice works to prevent oxidation as it contains citric acid, which is an effective anti-oxidant. You can use either freshly squeezed or reconstituted lemon juice, though it may be best to only use this method on sweeter types of apple, as the lemon juice will add tartness. To use lemon juice to stop your cut apples from turning brown, you can use one of two methods:  You can also use lemon to prevent browning by soaking the apple pieces in a bowl of cold water and lemon juice. You should use a ratio of one tablespoon of lemon juice to one cup of water. You only need to soak the apple for 3-5 minutes, before draining and rinsing. Lime juice can be used in place of the lemon juice in these methods, as limes also contain the citric acid necessary to prevent oxidation. Pineapple juice is another good option.

Summary in Vietnamese:
Sử dụng nước chanh.