Why do apples not brown when you put lemon juice on it?

The primary reason apples do not brown when you put lemon juice on them is due to the presence of citric acid in the lemon juice. Citric acid, a natural preservative, acts as an antioxidant that helps slow down the oxidation process that causes fruits and vegetables to turn brown. Here's the science behind it:

Oxidation and Browning:

When fruits or vegetables are cut or exposed to air, they come into contact with oxygen. This exposure initiates an enzymatic reaction known as oxidation, where the oxygen reacts with compounds called polyphenols in the fruit, leading to the production of brown pigments called melanin.

Role of Citric Acid:

Lemon juice is rich in citric acid, a weak organic acid. When you apply lemon juice to the apple slices, the citric acid reacts with the polyphenols present on the apple's surface. Citric acid donates hydrogen ions (H+) to the polyphenols, which results in the formation of a slightly acidic environment.

Acidic Environment Inhibits Browning:

The acidic environment created by the lemon juice inhibits the activity of the enzymes responsible for oxidation. The polyphenols remain in a reduced state, preventing the formation of melanin pigments and thus slowing down the browning process.

Additional Factors:

In addition to citric acid, other factors may contribute to the effectiveness of lemon juice in preventing browning:

- Ascorbic Acid (Vitamin C): Lemons also contain ascorbic acid (vitamin C), another antioxidant that can help prevent browning.

- Lower pH: The acidic nature of lemon juice can lower the pH of the apple's surface, which further inhibits enzyme activity and browning.

- Chelation of Metal Ions: Citric acid can bind to metal ions present in the apple, such as iron and copper, which are involved in the oxidation process. By binding to these metal ions, citric acid reduces their availability for oxidation reactions.

Overall, it is the combination of the acidic environment, the antioxidant properties of citric acid and vitamin C, and the chelation of metal ions that effectively prevents browning in apple slices treated with lemon juice.