Why does lemon juice slow the browning of an apple?

Lemon juice slows the browning of an apple due to the presence of ascorbic acid, commonly known as vitamin C. Ascorbic acid is a natural antioxidant that inhibits the oxidation process responsible for browning. Here's a detailed explanation of the process:

1. Enzymatic Browning:

When an apple is cut or bruised, it exposes the inner flesh to oxygen. This reaction triggers the activation of an enzyme called polyphenol oxidase, which starts a chain reaction leading to the formation of brown pigments called melanin.

2. Role of Ascorbic Acid:

Ascorbic acid, or vitamin C, acts as an antioxidant by donating electrons to free radicals, including the ones generated during the browning process. By neutralizing these free radicals, ascorbic acid prevents them from reacting with the polyphenols, thereby slowing down the browning reaction.

3. Mechanism of Action:

Ascorbic acid donates its hydrogen atoms to the free radicals, converting them into harmless molecules like water. This process prevents the free radicals from initiating or propagating the chain reactions that lead to browning.

4. pH Influence:

Lemon juice also lowers the pH of the apple's surface, which further inhibits the browning reaction. Polyphenol oxidase, the enzyme responsible for browning, is less active in acidic conditions.

5. Antioxidant Properties:

In addition to ascorbic acid, lemon juice contains other antioxidant compounds like citric acid and flavonoids, which contribute to the overall antioxidant effect and help in slowing down browning.

It's important to note that while lemon juice can delay browning, it does not completely prevent it. Over time, the enzymatic reactions may still occur, leading to gradual browning of the apple. Storing cut apples in acidic solutions, such as lemon juice or ascorbic acid-based solutions, can extend their shelf life and maintain a fresher appearance for a longer period.