Why does lime juice keep an apple from turning brown?

The browning of apples, also known as enzymatic browning, is a natural process caused by the exposure of apple flesh to oxygen. This process is triggered by an enzyme called polyphenol oxidase (PPO), which is activated when the apple is cut or bruised. PPO reacts with compounds called polyphenols, which are naturally present in apples, leading to the formation of brown pigments called melanin.

Lime juice, which is acidic in nature, helps to inhibit the enzymatic browning reaction in apples. The acidity of lime juice lowers the pH of the apple flesh, creating an environment that is less conducive to PPO activity. Additionally, the citric acid present in lime juice acts as a chelating agent, binding to metal ions such as copper and iron that are essential for PPO activity. By reducing the availability of these metal ions, lime juice further inhibits PPO activity and helps to keep the apple from turning brown.

It's worth noting that while lime juice can be effective in preventing browning, it may alter the taste and texture of the apple. To minimize these effects, it is recommended to use a small amount of lime juice and apply it evenly to the cut surfaces of the apple. Alternatively, other methods such as storing apples in airtight containers or using antioxidant treatments can also be employed to prevent browning.