Why does lemon juice fade in the presence of sunlight?

The fading of lemon juice in sunlight can be attributed to a photochemical reaction involving several compounds present in the juice, particularly ascorbic acid (vitamin C) and certain flavonoids. When exposed to sunlight, these compounds undergo chemical changes that lead to their degradation and loss of color.

Here's a breakdown of the photochemical processes involved:

1. Ascorbic Acid (Vitamin C) Degradation:

Ascorbic acid, a potent antioxidant, is susceptible to oxidation in the presence of light. When exposed to ultraviolet (UV) radiation from sunlight, ascorbic acid undergoes photooxidation, where it reacts with oxygen to form dehydroascorbic acid, which further breaks down into other products. This degradation process leads to a loss of ascorbic acid's antioxidant activity and results in the discoloration and fading of the lemon juice.

2. Flavonoid Phototransformation:

Lemon juice contains various flavonoids, which are natural plant pigments responsible for their bright yellow color. When exposed to sunlight, flavonoids undergo phototransformation reactions. These reactions involve the rearrangement and breakdown of the flavonoid molecules, leading to the formation of colorless compounds. As a result, the intensity of the yellow color in lemon juice decreases, causing the juice to appear faded.

3. Interaction with UV Light:

The ultraviolet (UV) component of sunlight plays a crucial role in the fading process of lemon juice. UV radiation has high energy and can break chemical bonds within molecules, including those present in ascorbic acid and flavonoids. This disrupts the molecular structure and leads to the formation of colorless or differently colored compounds, contributing to the overall fading effect.

4. Effect of Temperature and Oxygen:

The fading of lemon juice is accelerated by higher temperatures and the presence of oxygen. Heat can increase the rate of chemical reactions, while oxygen acts as an oxidizing agent, facilitating the degradation of ascorbic acid and flavonoids.

It's worth noting that the rate of fading can vary depending on several factors, such as the intensity and duration of sunlight exposure, the concentration of ascorbic acid and flavonoids in the lemon juice, and the temperature and oxygen levels in the environment. Refrigeration and storage in dark containers can help slow down the fading process and preserve the color and flavor of lemon juice.