Question

When fruits such as apples and pears are cut, the exposed parts begin to turn brown. This is the result of an oxidation reaction catalyzed by enzymes present in the fruit. Often the browning action can be prevented or slowed by adding a few drops of lemon juice to the exposed areas. What is the chemical basis for this treatment?

Short answer

The chemical basis for applying lemon juice on cut fruits to prevent or slow down browning is twofold. 1) The ascorbic acid (vitamin C) in lemon juice acts as an antioxidant, which reacts with oxygen that would have otherwise reacted with the fruit compounds to cause browning, thus reducing the oxidation reaction. 2) The low pH of lemon juice inactivates the enzyme (polyphenol oxidase) that catalyzes the browning reaction, thereby slowing down the browning process.

Step-by-step solution

Understand oxidation

When fruits like apples and pears are cut, an enzyme called polyphenol oxidase (or tyrosinase) in the fruit reacts with oxygen to convert phenolic compounds in the fruit into melanin, a pigment which is responsible for the brown color. This is an oxidation reaction. The oxygen is supplied from the atmosphere once the fruit is cut and the cells are broken open.

Understand the effects of lemon juice

Lemon juice is a great source of ascorbic acid, also known as vitamin C. When a few drops of lemon juice are added to the exposed areas of the cut fruit, the ascorbic acid works by reducing the pH on the surface of the fruit. At this lower pH, the enzyme responsible for the browning (the polyphenol oxidase) is less active.

Chemical basis of treatment

The lemon juice acts as an antioxidant, reacting with the oxygen that would have reacted with the phenolic compounds in the fruit. In addition, the lower pH from the ascorbic acid denatures and thus inactivates the polyphenol oxidase enzyme, slowing down the browning process.

Key concepts

Polyphenol Oxidase

Polyphenol oxidase is an enzyme found in many fruits like apples and pears. It is responsible for the browning you often see when these fruits are cut open and exposed to air. This occurs because the enzyme reacts with oxygen in the atmosphere. When polyphenol oxidase combines with oxygen, it catalyzes the conversion of phenolic compounds in the fruit into melanin. Melanin is a pigment that gives the fruit its brown color.

• In normal, uncut fruit, the skin acts as a barrier, preventing oxygen from reaching the enzyme.
• When the fruit is cut, this barrier is broken, starting the browning process.
• Reducing exposure to oxygen or decreasing enzyme activity can slow or stop this reaction.

By understanding this, we can better manage and control fruit browning in everyday situations.

Ascorbic Acid

Ascorbic acid, also known as Vitamin C, is a natural antioxidant found abundantly in lemon juice. When you apply lemon juice to the cut surface of fruits like apples, the ascorbic acid plays a crucial role in preventing browning.
Here's how it works:

• Firstly, it lowers the pH of the fruit's surface, creating an environment where polyphenol oxidase is less active.
• Secondly, as an antioxidant, ascorbic acid reacts with oxygen that would otherwise promote the browning process.
• This dual action helps slow down and even prevent the browning reaction.

Ascorbic acid's ability to act as an antioxidant and a pH-modifying agent illustrates its importance in fruit preservation.

Enzyme Activity

Enzyme activity refers to how active and efficient an enzyme is at catalyzing a chemical reaction. In the case of fruit browning, the enzyme in question is polyphenol oxidase. The rate at which this enzyme catalyzes the conversion of phenolic compounds to melanin depends on several factors.
Some key factors include:

• **pH Levels:** Lower pH levels, like those caused by ascorbic acid, can decrease enzyme activity.
• **Temperature:** Higher temperatures can denature enzymes, making them less effective.
• **Presence of Inhibitors:** Substances that bind to the enzyme or react with its environment can slow down its activity.

By influencing these factors, such as adding lemon juice to lower pH, we can control enzyme activity. This slows down undesirable processes like fruit browning, helping to maintain a fruit's fresh appearance.