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Chapter 22: Problem 31

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

Expert verified
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

01

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.
02

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.
03

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.

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