A slice of apple left on a table slowly goes brown due to an enzymatic reaction. Dipping the apple slice in lemon juice prevents it from browning Which of the following best explains this result?
A.
Lemon juice has a pH which inactivates the enzymes.
B.
Lemon juice functions to dilute the brown material.
C.
Lemon juice functions to bleach the brown material.
D.
Lemon juice has enzymes which verse the browning reaction.
The Correct Answer and Explanation is:
The correct answer is A. Lemon juice has a pH which inactivates the enzymes.
When an apple slice is left exposed to air, it undergoes a browning process due to a biochemical reaction called enzymatic browning. This reaction is catalyzed by an enzyme called polyphenol oxidase (PPO), which is naturally present in the apple. Upon exposure to oxygen, PPO facilitates the oxidation of phenolic compounds in the apple to form melanin, a brown pigment. This process is a form of oxidation, and it is similar to how metals rust when exposed to air.
Lemon juice, which is acidic (with a pH around 2-3), effectively prevents the browning process. The reason for this lies in the nature of enzymes like PPO, which are highly sensitive to changes in environmental conditions, such as pH. Enzymes have an optimal pH range in which they function most efficiently. For PPO, the optimal pH is typically neutral or slightly alkaline, but when exposed to the acidic environment created by lemon juice, the enzyme becomes inactivated or denatured. In this denatured state, the enzyme’s structure is altered, preventing it from catalyzing the oxidation of phenolic compounds. Without the action of PPO, the oxidation process that leads to browning is slowed down or stopped, thereby keeping the apple slice looking fresh for a longer period.
Moreover, the ascorbic acid (vitamin C) in lemon juice also plays a role as an antioxidant. It reacts with oxygen before PPO can, which further delays the browning reaction.
The other options do not explain the process accurately. Option B is incorrect because lemon juice doesn’t dilute the brown material; it prevents the browning from happening. Option C is also incorrect since lemon juice does not bleach the brown material, and D is incorrect because lemon juice does not contain enzymes that reverse the browning reaction.