Lemon juice keeps fruit from turning brown primarily because it contains acid, which inhibits the enzyme responsible for browning.
Here's a breakdown of the process:
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Enzymatic Browning: When fruits like apples, bananas, and avocados are cut or bruised, enzymes within the fruit are exposed to oxygen in the air. This triggers a chemical reaction called enzymatic browning, causing the fruit to turn brown.
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Lemon Juice's Role: Lemon juice contains citric acid, which lowers the pH of the fruit's surface. This acidic environment interferes with the activity of the enzyme polyphenol oxidase (PPO), the main culprit behind browning. Enzymes like PPO function best within a specific pH range. When the pH deviates significantly from this range (in this case, becoming more acidic), the enzyme's structure changes, preventing it from effectively catalyzing the browning reaction.
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Mechanism: Specifically, the acid in lemon juice denatures, or changes the shape of, the PPO enzyme. This altered shape makes it unable to bind with its substrates and complete the oxidation process that leads to browning.
In summary, lemon juice prevents fruit from browning by lowering the pH and inhibiting the activity of the enzymes that cause browning.
