Why Fruits and Vegetables Turn Brown After Peeling

Summary

Fruits and vegetables turn brown due to enzymatic browning, a biochemical reaction triggered when the plant tissue is exposed to oxygen after being injured. This process involves an enzyme called polyphenol oxidase oxidizing polyphenols, producing brown-colored melanins. Though this leads to food waste, it’s also responsible for desirable colors in items like prunes and coffee. Various methods, such as refrigeration, freezing, blanching, and adding acids like lemon juice, can slow down this process, preserving food's appearance and nutrients. These techniques are commonly used to prevent spoilage and maintain quality during food storage and preparation.

Takeaways

• 😀 Enzymatic browning is a biochemical reaction caused by enzymes and oxidation, which makes fruits and vegetables turn brown.
• 😀 The reaction is triggered when plant tissue is exposed to oxygen, often due to injury or cutting.
• 😀 Polyphenol oxidase is the enzyme responsible for catalyzing the oxidation of polyphenols in fruits and vegetables.
• 😀 The brown color is caused by the formation of melanins, the same pigment found in human hair, skin, and eyes.
• 😀 Enzymatic browning can be observed in fruits like apricots, bananas, and grapes, as well as vegetables like potatoes and mushrooms.
• 😀 While enzymatic browning leads to food waste, it is also responsible for desirable colors in products like prunes, coffee, and cocoa.
• 😀 Exposure to air can also reduce the vitamin C content in fruits and vegetables as it oxidizes over time.
• 😀 Refrigeration slows down enzymatic browning by inhibiting enzyme activity, but it does not completely deactivate it.
• 😀 Freezing inhibits enzyme activity as well, but it does not inactivate the enzymes, meaning browning can resume after thawing.
• 😀 Methods like covering fruits with honey, caramel, or sugar syrup can block oxygen exposure and slow down browning.
• 😀 Blanching is a short heat treatment that destroys or inactivates enzymes before freezing, preserving the quality of vegetables.
• 😀 Lowering the pH of fruits and vegetables, such as by adding lemon juice or vinegar, can inhibit enzymatic browning and slow the reaction.

Q & A

What is enzymatic browning and why does it occur?

Enzymatic browning is a biochemical reaction that occurs when fruits or vegetables are exposed to oxygen after being cut or damaged. This process involves an enzyme called polyphenol oxidase, which oxidizes polyphenols in the plant tissue, leading to the production of brown-colored melanins.

What role does the enzyme polyphenol oxidase play in enzymatic browning?

Polyphenol oxidase is the enzyme responsible for catalyzing the oxidation of polyphenols in fruits and vegetables when exposed to oxygen, resulting in the formation of brown melanins.

What are some common examples of fruits and vegetables that undergo enzymatic browning?

Fruits like apricots, pears, bananas, and grapes, as well as vegetables like potatoes, mushrooms, lettuce, and seafood like shrimp, tiny lobsters, and crabs, all undergo enzymatic browning.

How does enzymatic browning affect the nutritional content of fruits and vegetables?

Enzymatic browning can lead to the oxidation of vitamin C in fruits and vegetables. The longer the exposure to air, the more vitamin C is oxidized, reducing the nutritional value.

How can refrigeration help reduce enzymatic browning?

Refrigeration helps slow down enzymatic browning by reducing the activity of polyphenol oxidase. At temperatures below 7°C, the enzyme's activity is inhibited, though it is not completely inactivated.

What is the effect of freezing on enzymatic browning?

Freezing inhibits polyphenol oxidase activity, much like refrigeration, but it does not inactivate the enzyme. After thawing, the enzyme activity may resume, potentially causing further browning.

Why does adding honey, caramel, or sugar syrup prevent browning in fruits?

Covering fruit slices with honey, caramel, or sugar syrup blocks their exposure to oxygen, thereby preventing the enzymatic browning reaction from occurring.

What is blanching, and how does it help prevent enzymatic browning?

Blanching is a short heat treatment that destroys or inactivates enzymes in vegetables before freezing. It helps maintain color, texture, and prevents discoloration during freezing.

How does lowering the pH of fruits and vegetables affect enzymatic browning?

Lowering the pH of fruits and vegetables to around 4 by adding acids like citric or ascorbic acid inhibits polyphenol oxidase activity, slowing down the enzymatic browning process.

How does lemon juice or vinegar prevent browning in fruits?

Lemon juice and vinegar contain acids that lower the pH of the fruit, which inhibits the activity of polyphenol oxidase. Additionally, antioxidants in these juices further slow down the browning reaction.