Enzymic Browning and Oxidation

This lesson covers enzymic browning and the oxidation of vitamins, as required by the AQA GCSE Food Preparation and Nutrition specification (8585, section 3.3). You need to understand what causes enzymic browning, how to prevent it, and how oxidation leads to nutrient loss in fruit and vegetables.

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What Is Enzymic Browning?

Enzymic browning is a chemical reaction that occurs when enzymes in certain fruits and vegetables are exposed to oxygen in the air. The enzyme reacts with phenolic compounds in the plant cells, producing brown-coloured compounds called melanins. This causes the surface of the cut food to turn brown.

Key Definition: Enzymic browning is the discolouration of cut or damaged fruit and vegetables caused by the reaction between enzymes (polyphenol oxidase) and oxygen from the air.

How Enzymic Browning Works

1. The fruit or vegetable is cut, peeled, bruised or damaged.
2. This ruptures the cell walls, exposing the enzyme polyphenol oxidase (PPO) inside the cells.
3. The enzyme comes into contact with oxygen from the air.
4. The enzyme catalyses a reaction between the phenolic compounds in the fruit and the oxygen.
5. This produces melanins — brown pigments — on the surface of the food.
6. The food turns an unappealing brown colour.

Conditions Required for Enzymic Browning

Three conditions must all be present for enzymic browning to occur:

Condition	Explanation
Enzyme (polyphenol oxidase)	Present naturally inside the cells of certain fruits and vegetables
Substrate (phenolic compounds)	Present naturally in the plant cells
Oxygen	From the air; comes into contact with the enzyme when the cell is damaged

If any one of these three conditions is removed, enzymic browning cannot occur. This principle is the basis of all prevention methods.

Foods Affected by Enzymic Browning

Fruits	Vegetables
Apples	Potatoes
Bananas	Aubergines
Pears	Avocados
Peaches	Mushrooms
Apricots	Artichokes

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Preventing Enzymic Browning

Since enzymic browning requires enzyme + substrate + oxygen, prevention methods work by removing one or more of these factors:

1. Adding Acid (Lowering pH)

• Lemon juice, lime juice or vinegar can be brushed onto or sprinkled over cut surfaces.
• The acid lowers the pH of the environment, which denatures (deactivates) the enzyme polyphenol oxidase.
• The enzyme cannot function in acidic conditions, so browning stops.
• This is the most common domestic method.
• Examples: squeezing lemon juice on cut apples; adding lime juice to guacamole.

2. Blanching (Heating)

• Plunging the food briefly into boiling water (or steaming) for 30 seconds to 2 minutes, then cooling in iced water.
• The heat denatures (permanently deactivates) the enzyme.
• Once denatured, the enzyme cannot catalyse the browning reaction.
• This is commonly used before freezing vegetables to prevent discolouration during storage.
• Note: blanching changes the texture slightly (softens the food), so it is not suitable for all applications.

3. Excluding Oxygen

• Submerging cut fruit or vegetables in water (or acidulated water — water with lemon juice) prevents oxygen from reaching the surface.
• Covering the surface tightly with cling film removes air contact.
• Vacuum packing removes almost all oxygen from the packaging.
• Sugar syrup: immersing cut fruit in a concentrated sugar syrup creates a barrier between the food and the air.

4. Reducing Temperature

• Storing cut fruit and vegetables in the fridge or freezer slows enzymic browning.
• Cold temperatures reduce the rate of enzyme activity (enzymes work more slowly at lower temperatures).
• This slows browning but does not completely stop it — the enzyme is not destroyed, just slowed.

Summary of Prevention Methods

Method	How It Works	Removes Which Condition?
Acid (lemon juice)	Denatures the enzyme (lowers pH)	Enzyme (deactivated)
Blanching (boiling water)	Denatures the enzyme (heat)	Enzyme (permanently destroyed)
Submerging in water	Creates a barrier against air	Oxygen (excluded)
Sugar syrup	Creates a barrier against air	Oxygen (excluded)
Cling film / vacuum packing	Removes air contact	Oxygen (excluded)
Refrigeration / freezing	Slows enzyme activity	Enzyme (slowed, not stopped)

Exam Tip: When answering a question about preventing enzymic browning, always explain the mechanism — say why the method works, not just what it is. For example, do not just write "put lemon juice on it." Write: "The acid in lemon juice denatures the enzyme polyphenol oxidase, preventing it from catalysing the browning reaction." This scientific explanation gains the marks.

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Oxidation and Nutrient Loss

What Is Oxidation?

Oxidation in food science refers to the reaction of nutrients — particularly vitamins — with oxygen from the air, causing their destruction or degradation.

The most important example for AQA GCSE Food is the oxidation of vitamin C (ascorbic acid).

Vitamin C Oxidation

Vitamin C is:

• Water-soluble — it dissolves in water and can be leached out during cooking.
• Heat-sensitive — it is destroyed by high temperatures.
• Easily oxidised — it reacts with oxygen in the air, particularly when food is cut, exposed to air, or stored for long periods.

How Vitamin C Is Lost

Cause of Loss	Mechanism	Example
Cutting and exposure to air	Oxygen reacts with vitamin C, destroying it	Cutting an orange and leaving it exposed
Soaking in water	Vitamin C dissolves into the water (leaching)	Soaking chopped peppers in water
Boiling	Heat destroys vitamin C; it also leaches into cooking water	Boiling broccoli in a large pan of water
Prolonged storage	Gradual oxidation over time	Storing fruit juice in an open container
Alkaline conditions	Vitamin C is destroyed more rapidly in alkaline conditions	Adding bicarbonate of soda to cooking water (an old-fashioned practice to keep vegetables green)

How to Minimise Vitamin C Loss

1. Prepare fruit and vegetables just before cooking or serving — do not leave them cut and exposed to air.
2. Use a sharp knife — clean cuts damage fewer cells than a blunt knife (fewer cells ruptured = less enzyme release = less browning and oxidation).
3. Do not soak cut vegetables in water.
4. Cook quickly using methods that use little or no water (steaming, stir-frying, microwaving).
5. Use the minimum amount of water if boiling, and use the cooking water in sauces or gravy.
6. Serve immediately after cooking — do not keep food warm for long periods.
7. Store fruit and vegetables correctly — in a cool, dark place or in the fridge. Keep containers sealed.

Exam Tip: AQA often asks: "Explain how to preserve the vitamin C content of vegetables during preparation and cooking." A strong answer includes at least 3–4 methods with explanations. Always link each method to the specific reason it works (e.g. "steam rather than boil because vitamin C is water-soluble and will leach into boiling water").

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Enzymic Browning vs Oxidation: Key Differences

Feature	Enzymic Browning	Oxidation (Vitamin Loss)
What is affected	Colour (appearance) of fruit/vegetables	Nutritional content (vitamins, especially vitamin C)
Cause	Enzyme (PPO) + oxygen + phenolic compounds	Oxygen + heat + water exposure
Visible change	Brown discolouration on surface	No visible change (nutrient loss is invisible)
Prevention	Acid, blanching, excluding oxygen, cold	Quick preparation, minimal water, short cooking time

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Practical Applications

In the Kitchen