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: