NDA Biology · Biochemistry

Food Spoilage — Rancidity and Browning

Food goes bad mainly through oxidation: fats and oils turn rancid when oxidised by air, and cut fruit browns when the enzyme polyphenol oxidase oxidises its compounds — both slowed by keeping oxygen away.

All Biochemistry notes NDA Biology strategy

Why this matters

The NDA tests this as everyday-science recall — what happens to fats left in air (oxidation → rancidity) and how to stop a cut apple browning (lemon juice). The common thread is oxidation by oxygen, and the fixes all work by blocking oxygen or an antioxidant. EASY recall.

Concept 1 of 2

Rancidity — the oxidation of fats and oils

Intuition

Leave butter, cooking oil or fried snacks out in the air long enough and they develop a stale, unpleasant smell and taste. That is rancidity: the oxygen in air slowly oxidises the fats and oils, breaking them into smelly compounds.

Definition

Rancidity is the oxidation of fats and oils by atmospheric oxygen, producing the off-flavours and bad smell of spoiled fatty food. Ways to slow it down — all work by keeping oxygen away or scavenging it:

Add antioxidants (e.g. BHA, BHT) to packaged fatty food.
Store in airtight containers and refrigerate (cold slows the reaction).
Flush packets with nitrogen gas (replacing the oxygen — why chip packets are puffed with N₂).
Keep food away from light.

What rancidity is

\text{fats / oils} + O_2 \;\to\; \text{oxidised (rancid) products}

Worked example

Why are packets of potato chips often filled with nitrogen gas instead of ordinary air?

Rancidity is caused by oxygen oxidising the fats/oils in the chips.
Nitrogen is unreactive and displaces the oxygen, so there is no O₂ left to oxidise the fat.

Answer:Nitrogen removes oxygen from the packet, preventing oxidation (rancidity) of the fatty chips.

From the bank · past-year question

Example 1BiochemistryEASY

What happens when the fat and oil containing food materials are left outside for a long time?

[Q109 · Apr · 2025]

Rancidity is OXIDATION, not reduction or freezing

Fats left in air get oxidised (not reduced, not 'ice-covered'). The whole point is that atmospheric oxygen attacks the fat — so every prevention method works by removing or blocking oxygen.

Concept 2 of 2

Enzymatic browning of cut fruit

Intuition

Slice an apple, potato or banana and the exposed surface turns brown within minutes. An enzyme in the fruit reacts with oxygen in the air to make brown pigments — and a squeeze of lemon juice stops it.

Definition

Enzymatic browning is the browning of cut fruit and vegetables, caused by the enzyme polyphenol oxidase (PPO) oxidising the fruit's phenolic compounds (using atmospheric O₂) into brown pigments. Ways to prevent it:

Lemon juice — its ascorbic acid (vitamin C) and citric acid are acidic antioxidants that inhibit the enzyme (the bank's answer).
Dipping in cold water or salt water — limits contact with oxygen.
Blanching (brief heating) — denatures the enzyme.

What enzymatic browning is

\text{phenols} + O_2 \;\xrightarrow{\text{polyphenol oxidase}}\; \text{brown pigments}

Worked example

A cook sprinkles lemon juice on freshly cut apple slices to keep them looking fresh for a fruit salad. How does this work?

Apple browning is caused by the enzyme polyphenol oxidase oxidising the fruit's compounds in air.
Lemon juice is acidic and contains vitamin C (an antioxidant), which inhibits the enzyme.

Answer:The acid/antioxidant in lemon juice inhibits polyphenol oxidase, preventing browning.

From the bank · past-year question

Example 2BiochemistryEASY

Browning of the chopped apple can be minimized by :

[Q73 · Apr · 2023]

Lemon juice stops browning — not sugar or milk of magnesia

The browning fix is lemon juice (acidic + vitamin C, inhibits the enzyme). Table sugar, a closed container alone, or milk of magnesia (a base) are distractors. Browning needs the enzyme + oxygen; an acid antioxidant is what stops it.

Rancidity vs enzymatic browning

Both are oxidation, but rancidity is the oxidation of fats/oils (no enzyme needed), while browning is an enzyme-driven (polyphenol oxidase) oxidation of cut fruit/vegetables. Don't merge them.

Summary — formulas & gotchas at a glance

A revision cheat-sheet for the formulas and gotchas above. Click any concept name to jump back to its full explanation.

Formulas (2)

Rancidity — the oxidation of fats and oils
What rancidity is
$\text{fats / oils} + O_2 \;\to\; \text{oxidised (rancid) products}$
Enzymatic browning of cut fruit
What enzymatic browning is
$\text{phenols} + O_2 \;\xrightarrow{\text{polyphenol oxidase}}\; \text{brown pigments}$

Watch out for (3)

Rancidity is OXIDATION, not reduction or freezing
→ Rancidity — the oxidation of fats and oils
Lemon juice stops browning — not sugar or milk of magnesia
→ Enzymatic browning of cut fruit
Rancidity vs enzymatic browning
→ Enzymatic browning of cut fruit

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