Dextrinisation and Caramelisation

This lesson covers two important browning reactions caused by dry heat — dextrinisation and caramelisation — as required by the AQA GCSE Food Preparation and Nutrition specification (8585, section 3.3). You need to understand the science behind each reaction, the difference between them, and be able to give examples of each.

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Overview: Two Types of Non-Enzymic Browning

Both dextrinisation and caramelisation are types of non-enzymic browning — browning reactions caused by heat, not by enzymes. They are distinct processes that affect different types of carbohydrate:

Reaction	Substance Affected	Cause	Result
Dextrinisation	Starch	Dry heat	Starch breaks down into dextrins; golden-brown colour and toasted flavour
Caramelisation	Sugar	Dry heat (high temperature)	Sugar breaks down; brown colour, distinctive sweet/bitter flavour

Exam Tip: The key distinction: dextrinisation affects starch, caramelisation affects sugar. AQA examiners may give you a scenario (e.g. toasting bread vs making toffee) and ask which reaction is taking place. Identify whether the main carbohydrate is starch or sugar to determine your answer.

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Dextrinisation

What Is Dextrinisation?

Dextrinisation is the chemical breakdown of starch molecules into smaller molecules called dextrins when exposed to dry heat. This causes the surface of starchy food to turn golden-brown and develop a characteristic toasted flavour.

Key Definition: Dextrinisation is the breakdown of starch into dextrins by dry heat, causing browning and a toasted flavour.

How Dextrinisation Works

1. Starchy food (e.g. bread, pastry, the surface of a loaf) is exposed to dry heat (e.g. in a toaster, oven or under a grill).
2. The heat breaks the long starch chains into shorter chains called dextrins.
3. The dextrins are smaller, sweeter and more soluble than the original starch.
4. The surface turns golden-brown and develops a crisp, toasted flavour.

Examples of Dextrinisation

Example	What Happens
Toast	Bread is exposed to dry radiant heat in a toaster; the surface starch breaks down into dextrins, creating golden-brown toast with a distinctive toasted flavour
Bread crust	The surface of bread in the oven is exposed to dry heat; dextrinisation creates the golden-brown crust while the interior remains soft (due to the moisture inside)
Top of a pie	The pastry lid is exposed to dry heat; starch on the surface dextrinises to produce a golden colour
Yorkshire pudding surface	The exposed surface browns through dextrinisation
Biscuits	Surface browning as starch on the outside is converted to dextrins

Key Points About Dextrinisation

• Only occurs with dry heat — the surface of the food must be dry for dextrinisation to happen.
• It is a surface reaction — it only affects the outside where the food is exposed to direct heat.
• The interior of the food does not dextrinise because it contains moisture.
• Dextrinisation produces a pleasant golden-brown colour and toasted flavour when controlled. If heat is excessive, the dextrins continue to break down and the food burns (turns black and bitter).

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Caramelisation

What Is Caramelisation?

Caramelisation is the chemical breakdown of sugar when it is heated to high temperatures. The sugar molecules break apart and recombine, producing a range of brown-coloured compounds and complex sweet, nutty, slightly bitter flavours.

Key Definition: Caramelisation is the thermal decomposition (breakdown by heat) of sugar, causing browning and the development of complex flavours.

How Caramelisation Works

1. Sugar (sucrose, glucose, fructose or other sugars) is heated to a high temperature (typically above 160°C for sucrose).
2. The sugar molecules break down (thermal decomposition).
3. A series of complex chemical reactions occur, producing hundreds of different compounds.
4. These compounds give a characteristic brown colour and complex flavour — initially sweet, then nutty, then (if heated further) bitter.

Stages of Caramelisation

As the temperature increases, the colour and flavour of the sugar changes:

Temperature (approx.)	Stage	Colour	Flavour
160°C	Light caramel	Pale gold	Sweet, delicate
170°C	Medium caramel	Golden amber	Rich, buttery
180°C	Dark caramel	Deep brown	Intense, slightly bitter
Above 190°C	Burnt	Black	Acrid, bitter (inedible)

Examples of Caramelisation

Example	What Happens
Crème brûlée	Sugar is sprinkled on top and heated with a blowtorch or under a grill until it caramelises into a crisp, golden layer
Toffee and fudge	Sugar syrup is heated to a high temperature; caramelisation produces the rich brown colour and distinctive flavour
Caramel sauce	Granulated sugar heated in a dry pan until it melts and turns golden-brown
Roasted vegetables	Natural sugars in carrots, onions and parsnips caramelise in the oven, creating sweet, golden-brown edges
Onion marmalade / caramelised onions	Slow cooking releases and caramelises the natural sugars in onions, turning them sweet and brown
Brûléed fruit	Sugar sprinkled on fruit and grilled or torched

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Comparing Dextrinisation and Caramelisation

Feature	Dextrinisation	Caramelisation
Substance	Starch	Sugar
Heat type	Dry heat	Dry heat (high temperature)
Temperature	Lower (begins at the temperature of dry-heat cooking)	Higher (above ~160°C for sucrose)
Products	Dextrins (shorter starch chains)	Hundreds of complex browning compounds
Colour	Golden-brown	Golden to deep brown
Flavour	Toasted	Sweet, nutty, caramel, potentially bitter
Key examples	Toast, bread crust, biscuits	Crème brûlée, toffee, caramelised onions

Exam Tip: A 4-mark question might ask: "Explain why toast is brown on the outside but white on the inside." The answer should include: the outside is exposed to dry heat → starch undergoes dextrinisation → starch breaks down into dextrins → golden-brown colour and toasted flavour. The inside remains moist → starch undergoes gelatinisation instead (absorbing moisture), so it stays soft and white.

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The Maillard Reaction (Extension Knowledge)

While not explicitly named in the AQA specification, the Maillard reaction is closely related and is worth understanding:

• The Maillard reaction occurs between amino acids (from proteins) and reducing sugars when heated.
• It produces a brown colour and complex flavours (the savoury browning on seared meat, the crust of baked bread, the flavour of roasted coffee).
• It is different from caramelisation because it requires both protein and sugar, not sugar alone.
• The Maillard reaction typically occurs above 140°C.

Understanding the Maillard reaction helps explain why foods that contain both protein and carbohydrate (such as bread) develop such complex flavours when baked or toasted — both dextrinisation and the Maillard reaction contribute.

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Common Exam Mistakes