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.

---

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.

---

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).

---

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: