Functional Properties — Proteins and Carbohydrates

Functional Properties of Proteins

Proteins are not just a nutrient — they are functional ingredients that change their structure when heat, acid or mechanical force is applied. The spec identifies four functional properties of proteins: denaturation, coagulation, gluten formation and foam formation. These four processes underpin almost every baked or cooked protein-rich product, from a Victoria sponge to a quiche Lorraine.

Denaturation

Protein molecules exist in a precise three-dimensional coiled shape, held together by weak chemical bonds. When those bonds are disrupted — by heat, acid, salt or physical beating — the protein unfolds. This is denaturation. Think of the moment a raw egg hits a hot pan: the transparent, runny protein rapidly turns white and opaque as the albumin proteins in the white denature. The process is permanent — you cannot unfry an egg. Acids denature protein too. A lemon juice and olive oil marinade applied to raw salmon begins to denature the surface proteins before any heat is applied. The flesh turns from translucent to opaque, and the texture firms slightly. This is why “ceviche” — raw fish marinated in citrus juice — appears cooked even though no heat has been used.

Coagulation

Once proteins have denatured, continued heating causes them to coagulate: the unfolded chains bond to each other and form a solid, set network. When you bake a quiche, the egg-and-milk custard filling is liquid when it goes into the oven. As the temperature rises above about 70°C, the egg proteins denature and then coagulate, turning the filling from liquid to a solid, sliceable set. Too much heat (or too long in the oven) causes over-coagulation: the protein network contracts, squeezing out water and giving you a rubbery, watery quiche instead of a smooth, creamy one. Temperature control is everything.

Gluten Formation

Wheat flour contains two proteins: glutenin and gliadin. Individually, these proteins are relatively inert. But add water and work the mixture — kneading bread dough, rubbing fat into flour for pastry, mixing a cake batter — and these proteins combine to form gluten, an elastic network of long, stretchy strands. Gluten is what traps the carbon dioxide released by yeast in bread-making, allowing the dough to rise. The more you knead, the stronger and more elastic the gluten network becomes. This is why bread dough needs ten minutes of vigorous kneading to develop its full structure, while shortcrust pastry is handled as little as possible — you want a crumbly, short texture, not a tough, chewy one. Over-mixing pastry develops too much gluten and makes the final result tough.

Foam Formation

When egg whites are whisked vigorously, air is beaten into the liquid. The albumin proteins in the egg white are denatured by the mechanical action and form a thin, stable film around each air bubble. The result is a foam — a gas-in-liquid system where air is the gas and the egg white is the liquid. As you continue whisking, more and more air is incorporated until the foam reaches soft-peak stage, then stiff-peak stage. At stiff peak, the foam holds its shape. In a whisked sponge, the foam is folded into the batter; the trapped air expands in the oven, giving the cake its light texture. Over-whisking eventually breaks the protein film and the foam collapses — the egg white turns grainy and watery.

Functional Properties of Carbohydrates

Carbohydrates, especially starches and sugars, also have key functional properties that affect cooking outcomes. The spec identifies three: gelatinisation, dextrinisation and caramelisation. These three are frequently confused with each other in the exam — precise vocabulary is essential.

Gelatinisation

When starch granules are mixed with a cold liquid, they remain intact and separate. As the mixture is heated above about 60°C, the granules begin to absorb liquid and swell. Around 85°C they burst, releasing starch molecules that tangle together and thicken the liquid into a gel — this is gelatinisation. A roux-based béchamel sauce begins as butter and flour cooked together into a paste; when warm milk is added and the mixture is heated with constant stirring, the starch granules in the flour swell and burst, and the sauce thickens. The amount of starch added relative to the liquid controls the final viscosity: a thick pouring sauce uses less starch; a very stiff sauce or a setting agent uses more. This ratio is critical in sauce making.

Dextrinisation

Dextrinisation happens when starch on the surface of food is heated in dry conditions to above about 110°C. The starch breaks down into shorter molecules called dextrins, which are brown, have a toasted flavour and are partly soluble. The golden-brown crust on a loaf of bread is the result of dextrinisation: the starch in the outer layer of the dough is exposed to the dry heat of the oven and browns. Toast is an even clearer example — the pale bread surface goes from white to golden-brown to dark as the starch progressively dextrinises. This is not the same as the Maillard reaction, which involves both sugars and amino acids; dextrinisation involves starch only.

Caramelisation

Caramelisation occurs when sugars — not starch — are heated above approximately 160°C. The sugar molecules break down and recombine into hundreds of new compounds that produce shades of amber and brown, and a range of flavours from sweet and nutty to slightly bitter. Sprinkle sugar over a crème brûlée and apply a blowtorch: the sucrose melts and then caramelises into a hard, glass-like sheet. Roasted parsnips or carrots also caramelise where their natural sugars are exposed to the high, dry heat of an oven, which is why they develop a sweet, brown, almost sticky surface. Caramelisation requires higher temperatures than gelatinisation or dextrinisation and only involves sugars.