Functional & Chemical Properties of Ingredients

Why Ingredients Behave the Way They Do

Every time you make a sauce, bake a cake or fry an egg, you are running a chemistry experiment. The spec identifies twelve named working, functional and chemical properties — specific chemical or physical changes that occur when ingredients are subjected to heat, liquid, mechanical action or acid. Knowing the correct term, the right ingredient and what happens at a molecular level is the difference between a one-mark recall answer and an eight-mark explanation. This lesson covers all twelve, grouped by ingredient type, focusing on familiar dishes and what goes wrong when they fail.

Carbohydrates: Gelatinisation and Dextrinization

Gelatinisation is what happens to starch in a liquid when heated above approximately 60°C. Starch exists as tightly packed granules containing two types of starch molecule: amylose (long straight chains) and amylopectin (branched chains). When the granule is placed in warm liquid and heated, the hydrogen bonds holding the granule together weaken, water molecules force their way in, and the granule swells dramatically. Above about 80°C, the granule bursts, releasing the amylose and amylopectin molecules into the surrounding liquid. These molecules form a tangled network that traps water — the result is the thickening of a sauce, custard, gravy or soup. As the mixture cools, the network becomes more rigid; a cornflour-thickened pudding sets solid in the fridge because the starch molecules form a firm gel on cooling.

A common failure: a lumpy sauce. If starch is added directly to hot liquid, the outer granules gelatinise instantly and form a sticky coating that traps dry starch inside. The fix is to disperse starch in cold liquid or fat first (a slurry or roux), then heat gradually while stirring.

Dextrinization occurs when starch is subjected to dry heat rather than moist heat. The long starch chains are broken by heat into shorter fragments called dextrins. These are sweeter than raw starch, browner in colour and more soluble. The golden-brown crust on toasted bread, the crust on a baked pastry, the coating on roasted potatoes and the surface of a pan of fried rice all involve dextrinization. Unlike gelatinisation (which requires water), dextrinization requires only heat and the absence of water — that is why bread goes golden in a toaster but not in a bowl of water.

Fats and Oils: Shortening, Aeration, Plasticity and Emulsification

Shortening occurs when fat coats the flour protein particles, preventing the glutenin and gliadin from combining with water to form gluten. The result is a product with a crumbly, tender, “short” texture. Shortcrust pastry and shortbread both exploit shortening: rubbing cold butter into flour coats the flour in fat before any water is added, limiting gluten formation and giving the characteristic melt-in-the-mouth crumb. The fat is literally “shortening” the gluten network.

Aeration is the incorporation of air bubbles into fat during creaming or whisking. When softened butter and sugar are beaten together (as in the creaming method for a Victoria sandwich), the sugar crystals cut through the fat, creating tiny air pockets that are trapped in the fat. In the oven, these air bubbles expand as the temperature rises, helping the cake to rise and develop a light, open crumb. Without adequate aeration during creaming, the cake will be dense and heavy — a common cause of a sunken, flat sponge.

Plasticity describes the spreadability and workability of solid fats at room temperature. Butter and margarine are plastic: they can be spread, rolled into pastry, shaped and piped because their crystal structure deforms without crumbling. Lard is highly plastic and was traditionally used for pastry, producing particularly short, crumbly results.

Emulsification is the process of combining an oil and a water-based liquid into a stable mixture. Normally, oil and water separate because their molecules are structurally incompatible. An emulsifier has one end that is attracted to water (hydrophilic) and one end attracted to fat (lipophilic). Lecithin in egg yolk is the natural emulsifier in mayonnaise: as the yolk is whisked with vinegar and oil is drizzled in slowly, the lecithin molecules surround tiny oil droplets, keeping them suspended in the vinegar. Remove the egg yolk or add the oil too quickly, and the emulsion splits — the oil and vinegar separate out.

Proteins: Coagulation, Foam Formation, Gluten Formation and Denaturation

Coagulation is the irreversible setting of protein after it has been denatured. When an egg is heated, the protein molecules (albumin in the white, phosvitin and livetin in the yolk) unfold (denature) and then bond together to form a solid network. A perfectly scrambled egg coagulates gently at around 70–80°C, forming soft, creamy curds. Overcooked, it becomes rubbery because excessive heat causes too many cross-links between molecules, squeezing out water. Coagulation also sets the crumb of a cake (egg protein sets around the air bubbles), thickens a custard and firms the interior of a quiche.

Foam formation occurs when egg whites are whisked. Physical action denatures the albumin proteins, causing them to partially unfold and form a film around the air bubbles introduced by whisking. Continuing to whisk builds the foam from soft peaks to stiff peaks. Fat in any form (even a trace of yolk) inhibits foam formation because it coats the protein molecules and prevents them forming a stable film — bowls and whisks must be completely fat-free for meringue.

Gluten formation occurs when wheat flour is mixed with water. Two proteins in the flour — glutenin (strong, elastic) and gliadin (extensible, plastic) — absorb water and link together to form gluten strands. Kneading develops and aligns these strands into a strong, elastic network that can trap carbon dioxide bubbles from yeast fermentation, allowing bread dough to rise and hold its shape during baking. The more a dough is kneaded, the stronger and more elastic the gluten network. Conversely, for shortcrust pastry, gluten development is minimised by using cold fat, cold water and minimal mixing — too much gluten makes pastry tough and difficult to roll.

Denaturation can be caused by heat (cooking an egg), acid (lime juice “cooking” fish in a ceviche — acid denaturation) or physical action (whisking egg whites). Importantly, denaturation is the first step before coagulation — protein first unfolds, then sets. Understanding the distinction matters: a gently warmed egg may partially denature without fully coagulating; an overheated scrambled egg fully coagulates to a rubbery solid.

Fruit and Vegetables: Enzymic Browning and Oxidisation

Enzymic browning occurs when fruit or vegetables are cut, bruised or damaged. Cells in the produce contain polyphenol oxidase enzymes that are normally kept separate from the phenolic compounds (substrates) also in the tissue. When the cells are disrupted, the enzyme and substrate come into contact, and in the presence of oxygen, a chemical reaction produces brown-coloured melanin pigments. This is why a sliced apple, a cut avocado, a peeled banana or a raw potato turns brown when left exposed to air. To prevent enzymic browning: coat the cut surface with lemon juice or ascorbic acid (vitamin C, which acts as an antioxidant competing for the oxygen), submerge in cold water (limiting oxygen contact), blanch briefly (heat deactivates the enzyme), or refrigerate (cold slows the enzymatic reaction).

Oxidisation is a broader chemical process in which oxygen reacts with food components, causing deterioration. In vegetables, oxidisation of chlorophyll causes colour loss — overcooked broccoli turns dull and khaki as the pigment degrades. Oxidisation of vitamin C during prolonged cooking also reduces nutritional value. In fats, oxidisation causes rancidity — the unpleasant smell and taste of stale cooking oil or old butter. Minimising oxygen exposure through airtight packaging, refrigeration or vacuum packing slows oxidisation in storage.