Updating guides…
Explains the functional and chemical properties of ingredients, including gluten formation, gelatinisation, denaturation, caramelisation, raising agents and emulsions.
When wheat flour is mixed with water and kneaded, two proteins called glutenin and gliadin combine to form gluten, a stretchy, elastic network that gives bread its structure and chewy texture. The more a dough is kneaded, the more gluten develops, which is why bread dough is worked thoroughly, while pastry is handled gently and kept cool, since too much gluten development would make pastry tough rather than short and crumbly. Denaturation is a permanent change to a protein's structure caused by heat, acid, or mechanical action such as whisking; this is why raw, runny egg white turns into a solid white when cooked, and why milk can curdle when an acid like lemon juice is added.
Gelatinisation occurs when starch granules are heated in a liquid; they absorb the liquid, swell and burst, thickening the mixture, which is how sauces made with cornflour or a roux are thickened. Dextrinisation is a different process, where dry starch is heated without liquid, causing it to break down and turn brown, as seen on the crust of toasted bread.
Caramelisation is the browning of sugar when it is heated to a high temperature, producing a nutty flavour and golden-brown colour, seen when making caramel or roasting root vegetables containing natural sugars. The Maillard reaction is a different browning reaction between amino acids (from protein) and reducing sugars, triggered by heat, and is responsible for the brown crust on bread, the seared surface of grilled meat, and the colour and flavour of roasted coffee.
Raising agents introduce gas into a mixture to make it rise. Biological raising agents, such as yeast, work by fermenting sugars in the presence of warmth and moisture to produce carbon dioxide gas, as used in bread making. Chemical raising agents, such as bicarbonate of soda or baking powder, react with acid and moisture to release carbon dioxide, commonly used in cakes and scones. Mechanical (physical) raising agents rely on trapping air, for example whisking egg whites to make meringue, or creaming fat and sugar together for a cake mixture; steam is also a physical raising agent, as the water in a mixture turns to steam and expands rapidly, as seen in choux pastry and Yorkshire puddings.
An emulsion is a mixture of two liquids that would not normally combine, such as oil and water, which are held together using an emulsifier — a substance with properties that bind fat and water molecules. Egg yolk contains a natural emulsifier called lecithin, which is why it is used to make stable emulsions such as mayonnaise and hollandaise sauce.