What Is a Leavening Agent? Three Different Ways to Put Gas Into Dough

Biological leavening uses yeast or bacteria, which consume sugars and produce carbon dioxide over hours, and which simultaneously generate the acids and aromatic compounds responsible for most of the flavour in bread. It is slow, temperature-sensitive and builds flavour, which is why it suits bread and not cake. Chemical leavening uses a reaction between an acid and an alkali, conventionally bicarbonate of soda with an acidic ingredient, producing carbon dioxide within minutes and contributing nothing to flavour except an unpleasant soapy note if the proportions are wrong. Mechanical leavening incorporates air physically, by creaming fat and sugar, by whisking eggs or by folding, and that trapped air expands with heat. Steam is a fourth mechanism sometimes counted separately, since water in a batter turns to vapour and expands enormously, which is the sole leavening in choux pastry, in Yorkshire pudding and in puff pastry, where the expansion separates hundreds of layers.

The chemistry of the powders is worth understanding because it dictates timing:

• •Bicarbonate of soda is alkaline and requires an acid in the recipe to react, which may be buttermilk, yoghurt, lemon juice, vinegar, treacle, honey, cocoa or brown sugar
• •Without sufficient acid it does not fully react and leaves a soapy alkaline taste and a yellowish tinge
• •Baking powder contains bicarbonate plus a powdered acid plus a starch to keep them apart and dry, so it needs only moisture to activate
• •Single-acting powder reacts entirely on contact with liquid, so the batter must go into the oven immediately
• •Double-acting powder contains two acids, one reacting with moisture and one only with heat, which gives a second lift in the oven and tolerates delay
• •Excess of either produces a coarse open crumb that collapses, since bubbles grow too large, join up and escape before the structure sets

Gas alone achieves nothing without something to trap it, and what does the trapping differs by product. In bread it is the gluten network formed from wheat proteins during kneading, which is elastic enough to stretch around expanding bubbles and strong enough to hold them, and which sets when the protein denatures in the oven. In cakes it is a combination of egg proteins, starch gelatinising as it absorbs water, and fat holding air in an emulsion, with gluten deliberately minimised since a tough cake is a failure. In meringue it is egg white protein alone, unfolded by whisking and stabilised by sugar. In choux it is a pre-cooked starch paste strong enough to contain the steam until the eggs set. That is why the same leavening produces entirely different results in different recipes, and why substituting flours changes what a recipe can hold, since flours differ substantially in protein content and behaviour.

The sequence in the first minutes determines the result. Gas already present expands as it warms, and yeast becomes briefly hyperactive before the temperature kills it, which produces the burst of expansion bakers call oven spring. Water turns to steam and expands by a factor of over a thousand, which contributes more lift than is generally appreciated. Alcohol produced by fermentation vaporises and contributes further. Simultaneously, proteins begin to set and starch begins to gelatinise, and the moment those processes complete determines when expansion stops, which means the whole art is in having the gas production and the structure setting happen in the right order. Too fast a set and the product is dense, too slow and it rises and collapses. Steam injected into a bread oven delays crust formation, allowing expansion to continue longer, which is why professional ovens have steam injection and why home bakers put a tray of water in. The crust then forms and browns, and after that nothing rises any further.

Gas comes from yeast, from an acid reacting with bicarbonate, or from air and steam beaten in mechanically, and the choice sets the timing, since a chemical batter may need to go straight into the oven while a yeasted dough needs hours. Bicarbonate needs an acid in the recipe or it tastes soapy. Gluten, egg protein and gelatinised starch do the trapping, and the whole art is having gas production and setting happen in the right order.