Quick answer: A raising agent (also called a leavening agent) is anything that puts gas into a dough or batter so it rises and becomes light. There are four main types: biological (yeast and sourdough), chemical (baking soda, baking powder and baker's ammonia), mechanical (air beaten in by creaming, whisking or sifting) and physical, mainly steam, which puffs up choux pastry, puff pastry and Yorkshire puddings.
Bakers rarely rely on just one. A sponge cake uses air from whisking and steam from baking; a scone uses baking powder and steam. This guide explains each type, how it works, where it is used and what goes wrong.
How raising agents work
Every raising agent produces or traps a gas, usually carbon dioxide, air or water vapour. When the mixture is heated, the gas expands. At the same time, proteins in flour and eggs set and starches gelatinise, fixing the risen structure in place. If the gas escapes before the structure sets, the bake sinks; if it expands too much, the structure tears and collapses.
Raising agents at a glance
| Type | Agent | Gas produced | Typical products |
|---|---|---|---|
| Biological | Yeast (fresh, active dry, instant) | Carbon dioxide from fermentation | Bread, pizza, buns, doughnuts, naan |
| Sourdough / natural starters | Carbon dioxide from wild yeasts and bacteria | Sourdough bread, idli and dosa batters (natural fermentation) | |
| Chemical | Baking soda (sodium bicarbonate) | Carbon dioxide when mixed with an acid | Cookies, soda bread, recipes with buttermilk or yoghurt |
| Baking powder | Carbon dioxide (soda plus acid in one product) | Cakes, muffins, scones, pancakes | |
| Baker's ammonia (ammonium bicarbonate) | Carbon dioxide and ammonia | Thin, crisp biscuits and crackers | |
| Mechanical | Creaming, whisking, sifting, folding, lamination | Air | Sponges, meringues, butter cakes |
| Physical | Steam | Water vapour | Choux pastry, puff pastry, Yorkshire pudding, puris |
1. Biological raising agents
Yeast
Yeast is a living single-celled fungus. In a warm, moist dough it feeds on sugars and produces carbon dioxide and alcohol. The gas is trapped by the stretchy gluten network developed during kneading, so the dough rises. The alcohol evaporates during baking but contributes to flavour.
Common forms:
- Fresh (compressed) yeast: moist, crumbly and perishable; kept refrigerated.
- Active dry yeast: dried granules, usually rehydrated in warm water first.
- Instant yeast: finer granules that can be mixed straight into the flour.
Yeast needs the right conditions to work well:
- Warmth: lukewarm liquid encourages it; very hot liquid kills it, and cold slows it down.
- Moisture: yeast cannot ferment in a dry mix.
- Food: sugars from flour, or a little added sugar.
- Time: slow fermentation gives better flavour and texture.
Salt controls yeast activity and strengthens gluten, but in direct contact with yeast in large amounts it slows fermentation, so mix it into the flour rather than straight onto the yeast.
Natural fermentation
Sourdough starters contain wild yeasts and lactic acid bacteria, which raise the dough and give a tangy flavour. In Indian cooking, idli and dosa batters rise through natural fermentation of soaked rice and urad dal, and some traditional breads use toddy or fermented batters in the same way.
2. Chemical raising agents
Baking soda (sodium bicarbonate)
Baking soda is an alkali. On its own it needs heat to release gas and leaves a soapy, bitter taste. When it meets an acid, such as buttermilk, yoghurt, lemon juice, brown sugar, honey or cocoa, it reacts quickly to release carbon dioxide. Because the reaction starts as soon as the ingredients are mixed, batters with baking soda should be baked promptly. Baking soda also encourages browning.
Baking powder
Baking powder combines baking soda with a dry acid and a starch filler that keeps it dry. Since it carries its own acid, it works in recipes that have no acidic ingredient. Most commercial baking powder is double-acting: it releases some gas when wet and more when heated in the oven, giving bakers more time between mixing and baking.
Baker's ammonia
Ammonium bicarbonate is an older raising agent still used in some crisp biscuits and crackers. It releases ammonia gas as well as carbon dioxide, so it is only suitable for thin, dry products where the ammonia can bake off completely.
Using chemical raising agents correctly
- Measure accurately. Too much gives a coarse crumb, a bitter or soapy taste and a bake that rises then collapses.
- Sift chemical raising agents with the flour so they are evenly distributed.
- Store them airtight. Moisture makes them react in the tin and lose strength.
- To test baking powder, stir a little into hot water; it should fizz.
3. Mechanical raising agents
Mechanical methods trap air in a mixture by physical action:
- Creaming butter and sugar until pale and fluffy creates tiny air pockets.
- Whisking eggs, egg whites or cream traps air in a foam, the basis of genoise, meringue and soufflés.
- Sifting flour aerates it and removes lumps.
- Folding gently combines ingredients without knocking out air.
- Rubbing in fat into flour, as for scones and shortcrust, traps some air.
- Lamination, folding layers of butter into dough, traps layers that later separate with steam.
Air on its own gives limited lift, but the bubbles act as starting points that other gases, such as steam and carbon dioxide, expand into.
4. Physical raising agent: steam
Water turns to steam in a hot oven and expands enormously in volume. Products with a high moisture content and a strong structure can rise dramatically on steam alone:
- Choux pastry for éclairs and profiteroles puffs up and becomes hollow.
- Puff pastry rises in layers as water in the butter and dough turns to steam.
- Yorkshire puddings and popovers rise from a thin batter in very hot fat.
- Puris and phulkas balloon when steam forms inside the thin dough.
Steam needs high heat. A cool oven lets the moisture escape slowly, and the product stays flat and heavy. Opening the oven door too early lets steam out before the structure sets.
Common problems and their causes
| Problem | Likely cause |
|---|---|
| Dense, flat bread | Old or killed yeast, liquid too hot, too little proving time |
| Bread collapses or smells strongly of alcohol | Over-proving |
| Cake domes and cracks | Too much raising agent or oven too hot |
| Cake sinks in the middle | Too much raising agent, underbaking, or oven door opened early |
| Soapy or metallic taste | Excess baking soda or too little acid to balance it |
| Choux buns do not puff | Paste too wet or too dry, or oven not hot enough |
Choosing the right raising agent
- Use yeast for doughs that can be kneaded and need time to develop flavour.
- Use baking powder for quick cakes, muffins and scones with no acidic ingredient.
- Use baking soda when the recipe already contains an acid.
- Use mechanical aeration for sponges and meringues where eggs carry the structure.
- Rely on steam for pastries with high moisture and a strong structure, baked at high heat.
Sugar affects how yeast and other raising agents behave, so it is worth reading our guide to the types of sugar alongside this one. To see where the pastry section fits in a professional kitchen, read about the kitchen brigade system.
Frequently asked questions
What are the four types of raising agents?
Biological (yeast and natural fermentation), chemical (baking soda, baking powder, baker's ammonia), mechanical (air from creaming, whisking and sifting) and physical (steam).
What is the difference between baking soda and baking powder?
Baking soda is a pure alkali that needs an acid in the recipe to release gas. Baking powder already contains both the alkali and an acid, so it works on its own.
Why is steam called a raising agent?
Water expands greatly when it turns to steam in a hot oven. That expansion puffs up choux pastry, puff pastry and Yorkshire puddings, often without any other raising agent.
Can I replace baking powder with baking soda?
Only if you add an acid and reduce the quantity, because baking soda is much stronger. Without an acid, the bake will taste soapy and may not rise well.
What temperature kills yeast?
Yeast is damaged by very hot liquid. Lukewarm liquid that feels comfortable on the inside of your wrist is a safe guide for activating it.
Is idli batter raised with yeast?
Traditional idli batter rises through natural fermentation of soaked and ground rice and urad dal, which happens through naturally present microbes rather than added baker's yeast.