Raising Agent / Leavening Agent

Basic rule of baking if you want something to rise you will either need to use eggs and/or a raising agent of some description. You need bubbles to make cakes, breads and even some cookies rise.  The bubbles get warm in the oven they expand and your baking goods get lighter.  So the obvious question would be why are there so many types of raising agents and whats the difference between them.  Can you swap them?

Yeast - Typically used in breads, pizzas and bread based buns (Chelsea buns & Cinnamon buns).  It's not suitable for cakes as a general rule because yeast has a fairly strong flavour, especially in the case of sour dough.  Using yeast means taking time to leaven and knead the bread.  Yeast is a living organism that needs feeding (sugar of some description) and warmth to be kicked into action.

• Fresh Yeast - The best way that I can describe it that it looks like putty and smells like beer.
• Dried Fast Action Yeast - Powdered version of fresh yeast that is much more predictable 
• Sour Dough Starters - This is nothing shy of magic in my books. In the new year I will tackle the whole Sour Dough Bread thing.

Bread Soda / Baking Soda -Typically used for scones, cookies and Irish soda bread among many others. If you've ever made a Baking Soda Volcano you will know that if you add an acid to Bread Soda (who for those want it's proper chemical term it's NaCl -Sodium Chloride) it will fizz up and release a gas called carbon dioxide (CO2 this is the stuff we exhale).  This is a fairly instantaneous release of gas so only add the wet ingredients to the dry ingredients when you're ready to pop it into the oven.  Cakes that have a high acid content in them such as a lemon based cake or a cake with either buttermilk, sour cream, or yogurt in them will use baking soda as either the main raising agent or an additional agent to balance the chemical process and improve flavour.  This seems to be much more common in American baking recipes than Irish or UK equivalents.  I wonder of the origins I'm all ears if someone can enlighten (see the way I 'lightened' the sentence with a baking pun) me.

Baking Powder - Typically used for cakes.  Baking powder is composed of everything that you need   (Baking Soda and an Alkaline, a stabiliser (quite often Cream of Tartar) and some cornflour (so its not as at risk to moisture) in the correct ratios, to make a cake rise.  Typically a recipe calls for 1 teaspoon baking powder per 200g Plain Flour.  The actual baking powder recipe changes from company to company including one discovered by the man who created Birds Custard.

Self Raising Flour - I'm including this because even though it's not a raising agent it contains the correct ratio of flour to raising agent to make a cake rise, this makes life much easier.

Just a note on Scones and  Soda Bread if you have buttermilk use baking soda if you have fresh milk use baking powder