1. Raising Agents  
Raising Agents are gases which are introduced into a food mixture :
During the mixing process
In a substance which is added to the mixture
The gas is incorporated or released when ingredients are combined.
On heating (baking), the minute bubbles of gas enclosed in the mixture expand, causing the mixture to rise.

2. The three most common raising agents are steam, air and carbon dioxide.
They are frequently used in combination with each other
e.g. in a creamed cake mixture :-
Steam from liquid (egg/milk/water)
Air beaten in (creaming butter & sugar)
Carbon dioxide from chemical raising agents in baking powder or self raising flour.

3. Steam
Some food products have steam as the main raising agent. These products must contain a high proportion of liquid, e.g. Choux pastry, batter used for Yorkshire puddings.
For steam to make the mixture rise, two conditions are needed;
A large proportion of liquid (Milk/Water)
A high baking temperature of 200oC or above
During baking;
Liquid reaches boiling point
Steam forces its way up through the mixture, stretching the mixture and making it rise.
The steam escapes and is replaced by air, the mixture bakes and sets into the risen shape.
Sometimes steam is trapped inside. It condenses on cooling and then the product collapses. This often happens in Yorkshire puddings.

4. Air
Mechanical action incorporates air into mixtures, it is the main raising agent in some food products, e.g. whisked sponge cakes and soufflé
Air can be mechanically introduced into mixtures by;
Sieving flour – air is trapped between fine particles
Rubbing fat into flour – air is trapped between breadcrumbs
Creaming fat and sugar – air is beaten in, to form an air and fat foam.
Whisking eggs/sugar or egg whites – air is whisked ino the egg to form a high volume foam
Beating batter – air is beaten in, but steam is the main raising agent.
Rolling and folding pastry – air is trapped between the layers

5. Carbon Dioxide This is produced in two ways;
Biologically – from yeast cells during the fermentation process
Chemically from the action of bicarbonate of soda with an acid

6. Biological Raising Agents
Yeasts are micro organisms, during the fermentation process, the yeast cells feed on sugar and reproduce to produce carbon dioxide.
Certain factors affect the action of yeast :-
Temperature – optimum temperature range is 25oC – 35oC
Above 60oC the fermentation process is destroyed
Below 25oC the reaction is slowed down
Fresh yeast can be frozen without killing it and it becomes active again once defrosted.
Moisture – This is provided from milk/water/egg
Food – During fermentation of yeast, the cells need to be fed on sugar to produce carbon dioxide. There is also a small amount of sugar in the flour

7. Chemical Raising Agents
Carbon dioxide is produced from chemical reactions. This can be made to happen in many recipes by the addition of a chemical raising agent.
Chemical raising agents must be measured accurately, they ;
Are used in small quantities
Are easy to use and often already combined – e.g. self raising flour
Will produce controlled amounts of carbon dioxide
to aerate mixtures evenly.
Three types of chemical raising agent are :-
Bicarbonate of Soda – known as sodium bicarbonate
Bicarbonate of Soda & Acid (alkali & acid)
Baking Power or baking powder in self raising flour

8. Bicarbonate of Soda :-
Simplest type of chemical raising agent. When heated in a mixture, it produces sodium carbonate (soda) steam and carbon dioxide.
The carbon dioxide and steam aerate the mixture. The soda leaves a dark yellow colour and gives a sharp alkaline taste to the mixture.
Used in recipes which have other strong flavoured ingredients e.g. chocolate cake & gingerbread
Bicarbonate of Soda plus Acid
The bicarbonate of soda reacts with the acid to produce carbon dioxide. The acid helps to neutralise and prevent the taste and colour residue left when bicarbonate of soda is used on its own.
A traditional source of acid is sour milk. This is still used in some mixtures, an other sources of acid is Cream of Tartar (Tartaric Acid), when combined with bicarbonate of soda this releases the carbon dioxide slowly, leaving a tasteless salt behind. The ratio used is 2 parts Tartar to 1 part bicarbonate of soda.
Baking Powder
Commercial mixture of bicarbonate and acid substances ready mixed in the correct proportions. A buffer is added in the form of cornflower or rice flour to absorb any moisture and prevent the reaction taking place in the storage container.
Baking powder is added to plain flour to produce self raising flour.